Cat hull Hydrofoil

[FarmerColin]

Just to throw a sum into this. If we assume the foil could achieve a lift coefficient of about 1 (which is fairly conservative) at a speed of 20 knots it could provide 5 tonnes of upthrust per sq.m of foil plan area. Since this force is proportional to the square of the speed the high speed of a cat could allow it to make better use of the idea than a monohull. Being able to reduce the freeboard needed would help reduce windage so there could be a performance advantage if the concept was built in at the heart of the design rather than as an afterthought.

[Meanz Beanz]

Originally Posted by deepsix

Your foil is drawn on the bow above the waterline and infront of the pitch axis. This means that as you pitchpole the foil will be submerged and will generate lift. If the boat continues to rotate, the angle of attack on the foil decreases and the foil produces less righting force. Eventually the foil will only contribute drag and a may even generate a force pushing the bow down. If the foil is behind the pitch axis, the righting force will increase as the boat rotates until the foil stalls.

This would be my key concern with the idea, the moment it stops providing lift it becomes a rapidly increasing drag and would accelerate any pitching moment. I'm not saying that I think it can't be done but I would think its an idea that requires a lot of testing and foil knowledge, after all you really need it to work very well otherwise it would rapidly become a negative force.

Echoing other comments on this thread... As for weight in the ends of a boat, any boat... NEVER, its a really bad idea. Go for extra length if you have to but keep the ends as light as is possible.

[FarmerColin]

There are a number of things one could do to overcome the possibility of the foil producing a downforce or drag through being stalled. If you use a low aspect ratio planform then the foil could be set at a high geometric angle of attack because the low aspect ratio planform reduces the lift slope and making the point at which the foil stalls significantly later. One could also camber the foil so that it had a very early inverted stall which would limit the downforce it could generate if the situation got that far. If you have a look at the ends of the hulls on Dick Newick's "Cheers" you will see 'plates' set up at about 30 degrees and strakes running back down the gunwales from these and this arrangement could be seen as an extreme form of the idea.

If one wanted to use a high aspect ratio planform (such as the front crossbeam) as the foil (which would have the advantage of producing a better lift/drag ratio) the foil could be pivoted and sprung in rotation so as to provide some degree of angle of attack self-adjustment which could allow it to feather if the lift force on the foil was negative.

[Spiv]

Originally Posted by deepsix

... if you manage to get it into the water there is alot of drag, the boat slows down and is more likley to pitchpole.

I have experienced that in small cats: you come suddenly to a stop and if the hull/decks are wide, they offer resistance, the rig still wants to go fast and you pitchpole.

Modern bows are narrow so they can cut through the waves, .... The high freeboard also gives good bridge deck clearance.

which is most important as it reduce waves slamming under the saloon.

If you want to go fast you need to go straight through the waves,

exactly, that will also help reduce that menace of open seas going: pitching.