Adjustable assemetrical foil cases

Building a 7 m trimaran , building the outside hulls at the moment ,before I put the decks on I want to throw in some straight asso centerboard cases. Planing on them canting inwards towards the main hull. My question is , is there a way of adjusting the angle of attack of the centerboard in the case after it is build,not expecting to do it on the fly ? I've read somewhere that there is a way of fine tuning a curved lifting foil /case , I want to go with straight boards for obvious reasons , just wondering how this adjustment is made ? Is it the cases or the board which is adjusted?

Thanks j

 

That's one of the downsides of straight foils - easier to do with curved ones!

Make your cases box shaped and only shaped for the foil at top and bottom. Then if you want to change anything, you only need to alter the accessible bits and not rip the whole case out to start again

 

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You can make the top of the trunk longer than the bottom and the use an adjustable sliding plate on top of the trunk to move the head of the foil fore and aft changing the angle of incidence of the lifting portion a curved foil or "L" foil but NOT an angled straight foil. Changing the angle of incidence of a straight angled foil requires an athwhartship adjustment of the leading edge and is very difficult to properly install. Not ony that, any adjustment of the angled straight foil will change the angle of incidence of the whole foil affecting both the lateral resistance and vertical lift whereas adjusting the head of a curved foil(or "L" foil) fore and aft adjusts ONLY the angle of incidence of the vertical lifting portion of the foil. Curved and/or "L" foils are superior from an adjustment point of view because of this.
When you say "canting them towards the main hull" do you mean the bottom of the deployed foil will be closer to the main hull? If you can possibly do it curved (or "L")foils would be much better......Curved foils are 100% retractable ,"L" foils are not. Good Luck!

Pictures: 1) Farrier trunk and foil-note how the trunk is longer at the top, 2) Adjustable plate for curved foils on an A Class cat:

 

Thanks Doug those pictures , especially the 2nd one pretty much answer my questions , from what I can gather , the curved foils and cases are the hardest to make but the easiest to tune , the the straight boards are easy to make and install but a ***** calibrate ! Tell me some advantages of the L boards , , I'm not looking to get this boat airborne , just looking for some support/stability whilst pressing it hard.

Jay

 

foils

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The "L" foil(tip pointing inboard) is a compromise that allows the same adjustability as a curved foil(for vertical lift independent of lateral resistance) but is somewhat easier to make. It won't retract completely but on a tri under sail the windward foil should be clear depending on the design dihedral of the crossarms. You have to decide if you want the lateral resistance portion of the foil toed-in or not. And that depends on whether or not you're using a daggerboard in the main hull. If not it can be done with the trunk though some tri's toe-in the amas so in that case the trunk can be on the centerline of the ama. I've seen a lot of stories how curved foils are more "difficult" to build but if you're very good with glass/woodwork I don't think there is much difference. There is obviously a bit more work at the plug building stage but it is straight forward. And cost won't be a whole lot different if you're making the straight or curved foil out of carbon. There is a thread in "Multihulls" on SA that goes into building curved foils. Also, Farrier sells his foils and cases separately...

http://forums.sailinganarchy.com/index.php?showtopic=129899 brainstorming building curved foils

 

board cases

I will second making the cases oversize. I built mine a bit too tight for my Buc 24, and I dont have as much adjustment as I wanted. My first set of straight boards have proved to be oversize (too much lift/drag), so I need to build a set of smaller boards that will also allow more adjustment. My first set had an 11.5" cord with about 42" exposed, the next set will have an aprox 8" cord with about 48" max exposed. I think they will be about right, and allow me some angle of attack trim- my current boards are fixed at 2 degrees. I haven't found that I need much fore/aft trim, but angle of attack might be useful. I highly recommend them, they completely change the nature of the boat. Look at some of my posts on the Buc 24 forum, I posted some pics and other details. I used a Naca 2408 foil profile- a sort of thin flat bottom foil- that seems to work very well. They are very powerful, even at slow speeds and seem to suit the Buc's performance envelope. Good luck with yours. B

 

Perhaps the J boards Gary Baigent build for Sid, or a pair inspired on them, are a consideration as well.

http://www.boatdesign.net/forums/multihulls/alternative-marvelous-buccaneer-24-a-32382.html