Foiler cat

I have looking at foil asisted sailboats. Biger part of them there tris or monos. Maybe cat has something against foils? If so what?

 

I've been doing a few sketches on foiler cats, actually- it's not something you see a great deal of. Keeping a cat on foils in balance is a tricky task- normally a cat will fly one hull, but on foils it's essentially flying both- so the windward and leeward foils need to provide opposite forces, working together to counteract the moment applied by the wind on the sail. The foils then need to be able to reverse their forces when tacking. For small craft like a Moth or a Hobie (if you were to put foils on it), moving the crew provides all the righting moment you need. For bigger cats, the foils themselves must do that work, and the geometry gets very complicated. Since there are no major racing classes for foiler cats (or many for cats, for that matter, especially at the maxi level) the incentive to develop the system really isn't there, despite the huge performance potential.

 

foiler cats

When you look at foiler cats from the Icaurus era of modified Tornado to Spitfire the big difference between the cats and the foiler tri's is beam. Bradfields 16'Rave trimaran is slightly over square as is his 40'SKAT tri. Both use foils to provide righting moment. Foils adding RM works better on wide tris because the load on the forward foils is reduced compared to a narrower cat doing the same thing. Tom Speer quotes a conversation he had with Mark Pivac, designer of the Spitfire(see below): " Question: why did he use ballast instead of foil lift to create RM on Spitfire? Answer: Because you only have to lift it once." If you take ballast you lift it right off the bat and no matter how fast you go it's still the same amount of ballast. If you use the foils to generate RM the drag due to RM goes up the faster the boat goes.
Using foils to generate RM seems like it might tend to be more beneficial on oversquare trimaran configurations where the beam itself helps to reduce the load ; on cats it may be best to use ballast-though using ballast can't hurt in either case especially if it's already there in the form of crew.
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The Predicted and Measured Performance of a 16' Hydrofoil Catamaran
Address:http://homepages.rya-online.net/ejcchapman/HobartPaper/HobartPaperWeb.htm
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Spitfire
http://foxxaero.homestead.com/indsail_017.html
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Inflatable foiler cat
http://www.dcss.org/speedweek/toastrack.html

 

but cat has huge RM by itself, foils can be used as method to reduce drag.

 

If a cat had the wand-adjusting mechanism that Moths use on each hull,as well as the rudder foils,wouldn't it be faster than forcing the hulls through the water?Obviously,such a scheme would only work with comparitively small cats and not larger cruising boats.

 

If it is a 60ft maxi racing cat I think it would work too. Because they are relatevely ligt and very fast. Hydroptere is pretty much the same size, and is flying!

 

The problem with foils on large cats is you're up against the cube-square law and cavitation. As you scale the boat up, the weight goes up by the cube of the size, but the foil area only goes up by the square of the size. If you're going to fly the big boat at the same speed as the small boat, then you need foils that are proportionately larger. Or you can fly at the same Froude number, in which case the foils have the same proportion to the boat, but the flying speed is higher.

However, subcavitating foils have an upper speed range of around 40 kt. The big cats are already hitting these kinds of speeds. Putting foils on them isn't going to let them go much faster. A small boat, however, can use foils to go to much larger Froude numbers. So foils are a way to let a little boat be a giant killer - ala the Moths.

I think it's a mistake to think of foils as making a boat fast. The best use of foils is to allow a fast boat to still go fast in roughter conditions. This, again, is of more use to the small boat than the large cat.