foiler: roll stability

[nico]

hi,

Watching some videos from moth foilers and the Flyak, i am quite impressed by the roll stability they have. Is there a restoring moment or is it only damping? The only restoring moment, for a t-foil fully submerged, that i could think of, is the loss of lift due to one part being closer to the free surface, but i expect that to be minor.

[Chris Krumm]

I would think it's basically roll damping in these configurations. Both the moth hulls and sprint kayaks have little form stability. I remeber an article in Woodenboat on Graham Kings rowing shells, where his latest hulls were so "tippy" he added pairs of small fins to effect roll damping.

Chris Krumm

[Tim B]

no reason that lift should reduce closer to the free surface unless the foil is cavitating. Check out the aircraft stabilty books around. It's the same case but with a higher-than-normal CG.

Tim B.

[tspeer]

Originally Posted by Tim B

no reason that lift should reduce closer to the free surface unless the foil is cavitating. Check out the aircraft stabilty books around. It's the same case but with a higher-than-normal CG.

Tim B.

The reason lift decreases at shallower depths is because of interaction with the surface. The downwash from a given amount of lift is doubled as the foil reaches the surface, and this decreases the lift curve slope, too. So the deeper side will produce more lift at the same angle of attack.

Russian river-running hydrofoils depended predominately on this effect to control their height. It's not strong, but it is present.

Another effect is from the leeway acting on the foils. The trailing vortex shed by the vertical strut will induce a rolling moment on the lifting foil. However, I think this may well be in the direction to increase the heel, rather than decrease it.

But I believe the main benefit for the Moths is the damping, not restoring moment.