UK-Cherub Forum
Cherub Chat => Tech Chat => Topic started by: john_hamilton on May 20, 2012, 08:16:01 PM
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As anyone who saw my boat at the end of the nationals will know, the tiller is quite petite... about a foot and a quarter long. with my new adjustable caps, we are faster in heavy wind, but i find myself not having enough time to do anythin but battle the tiller loads, leading to two broken tillers, an broken uj, and a dead footstrap, all in a matter of weeks.
I cut down the tiller to give myself room in heavy wind gybes, and have experimented with having it longer many times since, always ending up going back to the shorter one... so either me and digby are rfeall fat, stand too far back, or just have a shorter gantry than everyone else ( it was taken off a different mould made by phil kirk for the specific job, and could be slightly different? So would there be any massive problems with just building a longer gantry so i can get my easy steering back? heres a picture of one on the new killing 3 int 14
many thanks, john
(http://1.bp.blogspot.com/-09BZYcpHUvE/T6rUwkRfR7I/AAAAAAAAJTg/jgjETs4ZL4c/s320/IMG_20120508_193545.jpg)
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Halo had a 540mm tiller, which was pretty short as they go, and I don't ever recall getting complaints from helms about the loads, although maybe Will, who did an event with that setup would disagree. Of course that was with the 97 sails, which will change things a fair bit, but I can't help thinking that if you are running into such severe tiller loads as you describe then you need to look for a root cause in the boat setup.
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John, measure up your gantry from transom pin to rudder pin and I can compare it to the E5 moulded one.
I think that generally the gantrys have got shorter since the move to the swinging gantrys, I can't remember the measurements from the Badger but that had quite a sizeable one, maybe 600mm or something.
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We too have a very short tiller on the E5. What I have noticed is that it loads up mainly for 2 reasons:
1. not keeping the boat flat - especially on any other course than a beat
2. having any t-foil on a white sail reach
When the gantry is back on the boat I'll measure the gap and let you know but in general the tiller loads aren't that bad.
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The bending moment on the tiller will not be effected by the length of the tiller.
If you are breaking a short tiller you are just as likely to break a long one but will not have to pull as hard to do it.
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im not breaking the tiller clive, just the extensions. A longer one will obviously make problem less... and jim, this is only 380mm long, with 05 rules sails, and its nothing wron with the rig, just if we arent exactly flat (and we sail on a n inland, gusty lake), then the problems surface
i'll measure the distance tommorrow
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Very high tiller loads sounds like a problem with the balance of the rudder. For a balanced rudder it should be raked fwd with respect to the axis of the rudder pin. i.e. the opposite way to a Laser.
As you move the top hole in the rudder stock away from the rudder, it becomes progressively more balanced, and the steering loads drop. Up to the point where it over balances and they reverse which is V hard to sail with.
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Slop in the rudder stock can cause the appearance of high loads because the blade can snatch from one way to the other. Not nice.
Misplaced kite ratchets (too far forwards) can cause extreme lee helm downwind in marginal conds.
Mostly though it is keeping the boat really flat, and as Phil says, balancing the blade. You defnitely want some of the area to be in front of the continuation of the rudder pin. My memory says a third, but that seems like an awful lot. Thinking about Norwegain's, Atum's and Antidote's, 10% seems to be the minimum.
If the footstrap is breaking it suggests it is lee helm you are struggling with. If so, you can try using more main sheet downwind and/or kicker, which may help a bit. You can roll the boat to leeward a bit to balance the rudder downhill. Easing a it of jib may also help.
However, the magnitude of the problems suggest there is something big going wrong. If your main is very deep with a hard leech downhill you might be stalling the whole thing?
Good luck!
From memory, Atums gantry is 450mm, and I think Antidotes is 350mm. It might have grown a bit later too.
I don't know why the 14s have been going for such long gantries. It may be ergonomics.
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two things to consider.
A longer pivoting gantry will increase the loads on the control line purchase and the loads on the fittings on the boat.
This will also mean that the foil is further back and it will be harder to get the helm's weight over the foil.
If I am correct john i think you are saying that the boat is normally balanced but lee and weather helm are felt when the boat heels in gusts and lulls. The forces from lee and weather helm are larger and more difficult to cope with because you have a short tiller.
I think my tiller is just under 600mm from the rudder pin.
The gantry mould that I made for you is very similar in dimensions to the original E5 gantry mould. You would had drilled the pivot holes without access to an existing E5 gantry to measure from.
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I wonder (completely evidence free theorising) if there could be too much side load on the rudder and not enough on the daggerboard. Its good to have the rudder loaded up to an extent, but it must be possible to overdo it. Unfortunately that's a really major deal to resolve: I fear it likely the best solution is moving the daggerboard case.
This sounds as if its the sort of thing you need a second opinion on. I don't know who the current best setters up of boats are in the fleet, but if you can entice one to go for a ride in your boat to give you a second opinion it can only help.
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When are you getting the very high tiller loads, upwind or downwind. Kite up or down. Mark roundings, twin wiring. It it weather or lee helm.
It sounds to me that the rig and foils aren,t in balance, I expect it can be sorted by playing with the rake a bit assuming the foils are in the same place as the other E5s....
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Having re read the thread a bit more I think I understand your problem better
From what you have said the boat is nicely balanced most of the time, but you have problems with gybing in windy weather. You have shortened the tiller to give you more room which has worked but caused other handling issues that require possible major boat mods. Cure worse than the original ailment I think. A shorter tiller has increased the loads on the tiller plus it will make the boat much more twitchy as a small movement on the tiller has a greater effect on the rudder angle compared to the longer tiller.
In windy weather we have developed a safe gybe technique as follows :-
1. I tell Peter that we are going to gybe to make sure he knows what is happening and doesn't follow me in at step 2.
2. I come into the boat and sit on the wing and unhook.
3. Ease the t-foil off to get the bow out of the water to stop it steering the boat
4. Get the boat dead flat and more importantly still going fast
5. Tell Peter we are now going to gybe
6. Turn the boat in a smooth curve as Peter comes in, I cross the boat just before him and head for the new wing.
7. Both get on the wing and and steer as necessary to keep the boat dead flat.
8. Peter gets straight out on the wire and sheets the kite. I follow when I've sorted out the main sheet and pulled the t-foil back on.
I would suggest before you get into major boat work you put a longer tiller back on and spend an afternoon working on your gybing technique. The main things to get right are keeping the boat flat, steering smoothly and predictably, and keeping the boat moving quickly.
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interesting.
Phil and will - i found an off-cut of 8mm tube from the plumbing (or something) similar about a meter long, and put it through the rudder pin hole, when extended it touched the joint between the tip of the vert blade and the "torpedo" fairing for the t foil, so definitely something to look at as there is precisely 0% of the rudder in front of this line!
Mr Kirk - How do you sail with a tiller that long? i have found when it gets gusty in gybes downwind i tended to trip on the longer tiller. This may be due to Sarahs diminutive frame in comparison to 6ft2 digby... i have measured my gantry as 300mm from pivot to pin, and i have a 420mm tiller... so a bit different, i think i could manage alot better with a 400mm gantry and a 550mm tiller (the original size, suprisingly)
Jim - the dagger board slot and rake is identical to e-numbers, which seems pleasant to sail, and fast, when the flat, the boat is well in balance, all i am looking for is a soution to the tiller loads (and broken universal joints)
the plan for now is to play with the pin angle a bit, then see if i can cope with the short tiller in that set up, thanks for all the informative replies...
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I think Paul's answer was the best. I shortened Norwegians tiller for the same reasons you describe and we swam basically every gybe until I lengthened it again (one event!). I took to opportunity to crank the end of the new bit of tiller down a bit so the outboard ends of the extensions were higher. You might find a photo or two on the website of NB with seaweed decorating the shrouds. It was that event!
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Has anyone ever tried to get the whole leading edge of their rudder forward of it's pivot point? Keel boats do this and it makes the steering much lighter. It would be fairly tricky to do on a dinghy though.
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sorry paulj, i never saw your reply, looks like we posted at the same time! technique is definately the problem, combined me and digby are 12ft3 and so crossing the boat at different times will help, longer tiller going back on, you have convinced me! (its the crews fault and the not the fault of whoever built this speedy boat ;-) )
although i will be looking at pin angles thanks to will (i cant help but fiddle with things a bit ;-D )
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with the help of the baking temperatures today, i have extended the tiller, fixed the board trailing edge, made a kicker hardpoint and more! it all cured in less than 2 hours from mixing!
(http://a3.sphotos.ak.fbcdn.net/hphotos-ak-ash4/389179_3903975085484_1462451748_33408787_1435849004_n.jpg)
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Has anyone ever tried to get the whole leading edge of their rudder forward of it's pivot point? Keel boats do this and it makes the steering much lighter. It would be fairly tricky to do on a dinghy though.
I have seen some fixed rudders setup like that, and if you look at the setup of some pivoting rudders, they can have the same effect.
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pray tell?
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I 'normally' put 2.5 degrees of fwds rake on a daggerboard blade vs the the pivot line - giving a light feel.
But with a T- foil, there are big loads and changes as the boat heels, so it doesn't matter much now.
The i14 with a T from the hull will unload the rudder and make the tiller loads much less.
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Hi Andy,
Can you explain how adding a t foil can increase tiller loading please?
I understand the loads on the rudder fixings will increase a lot. I understand that if you hit something with the end of T foil there may be big tiller loads.
To add tiller loading you need a force which is acting tangentially to the rudder pin line, right? I cant see how adding a T foil can increase that. (Beyond increasing speed possibly meaning increasing rudder loads.) Endplating the rudder may make the rudder less draggy, but that means less angle for the same steering, which probable wont increase rudder loads.
Effects whereby leeway makes one half of the t foil work better than the other (I think the windward half) might cause an effect whereby there is more induced drag on the liftier half, but I would expect this to be small.
Thanks!
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The loads on the tiller increase as you rake the rudder forwards and backwards to alter the angle of attack of the t-foil. I guess the highest tiller loads will be experienced downwind with the t-foil right off in waves. If you add a bit of leeward heel at the same time they would be reasonably high.
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Loco's rudder loads are tiny...comparable with an Enterprise, which is the lowest rudder loaded boat I have ever sailed. I've not twin-wired or used the T-foil yet tho'
What was the load like twin wiring and with the T-foil John? Is it sailing Loco which has prompted your re-evaluation or does it load up with the power on?
The big difference between Loco and current boats is mast position and size of jib...and therefore main. Apart from a bit of extra rocker (which is mostly up front?) the hull is just a modern boat with wings not racks...I think.
It's worth noting btw that Ben and Daryl very quickly moved the dagger board back about a foot from its original position.
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The loads on the tiller increase as you rake the rudder forwards and backwards to alter the angle of attack of the t-foil. I guess the highest tiller loads will be experienced downwind with the t-foil right off in waves. If you add a bit of leeward heel at the same time they would be reasonably high.
It depends on the mechanism used to adjust, with mechinisum on the boat the center of pressure of the rudder does not move with respect to the steering axis of rotation of the rudder, so there will not be a change in the balance of the rudder.
If you have the mechinisum on the stock then you do change this relationship and do change the feel quite a bit.
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Unless raking the foil forwards which will not be possible all the time if rudder rake also controls the t-foil angle it will be difficult to get the leading edge of the rudder in front of the pin with the current dagger style stocks.
Perhaps we are destined to go back to blades which pivot in the stocks however controlling foil angle will be trickier. Most dinghy rudders i have sailed with on different styles of boats have felt light if the boat is kept flat.
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Not sure I can explain why... but i can report that:
Using a canting gantry ( so the pivot line is constant wrt the blade )
with neutral setting of lift, windward heel gives a little lee helm, flat is balanced, leeward heel gives a little weather helm, and very light tiller loads.
Using several degrees of lift on the T foil, sailing flat is still balanced and light tiller load, but windward heel gives lots of lee helm, lee heeling gives lots of weather helm, and with much higher tiller loads.
so perhaps it's just the amount of lee or weather helm, having to push/pull the tiller a long way off centre and keep it over to make the boat sail straight.
in moths, the windward heel changes the leeway on the fwds angled strut, so that the strut provides no side force - all the sideways force normally supplied by the daggerboard is transferred to the (now inclined) horizontal foil. ( this reduces the surface piercing drag from the strut considerably )
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Perhaps we are destined to go back to blades which pivot in the stocks however controlling foil angle will be trickier. M
Or, just get fixed rudders! what could possibly go wrong ;)
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Thanks Andy - I think I can offer an explanation for what you describe. When the boat is heeled the T foil will be giving inclined lift, but the weight of the boat is still acting straight down. This will give a resultant force pushing the stern of the boat to the side because the boat is constrained to be on the surface of the water. This steering force needs to be opposed with rudder movements if the boat is to stay going straight.
This process acts in addition to the other main force of heel related weather and lee helm, which is the centre of drive of the rig moving off the centre line (and has v little or nothing to do with chines, contrary to popular belief - try taking the rig off your boat and towing it around heeled and level).
This means there are genuine extra drag losses when the boat is not level because the T foil and the rudder are creating drag to make lifts which cancel each other out (not much though). This loss might be less than the losses induced by having some dihedral on your T foil. Hmmm. Any gains would get less as you get better at sailing the boat flat though.
Another thing about this that smaller rudders would be expected to suffer from this more, because more angle would be required. Don't what this does to tiller force though. Could be more, less, the same or (my favourite) it depends. I think it is the last one!
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in moths, the windward heel changes the leeway on the fwds angled strut, so that the strut provides no side force - all the sideways force normally supplied by the daggerboard is transferred to the (now inclined) horizontal foil. ( this reduces the surface piercing drag from the strut considerably )
I made this observation to a collection of foiling mothists on the beach in Saundersfoot in the NB days, so it must have been August 2004. I was subjected to hoots of derision and given many (internally contradictory, mutually contradictory, or just incompatible with physics) explanations as to why I was quite wrong. I take it mainstream Moth thinking has caught up with us? ;)
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I've just reviewed most of the posts on this thread...really interesting. Weight on steering seems to be coming from T-foil and I can see how... as soon as you heel it is pushing the stern one way or the other.
I thought T-foils were used mostly (exclusively?) upwind yet PaulJ talks about letting it off for gybing and it was to let the '...bow come up...' - I thought if T-foil was used down wind it was used to keep the bow up ie avoid nose dives.
A couple of questions:
Do people use them down wind?
If so +ve or -ve? (+ve = bow down)
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i use the t foil so that when we are both stacked at the back, the stem is a few inches out of the water, obvioulsy i use less in waves, etc/...
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A t foil boat should always be sailed with the hull at optimal trim, obviously
So on the most basic of levels, if your boat is at optimum trim, and you aren't stood at the back of the racks, move back, and add more +ve onto the foil
You will tend to use less foil downwind as your extra boat speed will produce more lift for the same angle of attack
The t foil even in positive stops nosediving as it hugely reduces pitching, the boat pivots roughly more around the transom rather than the middle, also, when the boat goes into a nosedive, a serious one, the max of 7 degrees you may have on stops giving lift and starts giving suck as soon as the bow nosedives more than 7 degrees
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Aha...thanks mate, that makes sense
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Thinking about it I have probably applied more balancing to my T-foil rudder stocks than I had on my std rudder stocks. Initially because the design of adjustment I used required it, and more lately because I know how I want it to feel.
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Yes - Antidotes last two t foils had the foil tipped down quite a lot so the foil would be further forward, but it would also have affected balance because I moved the stock against the pin rather than the whole stock and pin (because it risked hitting the water)
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Interesting thoughts on how it is the geometry of the axis of rotation and the foil - not the axis of rotation and the water than is altering the load. After filling a page or 3 with sketches this has some fun implications with flappy panel gantries always having almost the same load with foil +/- or off (there is a slight geometrical change and the load increases with speed due to lift/drag when turning).
Having sailed both slot and flappy T-foil systems I did notice the foil became lighter with more foil on in Born Slippy (system had a slot and fixed pin) and have not noticed much change in the flappy panel boats since. Although I have not thought about it much before this thread...
In essence the advice presented her appears to boils down to the suggestion:
"better to ensure the top hole (usually found in the in the tiller) is further from the foil than the bottom hole (i.e. bottom of stock). Especially for flappy gantry systems - but not too far".
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It may help if the lifting foil is near the front of the rudder blade and therefore underneath the extension of the pin when on positive and creating lots of lift. We are tending to keep the lifting foil in this position to make it easier to helm weight over the foil.
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The T-foils built by Demon Yachts (http://www.demonyachts.co.uk) are designed by Kevin to be raked well forward to put the foil as close to the boat as possible. The foil is angled relative to the rudder to give lift when its well forward and the rudder is also very long to help with this and keep the foil in the deepest water.