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T Foils

Started by Will_Lee, 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.


Will, would you go for a symmetrical foil again or an asymmetrical foil if you had an option?


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!



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.



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.


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.


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.

Phil Alderson

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? ???


Hi Simon,

Welcome! Loco Perro is quite a vessel and can be seen on this thrilling video by Daryl Wilkinson:

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.



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

Ben Howett

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....


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?


Do you have a crash helmet?