Why we dont control pitching with foils?

[xarax]

We could reduce pitching with horizontal foils and modify the hull shape accordingly. A double-ender with horizontal foils could be faster than current designs !

[koen]

Paul Bieker did something like this with an I14. The horizontal foil on the rudder changes the wave system which makes the boat longer.

http://www.employees.org/~usifa/gene...hydrofoil.html

[water addict]

Control with horizontal foils is a real problem in anything other than calm seas. Flucuating angles of attack due to waves on the surface cause varying vertical forces, which can lead to pitch instability at high speed, and catastrophic crashes.
Of course at low speed, the forces generated are small enough to not really matter, but then at low speed, one wouldn't need the pitch control.

It sounds like your goal is to stop the hobbyhorse issue that is common with older double-enders, and that once done the double-ender will provide better performance than a typical transom stern. This might possibly be the case in some sea/wind/ point of sail conditions. But adding complex geometries and/or mechanisms has it downside. Simplicity always seems to win out.

In small boats like Moths the rudder has long had a t-foil to reduce pitching and the hulls have been nearly double enders for some time as well. The foils definitely work. On newer foiler Moths pitch is 100% controlled by the rudder t-foil.
Some smaller cats such as the"Stealth" (I think) are now coming standard with rudder t-foils to reduce pitchpole. Steve Clark has used rudder t-foils on the C-Class cats (and I think the A-class and as I remember it he considers that they work very well.
At least one of the Open 60 trimarans use a rudder t-foil for pitch control.

[koen]

The open 60 trimaran was gitana X. Unfortunately it pitchpoled.

http://www.projectsomewhere.com/gal-...0434,4822.html

[CT 249]

As Doug says, Moths have used foils to prevent pitching and pitch-poles for ages and the 14s now use them partly for that reason as well. Hull shapes have changed as a result.

Nigel Irens, at an address in Sydney, said that the problem with relying on foils to stop nosediving in tris is that as soon as they lose effectiveness (ventilation, breakages, or the cursed plastic bag wrapping around) the boat potentially capsizes straight away so there are some problems with the concept, but that doesn't apply to your ideas.

Double enders can already be fast in dinghies; the International Canoe with assymetric is arguably still the fastest singlehanded non-foiling dinghy, while the effectively double-ended Moth is probably the fastest of them all and even without foils it's fast for its length.

[Andy P]

Int moths have been using T-foil rudders since ~ 1993.
The all beneficial effects are less pitching and also prevention of nosediving by keeping trim angle constant.
The hull designs have evolved together with the T- foil so that the latest boats are double-enders.

and of course the development of double T foil fully flying 'boats'

PB adapted the t foil for use in i 14, but for lifting the transom, and so reducing displacement. ( the moth T is used primarily for down force to prevent nosedive ).

The difference is due to the i14 being a planing boat, and the moth being a slender forced displacement ( ie beam : length < 10% ).

I made the prototype T foils for the stealth cat which now are fitted as standard - increasing performance and allowing the boat to be driven harder.
( bloodaxe T foil rudder for stealth 16 in pic )

Full Pelt F-40 used T foils also - a reduction in top speed but allowing more power to be used without nosediving.

On a large Tri --- the risks of pitchpoling may outweigh the performance advantages.

The pitch reduction of the T foil is well worth the slight extra drag - any downforce ( moth ) or lift ( i14 ) effects are a bonus!


No high speed crashes - the T foil actually prevents the crash , ( but in extreme conditions, there is the temptation to push too hard - eventually something gives and any crash is more severe. )



Andy P

Chris, interesting that Nigel Irens said that because he is an ardent advocate of foils DONE THE RIGHT WAY.(designer of Dr. Sam Bradfields SCAT) I wrote him a letter asking why he had not use bananaboards on Ellens tri. He said he would have preferred to go forward with full flying foils but the sponsors wouldn't hear of it. He is not too much of an advocate of the banana board concept because even with a rudder t-foil the axis of rotation of the boat then becomes between the t-foil and the banana board and the thing can simply rotate around that axis resulting in a crash. However, that could be managed with a foil on the daggerboard but there is a tradeoff there in light air performance. The banana board can also become a surface piercing foil with or without a t-foil potentially resulting in a ventilation induced crash, the rolling crash mentioned before or a crash resulting from no pitch stability because the ama is now flying.
There is no question that one of the best uses of a t-foil for pitch control is in COMBINATION with a small or large altitude controlled main foil--at least at high speed.
Funny thing about Mr. Irens comments is that the t-foil is seldom heavilly loaded.....
=========================
Andy, what is the reasoning behind the planform of your Stealth foils?

[tspeer]

Originally Posted by Lorsail

...
Funny thing about Mr. Irens comments is that the t-foil is seldom heavilly loaded.....

That's not a surprise. Even for a full-flying hydrofoil, the aft foil must be more lightly loaded for stability. It may even be negatively loaded for trim. Damping and stabilization have more to do with the change in lift with a change in foil angle of attack, and less to do with the steady state lift on the foil.

[Andy P]

> Andy, what is the reasoning behind the planform of your Stealth foils?


Long and thin may be more efficient, but more of a weed catcher and more damage prone.
I've tried various planform moth Ts - from ~ 350 x 150 x 15, to 800 x 50 x 6

The thin ones less draggy, and you can use them on one setting, but if you get stuck on a mooring line it's impossible to clear it.

The customer requested that sort of shape for the stealth -
for weed
damage
'cool' appearance
( the T is similar to double tip on my daggerboard)
Keeping to within the overall hull beam

Andy