foil shapes

hi guys,

has anyone successfully used a trim tab on a canting keel foil/strut to produce righting moment?

cheers
boogie

 

canting strut rm

I have used it on an rc model canting keel prototype-and it worked well. I found out during work on the model that CBTFco has a patent on a similar system.
I'm convinced that by using the idea ballast can be significantly reduced and that canting keel boats using it are likely to be able fly on hydrofoils using a two foil system similar to Moths-at least offwind.

 

Cookson 50

 

Cookson

Don't think the keel on the this boat cants far enough to have the flap deflection act as righting moment. To use the flap for righting moment requires that the flap be deflected the OPPOSITE way than it would be to improve lift to windward which is what it's used for on the Cookson.

 

right you are doug. I confused my interpretation of the question

 

Two problems with the downward-lifting canter foil:
First, it's subject to catastrophic failure (leeward broach) if & when the foil ever surfaces, for instance if the boat rides up on top of a wave and is heeled over by a gust.
Second, as Tom has pointed out, the lighter ballast does you no good anyway whenever you're using the foil, since it's increasing the boat's displacement by the same amount. Actually even more, since the lever arm of the foil's line of action is somewhere in the middle of the foil, which is not as good as a bulb at the end. And it's also introducing drag by disturbing the water flow.
Also, of course, it does you no good at all in a knockdown. I guess the main advantage is when you're not using it, i.e. on a run.

 

canter RM

Skippy, if you look at multifoilers like the Rave you'll see that, in fact, using a foil to generate RM does do good because it can dramatically increase the power to carry sail-in the Rave case to the point of the destruction of the boat if you don't heed the speed limit.
Using this system on a canting keel foiler might be quite fast but your points regarding what could happen in a seaway are good ones but probably not enough to stop development in this direction. Maybe you could have an "instant" sheet release that would save the boat....But in any case you still have a keel which might self rite the boat if too much ballast isn't removed.(!)
The reduction in ballast would help in another way for the application on a canting keel foiler: it would allow it to reach take off speed sooner with the flap neutral and then allow a gradual increase in RM as necessary as the boat goes faster.Not only that but the RM controlled via a flap would be much quicker to change than it would be to move the keel.

 

Okay, I came up with one advantage of the trim tab. As long as the foil is at an angle, i.e. not all the way up to horizontal, the foil's force is more perpendicular to the lever arm, whereas the vertical weight of the bulb isn't being used completely for righting moment.

Oh that's right, the foil's force increases with speed, unlike fixed ballast. Except without a comparable lee foil pushing down, you're also increasing the displacement with speed, which can't possibly be good. Of course, the Rave has a leeward foil, but does the weather foil really reverse its AOA to pull down?

 

foiler

Skippy, the windward foil on the Rave does pull down very hard. The main foils on the Rave are set at +2.5° so the down force is mostly due to flap action though the shrouds may contribute a little to the reduction of the angle of attack on the windward side.
And the foil on the canting foiler would create a lot of RM as the boat speed increased and would have a "wand"(or other altitude control) on the main foil on the hull that would resist any increase in displacement felt due to the flap on the canting strut.

 

just to spin a few more ideas....

the argument with the windward down-pulling possbility of ventilation with increased heel and the increase in displacement are very good arguments against it.

why not "just" add another canting foil with no balast that swings to leeward and the flat defection lifts the leeward side instead of pulling windward down....
you could make it retractable too to reduce drag at very low power requirements [light wind, downwind run].

cheers
boogie

 

Boogie,
It's part of the CBTF patent, so if you ever forget to change the tab on a regular keel when tacking, you'll have a lawsuit waiting for you on monday.

Yokebutt.

 

More random points:

One more strike against the partially raised foil: even though the force is better directed for righting moment, it has a leeward component, so it's not feasible without the CBTF-style fixed foils, and even in that case, they'll have to be bigger.
The lee foil could conceivably work if you could find somewhere to put it. But remember, we're talking about a canting bulb strut, so the two foils would have to cross each other when you tack. And if the lee foil is also at an angle, then you're talking even more leeway, so I would just go with a multihull and horizontal foils on vertical struts. Longer lever arm, no leeward force, no need for the canting bulb keel. Since that was the original question, I'll vote "non".

 

foiling keel boat

I think the canting keel system with a flap to generate RM has particular application on larger monohul foilers. And it should still offer what all monohull keelboats supposedly do: self-righting capability. A canting keel foiler ,as I envision it, would be nothing like a CBTF boat in that it would have one turning foil-the rudder and one fixed and probably retractable forward foil with the main hydrofoil fixed to it.Inside that would possibly be a daggerboard for use when not on foils. Using just two hydrofols plus the flap on the canting strut would be the simplest solution for a foiling keelboat and I'm convinced it would work.
If CBTF could be worked into it it would help in non foiling conditions but I don't see it yet-but maybe.
The patent-"Heel Control System for Sailing Yachts and Sailing Yacht Hull"(5622130) refers to a monohull with the CBTF system -not a foiler-using a flap to generate RM.
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edit: I think Stephen Ditmore a former regular contributor here also has a patent that could be very effectively used for this purpose; the flap could be deployed on his system with NO leeward component to the lift generated.....