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

and they'll make that pitchpole all that more exciting once the t-foils clear the water and loose all their downforce...wheeeee! Better opportunity to be thrown clear. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Amen to that! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I will add my experiences to this debate.

We first tried the T-foil rudders 5 years ago, it was immediately obvious that they completely altered the sailing characteristics of the boat.

This is not one of those changes that you make and you are not sure whether anything actually changes, I say again the boat is totally different with T-foil rudders.

Next we had to check whether or not the difference was fast. Like Darryl we carried out 2 boat tuning with identical boats except for the rudders. In no circumstances was the non T- foil rudder equipped boat any faster.

In any kind of wave pattern whatever the wind strength the T-foils were quicker both upwind and downwind and in winds > 12 knots the T-foils where again quicker.

We also tried them on our formula 18 HT boat and the effect was just as marked.

And more recently the current Sptfire European Champion purchased a set and tested them he also confirms my/Darryls comments

I have no idea how much drag the T-foils create, and quite frankly I don't care, catmarans go quicker with them on so whatever the figure it is less than the drag of a normally equipped hull travelling through the water.

We did no tank testing (I think you will find that not much tank testing time is bought by any beach catamaran manufacturer since it is cheaper to build 2 boats and do the job properly on the sea).

As to the weed question, obviously a dagger rudder is harder to clear than a kick up rudder, just as a dagger board is harder than a centreboard. Perhaps if your water is very weedy these are not for you.

My credentials for carrying out these tests are that I was a full time member of the British sailing Team racing Tornado, my ISAF World ranking got to 9th, I was paid to sail by the Royal Yachting Association and my job from 1997 to 2000 consisted of 9-5 most weeks 2 boat tuning, with 8 -10 regattas a year thrown in, I have done thousands of hours of this work.

I would entirely echo Darryls findings and since we are the only people I know of who have done this and we both completely agree the chances are that we are not mistaken.

There is one further effect that is also noticable although I didn't pick up on it for ages, and that is that not only is the boat smoother through the waves but it is also smoother through gusts. Put simply the T-foil equipped boat lifts a hull more slowly when the gust hits, we think that this is because as the gust hits and the rig drives harder, it tries to push the bows down, of course the T-foils resist this, and the windward t-foil obviously adds load to the windward hull slowing down the hull rise and so squirting the boat forwards.

I have already made the decision to go with foils. I see them as the future. In regards to the last part of your post. The windward hull lifts slower with the foil in a gust but the foil will break the surface at some point. Does this come with a rush of lift?

John,

How much are a set of T-foils?

[edit] I guess that question should also be directed to Darryl.

Gareth

Very good John, thanks for yours and others comments..... Would be interesting to try them on an F18.

Any idea of a price for these blades and rudder heads to hold them? I've heard about $1,000 US. Also, if the T foils make the rudders more effective, maybe the rudders could be shortened? That would give you a little more reaction time to see the bottom before you impact it and bust your $1,000 rudders!

Also, have there been any problems with the T foils coming off after long periods of hard use, or cracking where they join the rudder?

You all have noticed the America's Cup designers have been putting end plate type wings on their keels for years now, (but never on their rudders) of course for different reasons, but there must be some improvement in drag or they would have dumped them and they do computer testing, tank testing as well as side by side. It must work... <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" />

Mate, Know of any of the pointy end of the Interntional Moth class that doesnt have at rudder foils? Even those who havent made the move to "flying" International Moths.. Ok they may not be "professional" sailors but I suspect they are possibly some of the most skilled sailors professional or Am around..

As for I 14s..
I believe your misunderstanding why International 14s have rudder foils.. Its not to reduce nose diving although that may be a beneficial side benefit.. The reason Bieker started the I14 rudder foil tren is to reduce the aft wave and hence make the water "believe" the hull was 16 not 14 foot.. So trimming of the foil is required for the incline of the hull to the water.. As Im sure your aware I14s sail flat upwind and a slightly nose up position downhill.. As for benefits Bieker suggests med to light air will show the BI14 rudder theory.. Not when the I 14 is airborne..
This is evident in the positioning of the I 14 rudder foil.. Bs foils are above half way up the rudder not at the base like the Moths or cats..

Just a few thoughts..

S

Not quite true, the Foils are also used to remove the pitchpoling issues that 14's have as a result of going to more skill like shapes with fine(r) bows.

THe trim the nose of the boat down for upwind sailing and the trim the bow up for downwind sailing.

Hmm.. Well Bieker put the first foils on his B3 to increase water line "length".. If you look at his foil they
1:are assymetric...
2: Above half way up the rudder..
3: yes they are movable to maintain water flow aft of the stern.
BUT the pitchpoling issue wasnt the driving force behind Biekers motivation.. The B4 has a fuller bow and hull shape to the B2 & B3.. The bows of the I 14 is regulated by a mid length mininium measuring point. Since skiffs have a wedge shaped hull profile they cant really go any finer than the B3 under the current rules.. In fact the B3 is fuller than the earlier "Zero" hull which was legal under the old Australian Skiff rules..

I dont know if Bieker's thoughts are still on the I 14 website or his home website..

After Bieker was Luggy.. Who built the "flying" I14.. His foils were designed to fly so his original foils were at the base of the rudder and again assymetric to generate lift..
The lifting foil was replaced by symetric foil..Again at the base of the rudder becaues the rudder was already set up for foil at that position..

Must admit I havent kept up with the current state of play.. BUT Biekers foils are NOT designed for anti-pitchpoling.. Pitchpoling is secondary..

The 14's we had at Grafham (ages ago) were loving the foils because they could trim them to allow more drive downwind; if this is not how Bieker was expecting them to be used then hohu,- it is how they were being used, but I know from talking to the guys (and I am sure there was an article in the yachts and yachting mag over here stating that they use the foils to allow them to drive harder downhill) that they made the bear off at the top marj easier and that they could drive much harder down wind.

I have not read or heard too much into the I14 rudders, but from everything I have heard from the AUS fleet, Stuart is correct.

I believe the biggest gains were made upwinf where the foils tricked the boat into behaving like it had more waterline length by reducing stern wake.

On the downwing the foils are left pretty neutral or maybe a slight hint of raking them back. Up wind the raked the rudder forward. This also helped lift the transom out of the water.

Reduced pitching and nosedives was just a perk to the theory.

Downhill an Int 14 has its bow lifted by the kites. As do other skiffs.. Have a look at the 12s where they bounce down hill almost on their sterns with no wings..
At the top mark one doesn't have time (well I didnt) to tip the rudder, vang raise the plate and get set.. One has enough time to vang off while the crew pulls the plate.. get back and bear away.. Even then one bears off really when the kite goes up..
When the kits is up I would guess the lift by the kite is a 70 kgs or so..
The issue comes when one is jumping waves.. If the third or so wave is bigger you snuff the bow. Or the waves and the bounces are out of sequence and one lands in the trough and not the top of the wave.. If the kits remains full the lift pulls the bow out and up.. If not you swim..

I do know however the fleets here are probably more experienced than the UK fleets.. So maybe that is a factor?

Just relaying what I was told, don't shoot the messenger, if they were wrong then so bit it.

Now Darryl that is not the entire story, is it?

I love what you have done with the boat, but lets face it, adding the kite alone did not drop its vyc from 88 to 77. Making the boat a good 50% lighter and increasing its stiffness, improving sail plan, taking away the fore beam and fore tramp, moving the position of the dagger boards and changing thier design. have all contributed to the performance gains.

I would suspect the new boat would have been low eighties/ high seventies without the kite.

Please keep experimenting Darryl, when you crack 75 vyc I'll buy one!

p.s. I have watched a lot of video of the boat, and none of it shows the boat tacking, is tacking still an issue for the AO design?

One thing that has been stated over and over is that these rudders "must" have more drag than a non-T foil rudder and this seems to be one of the main arguments against them. One preson alluded to the fact the A Cup boats use end plates on their keels and it was noted almost all new airliners use winglet foils on the ends of their wings. The SOLE purpose of these is to reduce the drag of the attached keel/wing. It has also been mentioned that the rudders are almost never in a perfectly neutral position in regard to the boat- they are almost constantly at an angle (ie loaded). Going upwind they are certainly loaded. The "endplate" effect of the foils in these conditions serves to decrease the tip vortices and drag and could even potentially result in a rudder with less drag than one without the foils. This would completely negate all the "They have more drag" arguments against them and may explain why even in light air T foil equipped boats seem to be at least comparable to non-T foil equipped boats.

Kirt

Now that is a very good point !

Parasitic drag will surely be larger for the T-foil rudders, but the end-vortex of a single foil will be reduced by the T-foil. Winglets on airplanes give a small drag reduction, so small that some modern designs dont even bother with them. In addition the angle and shape of the winglets are critical to make them perform, and this is on airplanes which operate in far more stable conditions than I think a rudder do.
I dont have the math to calculate the differences in drag, but dont believe the drag is the key to this. I believe it is the increased effectiveness of the rig.

Here is a very good article on the development of winglets.
http://oea.larc.nasa.gov/PAIS/Concept2Reality/winglets.html

Is it really so important as to why "T" foils improve the performance of a boat? Isn’t it quite sufficient that they DO? (And by no small degree) As I see it, there is a LOT of improvement from the fact that the sails are always at a more constant attitude to the wind and therefore they are always working more efficiently in all wind strengths than for a boat without foils. The rest is the foils also maintaining the hull in an attitude where it is able to accept that extra "power" and still "put it down" through the water retaining controllability/stability.
Is it necessary to fully understand the workings of a computer/calculator before accepting that by using it you can do more complicated computations with it than you can in your head?
THEY WORK guys, surely that’s enough?
In some ways I hope that sailers DON’T put T foils on their cats. The less that are out there, the bigger the advantage we have. If everyone “puts them on”, we’re all “back to square one” and we have to look elsewhere for efficiency improvements.
Someone asked about the cost of the T foil rudders? Our cost for a T foil rudder is only about 5% to10% more than for our “normal” ones, and this is only due to the fact that they are slightly larger in surface area and take a little longer to lay up. We don’t intend to charge any extra for an F14 with or without T foils.
DAZZ, I think you are confusing the yardstick for the 4.4 Alpha, which was a heavier “older” design than the 4.3.
The 4.3 was the cat that we built primarily the same as the F14 (as a “prototype test” boat). Although it was constructed in “polyester”, “normal CSM” and had an aluminium mast, it was “comparable” to the F14 and approximately only 15 Kgs heavier. We sailed it competitively for three seasons, established the 88 yardstick, then sold it on to a “non racing” sailer, it is a “unique” cat, a “one off”
I will go out on a limb here and suggest (only suggest) that the yardstick for the F14 WILL settle somewhere around the 73, 74, mark (maybe, just maybe, lower?). There certainly is much more potential for them to perform better than they are at present. As with any class, their “final” yardstick is dependent entirely on just how many of them eventually get out on the water regularly competing.
P.S. The Alpha F14 tacks "on a pin" (I am not sure which video you have been watching DAZZ, but in the DVD that I have been sending out on request, it is shown tacking and jibing). As to the "strength" of the T foils, we have given them the "crash" test many times as well as "loading" them repeatedly against up and down "flex" and so far they have proved strong to the point of being, "over engineered"
STEVE, I think someone wrote earlier that because it wasn't expressly mentioned/allowed in the F18 rules, then "T" foils are disallowed on F18's (Tornadoes as well) -they will never know just what joy they are missing.