T Foils

[Stuberry]

Can we a play a game of spot the difference with those pics, because I'm struggling to see any!

[Will_Lee]

You can't see even ONE difference between those pictures? 

[Stuberry]

I suppose you think you're funny? I warn you, I'm in East London tomorrow!

T-foil looks good, nice one Paul!

[paul_croote]

This pic might make it clearer

[simon_jones]

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

[kevin_ellway]

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

[Phil Alderson]

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.

[Debi]

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?

[roland_trim]

I like the idea of trim tabs applied by the crews trapeze elastic.

[tim_unerman]

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.

[kevin_ellway]

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.

[paul_croote]

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

[simon_jones]

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

[roland_trim]

(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....

[Will_Lee]

This from the Born Slippy crew just now: