Foil assist cruising cat possible?

This Dufour 82 with 30 ton displacement has keels and needs more waterline. I'm about to put on sugar scoops but wondered if a foil would help. I see even cargo ships are claiming better fuel efficiency with transom foils so maybe I can lift my transom like this but with T foils .


Getting lost in the foil forums so I'm thinking about building a bracket on the transom and folding down the foil while dialing in the fixed angle of attack.



If this is not a total waist of time then I'm looking for guidance on starting dimensions for stainless steel.

If this works then I'll next consider this idea. Comments?

 

Why do you think that lifting the transom will make your boat faster?
The best way to add foil assist is to use a lifting foil at about the same position a daggerboard would be located*-not at the transom. A rudder T-foil can be used to enhance pitch stability but is not generally needed for foil assist.
*just forward of the longitudinal CG
Catana 59 cruising cat with foil assist:


Catana 59 curved daggerboard foil:

 

82...


So if your boat is 30 ton, and you're looking for some sort of assist, that means about 10-20 tons of lift. Maybe less as the hull itself is likely generating some lift. But in any case, I suspect it's going to require a large foil. I'd have to do some more work to figure out how big...

 

And a large foil, especially at low speeds = drag.

Thus it is the usual lift v drag issue.

 

Back of the napkin seems to give a foil area of .8m^2 (for 10 000 kgs of lift at 8m/s -- roughly 15 knots --for a lift coefficient of .4). This seems small to me, but seems to equate to an 8 ft x 1 ft horizontal surface, which I suppose makes sense. I wouldn't be shocked if my approximations are wrong, though

 

Well, using your simple numbers

Lift = Cd x 1/2 x rho x Area x Speed^2 = 0.4 x 1/2 x 1025 x 0.8 x 8^2 = 10496 N = 1070kg.

But still doesn't take into account the drag of the foil and its supporting structure! Ergo...what is the quid pro quo...?

 

Thanks all for the comments.

As to why the stern foil first I think the boat is loaded beyond the design weight because the waterline is at the top of the transom at rest and is creates what seams a hole with violent vortexes in the wake. This seems a case for sugar scoops to add waterline and improve the bottom release angle.

Maybe I should start with a forward foil on the center but I'm not yet convinced there is a benefit to the basic concept to submerged foils making the boat faster at these speed . I've seen power cats with back swept v foils and dihedral in this position but no sailing cats with a center foil. There is a convenient structure to connect a swing down foil below the mast. Experimenting here would be more expensive. Do you think it more likely I would have a results here?

 

My recollection is the idea of a foil at the stern of a ship is to reduce drag by interacting with the wake, not simply to provide lift.

 

You should probably have a naval architect(that really knows foils) help you with the design and engineering-or else you're asking for trouble. You definitely don't want a single lifting foil aft!
This is "Happy Feet"-it uses a unique foil system that slides athwartship and consists of a wand controlled main foil forward and a rudder t-foil aft.

 

That was my problem: didn't work out the units! Newtons =/= kilograms.

So to lift the boat significantly out of the water (10000kg) , you're talking ten times the foil size = 8m^2.

That's a huge foil area. I would be significantly less worried about the parasitic drag because a) you could potentially make it retractable and b) the difference in drag by making your cat essentially weigh 2/3 of what it does would be substantial.

That said, I think OP might be better served by attempting to put said cat on a diet. Replace everything that's not a structural part of the hull with a lightweight version of the same. You could also increase the angle of attack of the foil to reduce the size needed.

 

Exactly!