There is also another way of looking at it :

-1- The T-foils are at the very back and so could acquire large effective stabilizing with only small forces.

additional comments. Roughly the centre of volume on a cat is below the main beam or just in front of it. This means that its leverage arm can easily be about 2.5 mtr = 6 ft. That is quite alot. The leverage of the sails is about 4 mtr = 10 ft. Say a gust that requires a 150 kg crew to move back 1 meter (large gust) can also be resisted by only some 60 kg downforce on the T-foils. At the typically effeciency of a hydrofoil (Daggerboards/rudders) the increase in drag (momentarily) is only about 6 kg's which is only a portion of the increase in saildrive due to the gust (about 50 kg's). So you invest

So yes there is more drag (momentarily) but the setup can still harness a net increase in drive that a more conventional crew probably could not harness in the same way. Because How quickly can you move backwards and smoothen out the gust with the mainsheet ? The T-foils can do that immediately. It sure felt that way when sailing the Stealth personally.


-2- The T-foils are only "ON" when they are tilted sufficiently relative to the water surface. So when the crew learns to place their weight such that in stable winds the T-foils are not working then the basic drag linked to them can be quite small indeed. The T-foils when then only see large drag when they are being used due to a gust or something, but here their positive effects may well outweight the increase in drag.

Additional comments : So it could well be a case of wehn you don't need them then the added drag is just too small to notice. Afterall how many of you could notice the speed difference if they sailed with rudder boards that are 4 inches longer ? The increase in area will be about the same in both cases. And ofcourse when you do need them then their effect is immediately noticed. Also note that the switching between "on" and "off" is always alot faster then any crew can react manually.


-3- The T-foils counter forces are proportional to the angle of attack the hulls make relative to the water surface (travelled direction). Meaning that the drag is proportional to the exitation of the boat as well. Small exitation = little effect to be won but also drag = small and the other way around. It could well be a case of only paying the penalty when the positive effects are large as well.


-4- A 4th benefit I always envisioned is that T-foils can act as a 3rd weightless crew member. Meaning it has the effect of putting more weight on the back of the boat allowing significantly more drive in a blow while not despressing the hulls in deeper when its effect is not needed. Also because it is at the far back of the boat it will achieve more resistance to pitching for a given depressing of the hulls the any additional crew weight. As such it is very effective in a big blow.

Also not that a T-foil with a 10:1 ratio in acquired lift (downforce) will always allow nett more saildrive when in use.

Say 50 kg more saildrive by a 4 mtr leverage arm will require the T-foils, 2.5 mtr from bouyancy pivot point, to work at a laoding off 4/2.5 * 50 = 80 kg's leading to a drag increase off 80/10 = 8 kg + plus something for depressing the hulls in deeper say about 5 kg. Therefor the nett increase in saildrive is 50 - (8+5) kg is 37 kg's.

Now you try to harness that increase without T-foils by moving your crew weight around and trimming your sails. From personal experience I can tell that that requires alot of skill and concentration and even then you may well not achieve it. All the while the T-foils do it automatically.

I would love to try the T-foils on my Taipan as my boat does require alot of attention on fast reaching in considerable wind. And I don't have weeds to worry about.

Wouter


Wouter