Curved Lifting Foils on Cruising Cats??

I think that you are right, Steve. The process of lifting a board out of its trunk by hand limits the discussion to the cruising boats that are small enough to allow such an effort. Big boats have big boards and the attachments for lifting that board become the design as much as is the foil in the water. Offering a single spare board for bigger boats requires that the lifting points be completely integrated in the design, that the fixtures not interfere with the immersed hydrodynamics and the design puts a decent skill set requirement on the person effecting the repair. I think it might be difficult to find that guy in Zanzibar.

It seems that the application requires a lot of specialized solutions compared to the typical straight board answer. It also seems that the benefit does not make a lot of sense compared to the assumed potential problems.

 

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I see no reason why a constant chord symmetrical or asymetrical curved board can't be designed to be used on either side of the boat regardless of the method of moving the board. Especially a board on a "cruising" boat.
However, based on the information provided about the Catana 59 I'm very skeptical that curved foils will provide any substantial benefit like they do on race boats. We'll see....

Interesting comparisons:

The Catana is 11% heavier than the Gunboat at light displacement and the Gunboat has a reputation for speed.

Gunboat 66 racer/cruiser
--light displacement=34,000lb
--Upwind SA- 2232 sq.ft.
--Downwind- 2583 sq.ft.
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Catana 59 racer*/ cruiser
--light displacement=37,800lb
--Upwind SA- 2120 sq.ft.
--Downwind- 2583 sq. ft.

From the Catana website:

*All this has culminated in the new Catana 59. A Catana with incredibly streamlined hulls, classically elegant, and offering astounding performance. The sail plan is borrowed from racing vessels, with a short mainsail that is easy to manage and a larger foresail. Moving away from the legendary daggerboards that made the Catana shipyard's name, this catamaran is fitted with revolutionary curved daggerboards, to create a hydrofoil effect. The result is an extremely safe and comfortable boat that achieves astonishing acceleration and an extraordinary cruising speed.

 

Perhaps I'm just a retrograde old sailor. But for cruising multihulls, what is wrong with fixed low aspect ratio keels. They have many other advantages than just preventing leeway.

 

I like that you can sail in thinner water, that you can let the tide go out and sit on the keels and that you don't have to worry about a trunk getting mashed from a grounding, but don't they also have a collection of problems, as well?

 

All leeway preventing devices have their good and bad points.
I scrapped the dagger board on my Piver Nugget to get space for a fold down double berth. I replaced it with twin surface piercing boards pivoting down from sleeve boxes at the folding deck joint. They proved to be a real pain.
I subsequently removed them and fitted a Norm Cross style LAR keel.
It transformed the boat.
I armoured the leading edge and bottom of the keel with Half round metal strip and had no problems with grounding. We used to sail it straight up the pebble beach on Fox island and tie up to a tree for the night.

 

post #1 pics...​

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It's a bit of an old thread, but since the topic runs here I'll reply here too, saw that Tony Grainger on the RAKU 32 get straight dagger trunks out of the way in the interior by just putting them at an angle, but in the up position the daggers do hang out past the line of the hull rub rail, which might hurt them along a high dock...

 

Curved boards would help alleviate that issue. A simple circular curved board should not be too much additional cost over a straight board, but might be difficult to replace as you'd have to get the curve spot on.

I wonder how much of an issue it really is though. With the board raised to just inside the rail, how much is left protruding? Maybe some fat fenders would help too: boats with outwardly curved boards/foils have massive fenders.

 

Yes, actually the same precautions as with leeboards along a dock or a jetty would work well . . .

(pic source)​

 

I just noted that the MOCRA rating rule calculations provide that full foilers are 4% faster than "normal" boats of the same design. A boat with rudder winglets and C foils is rated as 2% faster than a "normal" boat.

 

The foiling phenomenon, the history of foils – Yachting World https://www.yachtingworld.com/special-reports/the-foiling-phenomenon-66269

Excerpt:
Cruising catamaran manufacturers Catana launched a 59ft luxury cruising cat that has curved daggerboards that the builders claim produce half a tonne of lift at ten knots.

Because lift varies as the square of speed that means over a ton of lift at 15 knots.....dl

 

The luxury Catana 59 is a boat of about 20 tons cruising weight, so half a tonne is 2½ %, and 1 tonne is 5 % of the weight, and there is the extra drag of the lifting force, so what part would the speed gain of the curved daggers foils be when she's doing 15 kt . . ?

 

I agree, Catanas are (very) heavy cruising cats, It would be much more productive to lighten them half a ton !

 

I know on carbon racing bicycles and recumbents losing 1 gram costs about € 1, and that's going up when it gets lighter, unless you order them directly from Alibaba ^{(FB video)}

 

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Hard to figure when I'm not clear if the article referenced both foils each lifting half a ton or both foils together lifting half a ton?
At any rate, every little bit helps.......

 

Agree to that, and for Catana it ain't much of an effort to make the boards and their trunks curved instead of straight I think, so the little gain looks to be an easy catch.