Curved foils?

[andycarr9999]

Since starting the design process of my cat i have been recommended by various people to use curved foils like the new Nacra f20 carbon uses, i have watched a couple you tube vids of the boat and they look excellent, making the leeward hull appear lighter in the water whilst still flying the windward. I have contacted Nacra to ask if they would sell me a set of these foils but would like to know if there would be any major disadvantages i.e would it make the boat more unstable in large seas? Also it is my assumption that if i was to put those foils on it would limit the size of my rig to that of the nacra f 20??? For anyone interested so far the basic plan is to use two 25 foot moulds i already have the use of to make some very sleek 25 foot wave piercing, then to join them together with some curved beams to keep them out of the chop. this is by no means final but we are working on the hulls being 14 foot apart in total. I have booked in some design time with a friend who is a boat designer and we are going to use his computer to see what might work. The hull design i am most likely to use can be seen as the outriggers of this cat....

http://www.derickreynolds.co.uk/Ozone.htm

[Doug Lord]

Originally Posted by andycarr9999

Since starting the design process of my cat i have been recommended by various people to use curved foils like the new Nacra f20 carbon uses, i have watched a couple you tube vids of the boat and they look excellent, making the leeward hull appear lighter in the water whilst still flying the windward. I have contacted Nacra to ask if they would sell me a set of these foils but would like to know if there would be any major disadvantages i.e would it make the boat more unstable in large seas? Also it is my assumption that if i was to put those foils on it would limit the size of my rig to that of the nacra f 20??? For anyone interested so far the basic plan is to use two 25 foot moulds i already have the use of to make some very sleek 25 foot wave piercing, then to join them together with some curved beams to keep them out of the chop. this is by no means final but we are working on the hulls being 14 foot apart in total. I have booked in some design time with a friend who is a boat designer and we are going to use his computer to see what might work. The hull design i am most likely to use can be seen as the outriggers of this cat....

http://www.derickreynolds.co.uk/Ozone.htm

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Andy, curved foils are cool because of their adjustability:
1) They can be retracted slowly reducing the area of vertical lift to suit the conditions and performance you're looking for without significantly reducing the area of the foil devoted to lateral resistance.
2) Some applications of curved foils, including the DNA and Bimare A Class cat, allow the head of the board to move fore and aft changing the angle of incidence of the lifting portion of the foil, again, without affecting the portion of the foil developing lateral resistance.
3) From a design standpoint, the radius of the curve of the foils can be matched to the anticipated use on a particular boat. For instance, the foils used on the Farrier have a tighter radius than the Nacra 20 since the Farrier boat also has a daggerboard and is looking to maximize vertical lift.
4) The foils on the Nacra are designed for that boat to develop the required amount of lateral resistance and vertical lift. If your boat will have similar speed at similar weight and SA they could work well.
4) The more adjustability you can incorporate the better-at the very least design the trunk(or modify it if necessary to allow the head of the board to be moved fore and aft) while pivoting at the bottom of the trunk. Other adjustments used on some boats with curved foils include canting the head of the foil side to side(Steve Clarks C Class) and a facility to to adjust the toe in of the leading edge of the foil (that affects only the lateral resistance portion of the board).
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You could PM Steve(a member here) and he might even have some boards he could sell or build for you.

some more info here:

Advantages/ Disadvantages of Curved Lifting Foils on Monohulls/Multihulls

pictures, L to R: 1) Steve Clarks foils for Aethon, 2) & 3) illustration of canting facility Steve built in. increases or decreases lifting area w/o retracting board-may allow less area than a board w/o this facillity(ask Steve), 4)Farrier "tight curve" foils, 5) Bimare A Class angle of incidence adjustment system-changes lifting portion only-facillitates tuning foil for conditions, 6) DNA cat/ curved foils

[Corley]

The page you've linked to shows an 8m micromultihull trimaran (nice boat btw). Trimaran floats dont generally make great catamaran hulls without quite a few mods (the hull shape isnt right and lacks displacement around the rear of the hull needed to carry crew weight) I've seen some larger cats based on tri floats (bluey zarzoff comes to mind) that are quite succesful but I'd prefer to start with a clean sheet the designer seems to have a few small cat designs that could be adapted more easily to what your trying to do.

[andycarr9999]

Originally Posted by Corley

The page you've linked to shows an 8m micromultihull trimaran (nice boat btw). Trimaran floats dont generally make great catamaran hulls without quite a few mods (the hull shape isnt right and lacks displacement around the rear of the hull needed to carry crew weight) I've seen some larger cats based on tri floats (bluey zarzoff comes to mind) that are quite succesful but I'd prefer to start with a clean sheet the designer seems to have a few small cat designs that could be adapted more easily to what your trying to do.

I would to be honest prefer to start with a clean sheet also but money is an issue as it so often is... I am told that when the outriggers were deigned they were designed with a view to selling off the moulds to the then even more popular c-class sailors. Hopefully i can sort these possible problems out on the computer when we finally get to sit down with it.

Doug, i will certainly be getting in touch with the person you recommended, thanks again for all the very interesting food for thought!

[Corley]

Good luck with it and let us know what eventuates I can see how the hull shape could be made to work with foils t foil rudders might be needed though to keep the transom stable. I can recall reading Steve Clark posting that driving a thin torpedo through the water seemed to work better than fully flying the boat but his later C class cat had curved foils that lifted some percentage of the boats weight. C class catamarans are limited to a sail area of 300sqft are designed for a fairly limited set of wind and wave conditions and normally carry a very high aspect rigid wing rig so some tweaking of the boat will be required to fit your needs.