UK-Cherub Forum
Cherub Chat => Tech Chat => Topic started by: Will_Lee on August 13, 2007, 08:19:20 PM
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The place to discuss T foils.
Atum Boms T foil is 900mm x 150mm x 15mm with a section created from ellipses and straight lines, with the widest point 30% back from the front. It is made on 200 kg/cubic metre foam with a layer of 200g weave over everything, and under there a 1/2 layer of 200g unis, and a 3/4 layer of 200g unis. This was vac bagged at the December 2005 Sticky weekend.
The measurements were decided upon simply because we wanted to build it using the same techniques as we made the rudder, and we wanted one like Aqua Marina's, but a bit bigger. It was only the prototype but when we made a thinner one with a huge span of 1.4m, thinking it would provide huge medium wind advantage with costs at low and high speed (although the plan is to still cut it down to 900mm), it was v draggy and unimpressive. We then got the old one back in service pronto.
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Will, would you go for a symmetrical foil again or an asymmetrical foil if you had an option?
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Hi Tim,
Sorry for delay in answering. An asymmetric foil means less drag (for a certain lift) when the T foil is working, but more drag when the T foil is not working (ie feathered for min drag). A symmetric foil is easier to make, and easier to make right, but (if it is done right), an assy one is probably better.
Our next one will be assy I think, unless I change my mind!
W
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Will, What size rudder blade did you have on Atum? and was it too big, too small?
I am thinking that with the T on the bottom it gives a good endplate for the rudder, however at the point where you need more area (slow and pulling hard) I doubt if the end plate makes a difference
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The blade I think is 740mm from the bottom of the stock downwards.
T-foil positions:
I think the 14's keep them high because the rising water near the surface is so marked. We have loads less of this, so I think lower is better.
That said, there is recovery of energy to be made, and a T foil on the bottom has to be very strongly built because of interference between the high pressure side of the rudder and the top of the T foil causing a huge twisty load on the join. If the T foil is moved up about 300mm from the bottom then the twisting disappears.
I do not know about fore-and-aft positioning of the T foil.
Will
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Obviously the T foil changes the handling and performance of the boat but
should different hull forms be fitted with different T foils.
ie. does a low rocker boat such as a DOG need a bit more downforce off wind to keep the bow up than a more rockery boat.
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Hmmmm possibly, but the thing about a t foil (unlike many things) is that even one which is not so good is loads better than not having one.
The optimal section of the T foil is dictated by the speed you go upwind (more=thinner), how heavy you are (more=fatter), and how far you can get back (more=fatter). The fatter you go, the more drag you will have when you are not using it.
The optimal vertical position is determined by the displacement of the boat (more disp=higher foil), and the speed the boat goes upwind (/sqrt (lwl)) This is why i14s have them so high, as they go slower upwind (/sqrt(lwl)) than us and weigh vastly more.
I think the message is that cherubs are all light and fast, and unless you think your optimal route upwind is more footed off than other people, then you probably want a similar section to everyone else.
The issue of the Dog needing more control may be important and may indicate a symmetric section with a control system which allows for lots of downpull as well as uplift.
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Thanks Will,
Was talking to Tim U last night about the fun he had crewing cheese last weekend. Once we have the kitchen sorted i will be making one.
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With a flat rocker like the Dog I think that being off trim slightly will be slower than being off trim in a rockery boat. So you may need to be more active in adjusting the t-foil to keep you going fast.
For fore and aft position of the blade I am thinking as far forward as possible as this lifts the boat more than rotating it to a bow down trim
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We have just bought Loco Perro and are soon to be getting a T foil. Is it easier to learn with or without it?
Also how easy are they to set up? ???
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Hi Simon,
Welcome! Loco Perro is quite a vessel and can be seen on this thrilling video by Daryl Wilkinson:
http://www.sailingsource.com/cherub/video/clips/punkitup.mov (http://www.sailingsource.com/cherub/video/clips/punkitup.mov)
A T-foil makes sailing at mach 10 in loads of wind easier, but apart from that there's not much difference in terms of ease or difficulty.
W
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I find the asymmetric section on Cheese very effective upwind, as soon as we are twining our speed is around 8-9kts over a force 5 we are over 10kts a lot of the time. But down wind is another story. In any reasonable breeze the dog does not like the stern being lifted at all. I have tried twining with me on the wheelie bars and my crew long side the T Foil (my rudder is rake forward a lot) and with any T foil on at all the bow ends up going through the chop which is slow and we normally end up pitch polling. With the T foil at a large negative angle to stop the asymmetric section creating lift it seems to be very draggy. It is hard to get cheese over 16kts with the T foil without theT-foil in similar conditions it would be around 19kts+. The T foil hums at over 12kt which is probably not helping. Overall we are much better off with it than without it but the next T-foil I put on Cheese will be symmetrical.
Paul C
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My foil is about 950mm span with a 100mm chord in the center, tapering to about 35mm at the tips (which are rounded off)
Im not sure on the thickness but id guess around 10mm. I used symetrical solid carbon windsurf fins (the same as Wills huge prototype foil - but cut down)
The foil is mounted at a slightly positive angle on the trailing edge of the rudder foil about 350mm up from the tip.
Ive heard that the ideal placement is to have the horizontal foil behind the max thickness of the vertical foil but im not too sure of the science behind it (Something to do with the pressure recovery zones) butputting some distance between the leading edges of both foils should reduce the interferance drag at any rate, hence my choice.
No real science went into the vertical placement - It was a relitively quick job with the priority on making something solid to improve on later and it seemed easier to build a solid foil like that than mounted on the tip as most of the fleet seem to be doing.
Im thinking about trying to do something a bit more serious in uni at some point....
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I have designed a main foil for a cherub. It is 1289mm span and has a 181mm cord in the centre, the leading edge is eliptical and the trailing edge is straight. I have based its size on the scaling up of a Moth main foil. The theory I have used is based on my knowledge of aerodynamics. I have taken the all up weight of a Moth and divided it by the foil area to get a weight per square metre (wing loading in aerodynamic language). I then multipled it by the all up weight of a cherub to get the total area needed. I added about 10% so if anything it takes off a bit earlier. I can't think of a reason why this theory wouldn't work, can anyone else?
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Do you have a crash helmet?
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Phil do you have twisty grip adjustment on Primal as well as "push rod" adjustment?
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Hi guys, we received our T/foil today. Only it looks like some fool has bashed it in transit. BUGGER! It has cracked on the front edge of the join and there is some degree of flex/movement. I don't imagine this would be much fun at mach 10. Although it might be spectacular to watch if it gave up altogether, never mind I'll have to get in touch with Aardvark and get it sorted.
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That's a pity!
Hope it's all sorted out soon. Have you been sailing yet?
Will
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Hi Will,
Not yet but we are planning to go to Draycote on sunday if the blast is still on. We can't go on sat as my Little boy is 4 , and I think my wife might go madder than a cherub with a broken T/foil if I don't make the party. It does all depend a bit on the weather as well.
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Hi Will,
Not yet but we are planning to go to Draycote on sunday if the blast is still on. We can't go on sat as my Little boy is 4 , and I think my wife might go madder than a cherub with a broken T/foil if I don't make the party. It does all depend a bit on the weather as well.
Blast weekend this weekend?
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Hi Ross,
Tim N has sorted it out . This weekend at Draycote. Are you going to be there? I think Hoppy is looking for a crew for Atum.
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BAH! I leave for a weeks skiing in Switzerland on Saturday :(. Skiing or Cherub blast weekend? Tough call. If the ski trip hadn't already been paid for I would be there.
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Pressure recovery? Can any one explain this one or should I just wait until the dinghy show for a chat?
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Energy recovery from T foils... The quick answer is that with the foil mounted closer to the free surface (a la I14 as opposed to the current cherub trend of mounting at the tip of the rudder) the low pressure above the foil has the effect of flattening the stern wave and you end up with the effect of a longer boat.
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Phil Alderson wrote:
For fore and aft position of the blade I am thinking as far forward as possible as this lifts the boat more than rotating it to a bow down trim
[/quote]
OK, here's a question from a T-foil virgin which will doubtless seem daft to foiling experts, but...
I've never understood why the foil goes on the rudder. If the idea is to use a smallish foil to reduce the displacement of the boat (as opposed to making the thing fly completely like a Moth), and instead carry a lot of the crew weight on the more hydrodynamically efficient T-foil, why not put it really far forward, i.e. on the end of the daggerboard? What's the point of putting the "apparent" centre of buoyancy way out the back of the boat?
N.
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A boat without a T-foil pitches around the hulls centre of floatation. A t-foil rudder moves it much further back in the boat, almost round the foil its self. This makes the boat behave like it’s much longer which helps in a small boat with a tall rig. This is the main reason for having it on the rudder, I think. If you we're to have a foil on the dagger board you could get upwind lift like the moths do (you always see them healed to windward). But this would call for different hull shapes for effective use. A foil on the dagger board would mean that the boat would pitch around it and make it very see-saw like. It's much easier to control the pitch of the boat from one end, where as if it was in the middle it would be a very difficult balancing act. If you read Kevin Elways T-foil article in the members section it makes the whole t-foil thing very clear.
Unless someone would like to add to or correct my thinking...
I have other thoughts but I don't know how to word them. ::)
A 49er with a foil on the dagger board.
(http://img352.imageshack.us/img352/7538/2008010516375249erkh4.jpg)
(http://img204.imageshack.us/img204/5490/20080107210715flyinguiuxn6.gif)
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There's a whole variety of reasons for the T foil being on the rudder than on the board. Here are two.
1) Self trimming: The feathers are on the back of an arrow or dart: When the arrow does not fly straight there is lift (and extra drag) made by the feathers at the back. This pushes the arrow back to flying straight. Now try that with the feathers on the front of the arrow - works fine until there is a tiny error in direction and then the feathers make those errors worse and it ceases to work! Now imagine a foil on the centre board (with no wand system) and it is the same. Everything is fine for approx one second until a wave/gust/comes and disturbs the system, then it is all over. (In fact, you don't have to dream, all you have to do is ask Fliptop how his uncontrolled foiling system worked on his Indian Takeaway. He sent some pics of the boat on the grass with foils in but oddly none of the boat in flight).
Similarly, if you overdo the T foil, the bow goes down, which unloads the T foil, so you are OK. Try that with a foil on the board!
Again similarly, you can load up the T foil by moving back, and unload it by moving forwards.
2) Pitching moment: Pitching about the back of the boat allows the whole boat to act as 'front', stabilising the rig for a certain sea state etc. . Also means you can get the people further back which relatively unloads the front of the boat, meaning less ploughing and more
contouring.
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Phil do you have twisty grip adjustment on Primal as well as "push rod" adjustment?
Just the push rod lead out to the sides. If I ever get around to building a rudder with a moveable foil then I would use a twist grip but with a fixed blade I think the tradeoffs are not worth it.
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As our T/foil was damaged in transit and has to go back for repair. Is it worth asking for the T to be moved from the bottom of the foil to " A BIT" (technical term) further up. If so how far up the foil should the T be?
I know Will was talking about the pressures being equallised. Any thoughts ?
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You are geting into very complicated fluid dynamics here so I doubt that there is a solid answer. there are advantages and disadvantages to the end and the middle
The Only hard rule I would say is make sure that the tips will not break the surface of the water when heeled, or the rudder ventilates and lets go suddenly :o
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Stand by for some ASCII Art!
If you sail with the rudder loaded (ie a bit of weather helm), then there is high pressure on the leeward side of the board and low pressure on the windward side:
(looking from the bow, with the boat on Stb tack)
Low Rudder High
____________________________
- | | +
- | | +
- | | +
- | | +
- | | +
V
Imagine a T foil on it's own in the water (not attached to anything): When it is loaded, there is low pressure above and high pressure below:
Low Pressure - - - - - - - - - - - -
T foil =================
High Pressure + + + + + + + + +
Now put the two together and you get:
____________________________
- | | +
- | | +
- | | +
- | | +
- | | +
- | | +
- | | +
- | | +
- | | +
- | | +
-- | | 0
-- -- -- -- | | 0 0 0 0 0 0 0
| |
=================
+ + + + + + + + +
The minuses combine to become double-minuses on the windward side, but on the leeward side they minuses and the pluses cancel out meaning less lift from the foil (and less lift from the rudder too I think).
The major problem of this is the large twisting load on the junction of the T foil and the rudder.
This problem can be eliminated by moving the T foil up the rudder a bit. (About 300mm? I don't know, sorry). This leaves this situation:
- | | +
- | | +
- | | +
- | | +
-- | | 0
-- -- -- --| | 0 0 0 0 0
================
0 0 0 0 0 | | ++ ++ ++ ++
0 | | ++
- | | +
- | | +
- | | +
Here you can see that the extra low pressure on one side is balanced by the extra high pressure on the other: No twist!
Other solutions to the twisty problem are:
1) Build the T foil really reallly well.
2) Don't load the rudder too much.
Hope that lot helps,
Will
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I have been trying to figure out how far along the T-Foil the influence of the rudder goes,
I have not found anything useful yet other than Biplane wings should be at least one chord width apart.
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Kevin is your man for questions of that kind.....
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Old .... and new rudder.
Now I just need to decide where to put the T foil ?
I don't want to put it at the tip for obvious structural reasons.
Should the T foil be at the front on the leading edge or at the back on the trailing edge ?
The rudder is about 16 cm wide while the foil is 12.
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Nice looking new rudder!
I think that from a hydrodynamic point of view it doesn't make too much difference, however if the T foil is further forward then you can load it up more. Also it is probably a good idea to have the thickest part of the rudder and the thickest part of the T foil intact, so a short slot on the back of the T foil and a short slot on the front of the rudder would allow you to fit the two together neatly, while preserving the thickest (strongest and stiffest) part of both.
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I've been doing a lot of CFD analysis of T foils, centreboard and rudder interactions and on planing hull trim. So here is a quick summary of findings
1) T foil sections
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Best to use asymmetric section with about 10% t/c. Speer H105 is a good section because it gives low drag a zero lift and has an extended drag bucket at higher lifts.
2) T foil planforms
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Assuming you don't want a span >900mm, use an approx. ellipse. You can get approx elliptical loading by having a trapezoid with a taper ratio (tip/root) of around 0.5. Make tips rounded.
3) Position on rudder
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The T foil lift is of no benefit below about 6kts and so should be a zero lift below 6kts boat speed - see Y&Y article for reason.
If your boat is in bow down trim at say 7kts, best place is as close to the water surface as possible to take advantage of the upward flow off the bottom of the hull. Most cherubs are too fat in the bows to do this and start to adopt a bow up trim. One this happens, the near surface advantage is lost.
My thoughts are that it is more practical and easier to fix T if it is at bottom - makes launch and recover easier. See also 4)
4) Aspect ratio of rudder blade
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Given the normal ratio of rudder to CB sizes, the foils will give best lift to drag when the rudder is unloaded (i.e. practically neutral helm when boat is upright).
The trend on I14s for long skinny rudders makes no sense if the rudder is only lightly loaded. This is because only the induced drag (the drag caused directly by producing lift) is reduced by aspect ratio. If the rudder is lightly loaded, then the induced drag is very small.
For control at low speeds, pre start moves, and general control, you can do for a wider chord (25 cm or so) and a short blade. If you stick the T on this, it will be pretty near the surface anyway.
5) Using a T foil
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For any given speed, a hull will have an optimum fore and aft trim angle that will give least drag. It is important that this is maintained when using the T foil rudder. When at planing speeds, too much trim will be less detrimental to speed than too little, so don't pull the T on so that the bow is in the water. You must be able to place your weight sufficiently far aft to maintain the same optimum trim as you would without the T.
6) Notes on optimum hull trim
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When a boat is planing, the optimum trim angle depends upon the lift coeff of the hull's bottom. If your boat has a fair bit of chine rise in the back, trim will be around 7-8 degrees . For a real flatty like the Dog, it will be nearer 5 degrees.
Zero lift on a symmetric foil is at zero to the water flow. So on the Dog this would be at about -5 degrees to the boat. Zero lift for an asym foil is about -3 degrees, so this would be a -8 degrees on the Dog.
In practice, you will never use a T foil a negative lift.
7) Notes on nosediving
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If your boat is nosediving or sailing too bow down with the T foil fitted compared to when it is not fitted, then the foil is producing lift.
The balance of a Cherub offwind is a precarious one because of the ratio of rig height to boat length. The C of E of the rig with the kite is about 45% of the way up the mast. This produces a moment trying to pitch the bow down. You try to counter this by moving aft.
If the drive from the rig (with kite) was horizontal, practically any Cherub would become unsailable as the wind increased. Fortunately, the drive is not horizontal. If you have a long pole and a highly raked mast like on Atum the drive is upwards at about 34 degrees to the horizontal. This reduces the pitch down moment and lightens the boat, as does the T foil. (See WB sails website article on Lifting the bows of a Cherub)
If your boat is really divey, then you need to increase the rake, increase the pole length and lengthen the wheelie bars. If you imagine the line of the drive vector from the rig and extend this line backwards, it will eventually cross the line of the boat at sheer level. If, where it crosses, is in front of where you can stand (lots of rake and long pole), then the boat will never dive. If it's well behind you (short pole, upright mast) you'll be sailing a sub.
I hope these comments help.
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Excellent comments Kevin Thanks for those.
Regarding the fore and aft position I have heard that you should avoid having the high pressure zones of the two foils in the same place, so you should either put the T back behind the thickest section of the main foil, or sticking out the front so the main foil is aft of the thickest point on the T
Does this make any sense or is it just a myth?
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Thank you Kevin,
I believe that when you talk about trim angles you are assuming that everything is angled relative to the water surface. In practice as the boat trims up on to the plane downwind and accelerates the helm (grinnning from ear to ear) is not thinking about re trimming the t foil so that it is at zero degrees and may sail with the foil at the 5-7 degrees trim up angle of the boat. So in practice the helm has to re-trim the foil at a negative angle to the boat to avoid going swimming.
I like the logic on length of spinny poles and rig rake to surviving downwind.
Slippery (a dog) with her new longer pole feels more controlable down wind than before. I haven't used the new wheelie bars yet.
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Thank you Kevin,
:)No problem
I believe that when you talk about trim angles you are assuming that everything is angled relative to the water surface. In practice as the boat trims up on to the plane downwind and accelerates the helm (grinnning from ear to ear) is not thinking about re trimming the t foil so that it is at zero degrees and may sail with the foil at the 5-7 degrees trim up angle of the boat. So in practice the helm has to re-trim the foil at a negative angle to the boat to avoid going swimming.
:)Sort of. If you consider the axis of the boat to be the line between the base of the transom to the base of the stem, this will trim up by about 6 -8 degrees when you plane. So if you have a symmetric foil it will look to be a say -7 degrees relative to this line to get zero lift. Note also that an asymmetric foil produces zero lift when it is about -3 degrees to the water flow. So in above example, you'd need the foil at say -10 to boat to get zero lift.
You never want to have the foil pulling the back of the boat down. This just creates more drag, so the pitch down force of the rig increases and you go really slow!
I like the logic on length of spinny poles and rig rake to surviving downwind.
:)It really works both mathematically and practically. If you have enough rake to the rig as a whole, the boat will go bows up as it goes up a wave increasing the rake and thus lift still further. This puts the line of force in front of the boat's cofg. The boat will then float (rather than crash) down as you go over the peak. That's what 18s and 12s do.
Slippery (a dog) with her new longer pole feels more controlable down wind than before. I haven't used the new wheelie bars yet.
The solent is really choppy. D Beans was a real nose diver until I increased the pole length and in particular, added the wheelies. You could also try a shorter gantry to bring the lift from the T foil farther forward.
The big problem with pre 97 boats is not really the rocker, but the amount of planing surface - it's too big and the bows are too fat. So when the bows go in, the wetted surface shoots up, the resistance increases, the boat slows and the pitch down from the rig increases, pushing the bows down still further and so on, until the stern overtakes the bow. I actually have some software that simulates this.
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Thanks again for the really clear explanations.
We sail slippery at quite a low combined crew weight 115-120 kg so the boat planes quite early. We are still getting used to sailing in breeze so probably haven't needed to get so far back yet. I haven't fitted a T foil yet but your post above will help us choose our final soluiton.
We haven't really sailed the boat in much chop so I suspect the wheelie bars will get some use at Weston if there is some breeze.
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And now the finish product. ;D
Please note : also Bilbo is on the background, I haven't painted him ;).
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Shiney! I like the broadened roots of the T foil.
W
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TRES BIEN!! ;D
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Very nice! What do we think the optimum span of the T should be? as i'll be cutting down the prototype Atum Bom uber wide special soon.
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Surely that depends on the shape, thickness and depth?
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Basically, any more than 900mm is a practical problem. I recommend 900mm.
W
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My T-Foil is a moth rudder's one.
It is about 110 mm but really thin.
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no wider than the boot of your car
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no wider than the boot of your car
Phil I drive a saxo, it will never fit in the boot no matter how narrrow the foil!
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no wider than the boot of your car
Phil I drive a saxo, it will never fit in the boot no matter how narrrow the foil!
OK change that to the widest doorway.
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:P
no wider than the boot of your car
Phil I drive a saxo, it will never fit in the boot no matter how narrrow the foil!
Just make me think when I get my international 14 from Archie Massey : he forced out the T-foiled rudder with fixed tiller from the back of his Subaru estate. I had then to manage to get it into my 307 CC :D :D :D :D :D
I did Calais to Paris trailing the boat without any roof :D
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Very nice! What do we think the optimum span of the T should be? as i'll be cutting down the prototype Atum Bom uber wide special soon.
I suggest there are 2 factors you need to consider as well as transport.
The first is the area of the foil required. The max L:D of the foil occurs at around a CL of 0.5 (5 deg for a symmetric foil). You can calc the lift from 0.5*Cl*1000*V^2 where V is the boat speed in m/s. To get say 70kg of lift at around 8.5 kts I think I recall that sum gives you about 0.1m^2. The windsurfer foils Will stuck on the bottom of the rudder are way bigger than that which would explain why they were draggy, so I suggest you chop some off the ends to give an area of about 0.1.
The second thing is that for a given area, increasing length (AR) lowers the induced drag. But if you place the foils 1/2 way down the foils like Phil to gain lift from the hull's wash, then there is no point in going wider than 900mm. The reason for this is that the zone of water affected by the hull is roughly a semi circle of 1/2 chine width radius extending below the hull.
Finally, to clarify a point often made, the thickness of the foil doesn't affect its lift for a given angle of attack. Thick foils can operate at higher lift coefficients than thin ones, but have a lower max L:D. You don't need really high lift coeff on a rudder T, so a thickness to chord of 10% is a good compromise. Laminar flow sections are also good. These tend to be a bit sharper at the front and have the thickest point at around 40% back.
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Picture from SA showing how to run a twist grip to a push rod gantry system from RMW 14 at the dinghy show
(http://forums.sailinganarchy.com/index.php?act=attach&type=post&id=65285)
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You can't see the above unless you are logged in to SA. Here is it for everybody.
I like the way this 14 does it. Like Atum they have gone for string-round-a-shaft method of controlling the Tfoil, but they have done some extra things - The string wraps round a few times and then continues so that there is positive control upwards and downwards. They also have gone (a little while ago) to using ordinary UJs and click-stop control, which I do not much favour.
My current thinking is twisty grip with a single line, but with less purchase (less than 48:1 that Atum had for the fine control). I am currently leaning toward s 24:1 with a fat (~25mm) extension to allow for more positive control. There will be dual control of string + twisty grip, but I am keen for the on-the-water control to be all on the twisty grip because it is so good adjusting it while flat out - especially downwind!
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Could anyone whip up a quick drawing in paint or somthing similar showing the route of the line? Pretty please?
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Sorry Ross, I don't know how it goes.
However, here is our own T foil progress, as of now:
(and, yes, I did need to take the workmate apart to get the t foil out :-\)
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Random thoughts...
Using a T-foil with an adjustable flap at the back, like a moth foil, will have the benefits of the symmetric and asymmetric foil sections.
When the flap is centered the foil will be symmetrical and will have minimum drag in the light stuff.
When the flap is raised, to give lift, the section will be asymmetric, and give maximum lift for least drag.
Discuss...
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And easy and quick to actuate.
But difficult to build, difficult to organise the control system, hinge drag and hinge leakage.
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To get the necessary range of angles, you would need a coarse adjustment of the whole rudder assembly and a fine adjustment for the flaps.
In principle, this combination will give lowest profile drag. To achieve this, you need an asymmetric foil with a flap. Tom Speer [www.tspeer.com] kindly did a design for me. If you want to build it let me know. All you need to do is reference Tom and let him know how it performs.
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Adjuster loads.
To start with (and to hit the target of foil by Keilder) we are opting for a for a "pull string" adjuster for the T-foil for Slippy. Am I barking to think 4:1 will do me (that equates to 200mm of string from 0 to 10 degrees). What have people used before?
I know that 24-1 and 48-1 have been mooted for twisty grips and just remebered to chip in on this that remember the spindle diamer also has a big effect on the purchase of rope sysrtems.
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Using the push rod on the boat system,
6:1 is OK but a bit heavy to get fine adjustment when the boat is hammering
8:1 lead out to the racks is what I am using at the moment which is working well
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Adjuster loads.
To start with (and to hit the target of foil by Keilder) we are opting for a for a "pull string" adjuster for the T-foil for Slippy. Am I barking to think 4:1 will do me (that equates to 200mm of string from 0 to 10 degrees). What have people used before?
I know that 24-1 and 48-1 have been mooted for twisty grips and just remebered to chip in on this that remember the spindle diamer also has a big effect on the purchase of rope sysrtems.
I have 4:1 on my twist grip which is a bit hard in heavy weather.
I'm thinking of a double system : 8:1 (or 16:1) on twist grip with a control on racks.
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Our double system was 6:1 on the rope and 48:1 on the twisty grip. The last 2:1 on the twisty grip was the ratio of the spindle (9mm) to the extension (18mm). The first 24:1 was rope.
The new system is likely to be 12:1 on both, but I may change my mind.
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Is having two different mode of adjustments for fine and course adjustments?
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Yes.
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To move forward and backward a 20 cm long rudder cassettes by 10° (+3 - 7) you need at least to have a 13 cm course.
Basically with a twist grip haveing a 2cm mandrel it means you will do 13/(2pi*1) ~ 2 complete turns without purchase.
As honestly I wouldn't do it without less than 4:1 purchase it means you will need approx 8 turns before going from 1 extreme to the other (bear away for exemple).
A faster mode with a big purchase and 2 blocks on racks would allow to release faster when tacking/gybing/bearing away.
Not sure to be clear ... ;D
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To get the necessary range of angles, you would need a coarse adjustment of the whole rudder assembly and a fine adjustment for the flaps.
In principle, this combination will give lowest profile drag. To achieve this, you need an asymmetric foil with a flap. Tom Speer [www.tspeer.com] kindly did a design for me. If you want to build it let me know. All you need to do is reference Tom and let him know how it performs.
That's really interesting Kevin. I'll have to look at it closer when I'm back in the country.
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read through the thread but still hugely confused as what to do t-foil wise,
hope tomake one for my 14 not a full cherub sailor YET,
any help/ideas? and what would i be looking at spending if i made it?
and what system is best to go for?
Cheers
James
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Have you had a look at these pages?
http://www.sailingsource.com/cherub/test/doku.php/tech/t-foil_systems
http://www.sailingsource.com/cherub/test/doku.php/tech/t-foil_rudder
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Great to here about another dog getting out regularly, Me and Tim were sailing at Chew, The evening started well with the fairing mod to the T foil working a treat, We now getting the speeds downwind we used to have before the T foil. Due to the weed at Chew we sailed the race with the standard rudder. Started ok but then it picked to big time. The down wind half was all 2 sail reaches, seriously terrifying. It got the the stage were we couldn't bear away without pitch polling. 91 sails would have been very welcome.
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Our T foil works!
We know this because as soon as it tried to carry my weight it dismanteled the bottom of the stock.
It ripped the casset like a sheet of A4.
Oh well!
2 plans of action have resuted.
1. A stronger stock and we are going to build a complete spare stock rudder etc so we can just come in and replace it.
2. Try different T foil section.
Will, do you have any drawing section information on your rudder and T foil?
I think we would like to try something similar.
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Dave,
Seeing as we're using a stock which Bloodaxe rated as "will last 2 minutes" I am keen to see some more failures to evaluate what the next stock will be. Do you have any pictures of the carnage?
R
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My cassette seems to be bullet proof :
From inside to outside :
1 layer of 0/90 carbon/aramid 200 g/sqm
2 layers of UD carbon 200g/sqm
1 layer of 0/90 carbon/aramid 200 g/sqm
Some UD reinforcments at the top and the bottom of the cassettes.
The cassettes cross the tiller and is glued + 2 layers of biax carbon 300g/sqm
At the bottom the cassette cross a 1 cm carbon plate glued + 2 layers of biax carbon 300g/sqm.
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Our is two plates of at least 10 layers each - about 2mm thick and bagged. These have the shape of the rudder cut into them.
The sleeve is one layey of 300gsm 2x2 twill 0-90 weave.
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Paul Croote had a problem with Cheese: With the T foil on a speed limit was reached at about 13 knots. He suspected a too blunt rudder and root of t foil. He did a neat sharpening job on it and it worked!
Here's a couple of pics:
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Can we a play a game of spot the difference with those pics, because I'm struggling to see any!
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You can't see even ONE difference between those pictures? ;)
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I suppose you think you're funny? I warn you, I'm in East London tomorrow!
T-foil looks good, nice one Paul!
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This pic might make it clearer
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Hi Paul, is that an aardvark Tfoil? if so then 1)what is the core of the rudder blade made from? 2) Did you have trouble turning off the foil enough downwind? It looks as though we are having the same trouble. as at the weekend, in about 30mph of breeze we could only manage 14.5 knots with the kite up, the foil seems to be pushing the nose down 3) How much did you change the angle on the foil?
Any help would be greatly appreciated as we are complete novices in chopping things up and rebuilding them.
Cheers
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Hi
If you have a foils the same as Paul's. it is a H105 section. This is asymmetric. It needs to be at an angle of attack of about -3 degrees before it produces zero lift. If you're boat is not at its correct trim (i.e. it is too nose down) it will slow. Interestingly, it is less draggy at high planing speeds to have too much trim rather than too little. Note that when planing, your boat will adopt a natural bow up trim of around 6 - 7 degrees, so the T foil needs to be at a -ve angle of around 9 degrees to be 'turned off'. Note this is not -ve lift, it's just no lift. I would suggest that for a 20kt run, you probably want an angle of attack of 1-3 degrees, so this will be about -6-8 degrees re a straight line joining the base of the stem to the base of the transom.
Next we come to profile drag. At high speeds, foil drags are very sensitive to thickness to chord ratio (t:c) and the fatness of the leading edge. Where the foil is at the bottom and the rudder chord tapers, you tend to end up with a very fat, blunt, foil. Paul's mod sorts this out nicely - he's extended the chord width of the rudder tip, making it both sharper and reducing the t:c.
Hope this helps
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One thing I do not know about is parasitic drag from the intersections between the T and the main foil. I think the radius and shape of the intersection could make a big difference, but have no idea what would be best, Sharp corner or more rounded radius to the join.
If you look at an aircraft wing there are typically quite large fairings around the wing fuselage intersection.
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Not sure if anyone has seen in dinghy sailing magazine, but we have just done some sails for Joe Richards, as he has nearly finished his new national 12....with trim tabs on the centre board (operated by the crews toe straps!) and a t-foil that sweeps back - apparently so weed comes off easily if you wiggle the tiller etc...
having had issues with weed at carsington, just wondered would having it sweeping backwards affect the benefits/performance of the t foil? and how would it affect the performance?
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I like the idea of trim tabs applied by the crews trapeze elastic.
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Hi Paul, is that an aardvark Tfoil? if so then 1)what is the core of the rudder blade made from? 2) Did you have trouble turning off the foil enough downwind? It looks as though we are having the same trouble. as at the weekend, in about 30mph of breeze we could only manage 14.5 knots with the kite up, the foil seems to be pushing the nose down 3) How much did you change the angle on the foil?
Any help would be greatly appreciated as we are complete novices in chopping things up and rebuilding them.
Cheers
The 'T' foils on cheese has been chopped around a bit, but we still tend to sail down wind with positive on and when we tried it natural it was still very draggy the main difference was the fairing at the front which was made by adding a piece of carbon plate then fairing round it. Most of the time Paul change the angle of the t foil to help balance the rig as the original plan rudder was raked forward.
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One thing I do not know about is parasitic drag from the intersections between the T and the main foil. I think the radius and shape of the intersection could make a big difference, but have no idea what would be best, Sharp corner or more rounded radius to the join.
If you look at an aircraft wing there are typically quite large fairings around the wing fuselage intersection.
Hi Phil, here's the answer, courtesy of Tom Speer's web site:
Junction drag Cdj = 17(t/c)^2 - 0.05 no fillet
or Cdj = 1.2(t/c) - 0.05 with fillet, where t/c = thickness to chord ratio and 0.1<t/c<0.45
So to calc this aspect of the drag:
Junction drag = Cdj x tav^2 x 0.5*rho*v^2, where tav is av 1/2(rudd thick + foil thick) (m) , v is speed in m/s and rho is density of water.
Note that this is per junction. So if T is at base of rudder, you have 2 junctions. If it is 1/2 way down, you have 4.
You can see that foils with low t/c and low t and fillets give least of this form of drag.
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Hi Simon
My T-foil was about 90 degrees to the rudder when I first had it. To stop it lifting too much downwind I had to have the rudder swepped back which made it very heavy on the tiller. I now have it set at 95.6 degrees from the rudder trailing edge (-5.6degrees when the rudder is vertical); it now feels quite nice. As we discussed on the phone it is not as difficult to change the angle as it first seems. Or it could be a project for the next sticky weekend.
Paul
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Hi Paul,
Thanks for the advice, we spent saturday at Will and Lucy's chopping our rudder in half and re glueing it back together. I'm now in the process of sanding and fairing.I think we will wait untill after the nationals before we try to sharpen up the front of the T. Once again thanks to everyone for the help, see you all in Wales ;D ;D ;D
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(and, yes, I did need to take the workmate apart to get the t foil out :-\)
Following another night of stickyness I went to bed knowing my spanner set is 15 miles and 30 minutes drive away.
The dyselexia answered my problem in my dreams - Open wide, hook one side over, collapse and it spins the whole thing round....
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This from the Born Slippy crew just now:
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Next up is the new rudder stock tonight and it poses the following query:
- I have a shiny looking blade that I now don't want to scratch :D
- Is it worth lining the case with something.
- If I have a lining is there anything I can use that I can laminate in. I'm planning on making the case using the board as a mandrel, with a couple of layers of plastic to give separation. I'm toying with adding a layer of fabric before the carbon to act as a nice soft layer - but worried it will just soak up the resin and be just as bad.
Any suggestions appreciated.
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There is a possibly rudely named thing called jap tape, which is self adhesive and has (possibly rude) ribs on it to allow for non scratchy slidation (rude!).
Noycey may be able to help you out.
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The loop side of velcro works well also, but you have to pack round the blade for the thickness of the velcro, laminate and then glue in with contact adhesvie.
The Jap tape is also excellent but slightly harder and will mark(but not scratch) a foil
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Sounds promising. Which side is the loop side? Are they strong enough to hold the compression?
John lewis sells sheets, I could just laminate onto it and see if it sticks - this way I have the material tonight. Or I could just allow enough packer and push it in after it is cured. Never expected the board to be this shiny, hence the unpreparedness.
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I have used jap tape which wont compress and have seen a type of draught excluder used. both can be stuck on afterwards but you need to pack the mandrel out to make the sleave slightly oversize. a few layers of cereal packet covered in parcel tape could do the job.
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Sounds promising. Which side is the loop side? Are they strong enough to hold the compression?
The soft side, rather than the hard hook side.
It will compress a little but you need that to be able to get the blade up and down.
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The other stuff you could try is progrip, which won't mark the foil and also compresses to help a snug fit.
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The other stuff you could try is progrip, which won't mark the foil and also compresses to help a snug fit.
And we have a winner - see me in the bar next week :-)
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The other stuff you could try is progrip, which won't mark the foil and also compresses to help a snug fit.
It is not very slippy though, I used it on once and could barely get the rudder up or down, ended up ripping it out and replacing with velcro.
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Roland, I have loads of progrip left over if you want some.
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Going sailing tomorow with our slightly reshaped foil. Forcast 15mph and big smiles!
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Are we missing a trick by not moving the bottom pintel with our 'T' foil adjustment systems? Or is there too much load on the bottom pintel? A system like the design attached seems very simple to me and easier to fit. What do ya all think?
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I do not see any big reason for it not working.
The advantage of having the adjustment at the top like on aqua, Primal and I think Fuzzy Logic is that the lifting loads from the rudder can be taken by the bottom gudgion and so isolated from the adjustment mechanism.
With your big ram it has to take the lifting loads as well, so the adjustment mechanism needs to be stronger.
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The cam on the B1C (still can't bring myself to call her Ronin) is set up this way. Although with no pin, just blocks and string.
I think Pete has had some issues and has had to beef up some of the components but I think it basically works pretty well.
We spent a bit of puzzle time trying to suss the easiest way to push by pulling....and in the end ran out of time so pulling to pull won the day.
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Princey, can you remember how the cam system on Fuzzy Logic worked? All that remains is a mini block to the right and a hole to the left of the if the gudgeon.
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Cascade system pulling one corner of the cam. Running along the gantry down the middle then forward to split either side.
Princey
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Here's a vid of our new t foil system working.
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Is that the fine adjustment system? If it isn't what is to stop the system re-centering when your wrist can't hold the tension in the system anymore?
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It is the fine adjustment - though it is now no finer than the 'coarse' part, which is a turret cleat in the middle with a split line to each rack.
At the weekend it was a bit tight to hold the tension on by hand, but what I liked was being able to let it spin back a bit as we gybed or tacked, as well as being able to pull some on without moving.
To make it easier to hold we went for 25mm extensions on this boat rather than the 16mm ones we had before, which gives you another 3:1 purchase because the spindle is 8mm across. Also more area to grip with in your hand.
To overcome the other problem we had on Atum (which was the end of the tiller filling up with rope), we have a coarser purchase, a bigger volume for the rope to wind up in, and we've gone for v thin vectran rope which winds on there.
The master plan is to make a spindle like the picture though. This will hold itself every turn but does not need a cleat.
Sailing with the system we have at the moment was so good last weekend I can't wait for another go!
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ok seemingly daft question time, how do you attach the tiller extensions to the gearbox/spindle? I've been trying to think but i can't seem to think of a way that is both solid enough so it won't twist and snap yet will still be flixible enough to be used in real life. like i say probably a stunningly simple answer but the beer from the weekend has stopped my brain working...
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There are a few different solutions floating about - mine are made using short lengths of hydraulic hosing epoxied onto a standard universal joint connector to keel them removable. The hosing is effectively a reinforced rubber tube, probibly around 15mm diameter? - nice and flexy but virtualy untwistable.
If you want to go down a similar route I have some pictures somewhere. The hosing is free or very cheep - (certainly less than a couple of quid) if you find a local suplier and ask nicely.
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We use hydraulic hose which has been pre drilled with a stainless bolt through the end of the tiller extension. On the other end is a stainless hoseclip.
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We use hydraulic hose which has been pre drilled with a stainless bolt through the end of the tiller extension. On the other end is a stainless hoseclip.
Where am i best looking for the hydraulic hose? I'm guessing i won't find it in an average DIY place?
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Auto-spares shop
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Matt if you're going to Weston this weekend I can let you have some.
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Where are most people putting there 'T' foil adjustment cleats? On the aft rack beam seems easiest but to my thinking involves and interesting crossing arm manoeuvre to adjust it, holding the tiller in the front hand with dagger grip whilst adjusting it with the back hand?
Any thoughts?
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VIEW FROM THE BACK OF THE FLEET (we get to see many transoms)
There is a growing trend of "Next to the cunno and kicker" backed up with fine adjustement from the tiller.
Atom is sporting a really nice "Cam cleat along the tiller" system'.
BS has hers on the tiller next to the pin, with the line split out to either tiller. Yes, adjusting while on the fly can crossing hands, but is OK for the bear-away and H has made me stop fiddling with it when twinning as adjusting the weight around our preferred settings
i.e.
down hill - not alot, but slightly on
up hill with wind - on a bit more
uphill with no wind - on alot as the bow is down and needs to be corrected for or is effectively on negative (although at this point people say it does nothign for speed, I still find we are quicker with it - although we have not tried to old rudder in L&S for a few months).
LLC(ALMA) will have a kicker, cunno and foil in a set at the front, swept back so both of us can get to it.
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Ummm... so how are you adjusting it if it's on the tiller arm? When you are tacking / gybing or sitting in?
(see pic.)
I was going to put a cleat where 'A' is. But I think now I will turn the control line along the hull side so it is with the other cleats in the middle of the gunwale 'B'.
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I have a cleat at B and am happy with it. It is good for the helms front hand and can also be used by the crew when the helm is 'too busy' at the back.
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On the SK4 they have it up by the shroud, but with limits set at the back. This is quite neat because as you go forward to adjust it it unloads the foil, so you need less purchase. But is also means a perilous journay forward to reduce the t foil angle if it is on too far downwind!
We had cleats at A, but now have twisty grip for fine and a turret cleat on the deck under the tiller with split lines attached with bungee to each rack. That is used for coming ashore only really.
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On a similar topic, does anyone have a picture of how the control lines are set on Dan's boat?
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Thanks guys, I think it will be 'B'.
Off to exchange some more fittings... again.
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On a similar topic, does anyone have a picture of how the control lines are set on Dan's boat?
here's some....
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i was considering a course control through cleats led to the racks at B and a fine tune twisty grip on the tiller extensions.
i'm asuming that I wont get the course control right every time or that the conditions will change and a fine tune adjustment will be required at a point when you don't want to leave the rack or have your hands full.
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Ummm... I'm still trying to work out how to make a fine control on my system. ::)
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This what I am thinking about for the fine control system. but I've looked at this for too long and it's hurting my head. I think it works but other opinions would be good. Especially as I haven't sailed with a 'T' yet so have no experience of the twisting loads on the tiller extensions etc...
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.
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I think you will need some serious muscle to operate the fine control on that! Have you looked at Will/Lucys boat? That has a fine on the tiller. The fine looked like it once went to a twisty grip but is disconnected in this photo.
Sorry Will for posting spy photos of your lovely boat!
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I think you will need some serious muscle to operate the fine control on that! Have you looked at Will/Lucys boat? That has a fine on the tiller. The fine looked like it once went to a twisty grip but is disconnected in this photo.
Sorry Will for posting spy photos of your lovely boat!
sorry I'm failing to see what's going on in that pic.
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On Auntie, the coarse control line does not have a dead end. Instead it terminates in a line that connects into a pulley system in the tiller (via the plastic bobble and loop shown).
The twisty grip on the in the tiller has leverage from twisty grip being bigger than spindrel system and also a couple of blocks, resulting in an "umpty to one" force magnification by the time it leaves the tiller (and connects to the bobble) .
This way the flappy gantry can be moved by the coase control (on the cleat at "dumpty to one") and by the fine by twisting the tillers and moving the dead end of the coarse line (at umpty to one x dumpty to one ).
Connecting via the bobble means that if the twisty system fails it reverts to coarse only. I am sure someone else can supply the umpty and dumpty values , I think these are both of the order of 16.
Hope this helps.
EJ will have the same coarse fine system, except I have not yet managed to fit the fine bit. Last weekend it became obvious that now I am on the wire a lot more I am not able to work the cleat systme as I did in BS, so implimentation will be imminent.
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You see the bobble at the top of the purchase, that looked like it once went to a spindle on the tiller (extensions attached) which provided some fine adjustment. The other end of the purchase goes to a cleat on a swivel in the centre of the boat for coarse adjustment. Will should be able to confirm. IIRC the spindle was not a straight cylinder but had a shape similar to a crankshaft in an engine so that the twisty grip was sort of self locking at single turn intervals.
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Sorry for repeating Born Slippy.
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Computer is cooking something about some Swedes, so I can reply to this.
Rope 1 (3mm D12): Bottom of transom>block on bottom of gantry>block on bottom of transom>block on top of gantry>top of double floating block.
Rope 2 (4mm D12): Cleat for coarse adj on deck>half of floating double block>block on deck> other half of floating double block>another block on deck>block at top of gantry>bobble and loop.
Rope 3 (1mm vectran): Hole in middle of 8mm stainless spindle>round spindle a few times>ties to loop with bobble.
Action 1: Twist 18mm tiller extn>pulls vectran>pulls bobble>pulls Rope 2 from top. This is about 45:1 purchase.
Action 2: Pull rope 2 from bottom. This is 20:1 purchase.
This works ok, but the bearings on the UJs are the biggest problem. They are now balls of rust. New ones coming!
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rusty balls Will?
better than no balls at all, might explain why my twisty isnt perfect !
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Its not the ball bearings which are now balls of rust - its the entire ballrace! :-\
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Is this something that a dismantled block could solve?
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Thats an expensive way to buy ball bearings!
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We have already solved the problem, with ronstan sheaves i found and p and b happily ordered in
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http://www.ronstan.co.uk/marine5/product.asp?ProdNo=RF1020
Couple of these, axle between. £6 a pop I think.
Next is to sort out the UJ. The double pin joints wrapped up in bike inner tubes are ok, but not perfect:
http://www.ondrives.com/couplings-universal-joints-8-25mm-bores.html
I have some moth-style hydraulic pipe which may work, but if not I was thinking of a home bru one.
Oh yeah - we never made the cranked spindle - it didn't seem necessary.
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iv found those "on-drives" get stuck in a postion at really inappropriate times, especially when there is a large angle between the extension and 90degrees from the tiller, like when sitting very forward in light winds (not to bad) or standing at the back of the bus with the spinny up in ridiculous winds (worse), i have only sailed with theese kind of things on an 18 so im not sure this would happen to cherubs
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i have only sailed with theese kind of things on an 18 so im not sure this would happen to cherubs
You have been helming an 18 John? I'm impressed.
What about modifying a jib furler to sit on the end of the tiller as as the drum? The 14s use something similar but I have so far been unable to find one.
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Sorry in wrong place. E5 mould with Andy p in prep for joins build!
ON ferry on way back. Weather good but so many smokers!
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Rob, would one of those furling drums they use on SMODs work (Xenon, Vago, Vision,etc) or is it too big? i may hae a few lying around somewhere, and yes i have helmed an 18 a few time, you should try it, its incredible fun (apart from when tiller loads up and UJ is stuck at the same angle, flinging you into the other crew)
will, did you take it down this weekend, we are very gratefull!!!
also, does anyone know where i should post the roylaty to? (kevins address)
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Some time ago I think that I was quoted about £40 for an int 14 style drum, complete with flexible joints. I think that it was from Ovingtons but I am not sure.
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Would it be technically feasable to build a rotating gantry t foil set up that was controlled by a tension cable? The tension cable would then be adjusted via a srew thread within the tiller, which would then in turn be driven by a worm drive with the two tiller extensions running through it. This has the advantage of MASSIVE purchase, no expensive blocks, potentially quite light and simplicity. The Moths have proved that cable systems can be used in a marine environment, the question is would it handle the load?
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This is v like the daemon system, where a worm gear pulls on the top of the rudderpin to deploy the t foil. The worm gear seems like a good idea but poor efficiency and no feedback (as well as engineering problems of having a slot int he tiller) put me right off the idea.
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Personally I think this is the most elegant solution....
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I saw that at the show. The huge lump of metal could be quite a handy murder weapon, so I suppose it has some merit.
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I guess this is a feasible solution when you build a 12 foot boat out of carbon which has a minimum weight about 30kg heavier than a Cherub?! Just googled and the minimum all up hull weight is 78kg, so even with that scaffolding I am pretty sure they will need a fair bit of lead.
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Err.... I'm not suggesting the material use! Just the design of the system. A while back I talked about a 'might screw' moving the bottom pintle like the one you see in Joe Richards design here.
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From talking to the guys at the show i understand that Jo richards was pumping the t foil on the waves in some conditions and gaining from it. So the system has to be able to take a lot of punishment. I'm not sure how that is viewed under rule 42.
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The problem with the design used on the N12 is that the loads on the bottom pintle are significantly higher than those on the top pintle.
If you do not have a gantry then using the bottom of the boat to support the massive lump of metal needed to get it to work is feasible but if you are going to have a gantry then there are many much more structurally elegant design solutions.
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I have a 2 Rudders with lifting foils for sale:
A nearly new development set £500, and new latest design £700.
Pre-preg carbon, corecell foam, from Aluminium CNC tooling
Spring clear out - refer them on a boat going fast than gathering dust.
Bob 07769 715518
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Hi Bob,
Please can you post pictures of the foils?
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I think rich taylors gearbox is what you are looking for.
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not permitted to post pics yet, but email me and can reply.
bob@compositecraft. co. uk
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Hi Bob,
Do you mean you lack permissions on this forum to post pics? If you email them to anyone on the contacts page they can post them, or a moderator may be along in a minute and grant you permissions.
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Hi Bob,
Do you mean you lack permissions on this forum to post pics? If you email them to anyone on the contacts page they can post them, or a moderator may be along in a minute and grant you permissions.
New users can not upload attachments or post live links for the first five posts, to stop some of the unmentional pictures that have been put up by spammers.
As you said he could email pictures.
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Here are the pictures of Bob's foils:
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From the pictures they appear to be designed for a pivoting rudder stock which is pinned down in the vertical position.
There is nothing stopping someone from building a stock for these foils. they do look really nice.