GARDAGAST???
Robert, could you explain a bit what exactly the purpose of a hinged foil is? I’ve stumbled over it before, but never really understood it. In which direction is it producing hydrodynamic lift? Is it a bruce foil to counter the leeward drift and heeling moment of the boat, or is it just to get the ama out of the water? (The purpose of the hinge, as a means to keep the foil in the water, is clear to me).
Marco
In which direction is it producing hydrodynamic lift?
On a proa, to windward and normally down. If we can neglect weight and buoyancy, the resultant force of a hinged foil has to go from the foil’s centre of effort through the hinge. If it doesn’t, the resulting torque will rotate the foil until the force goes through the hinge again. The hinge is above the water, the foil’s centre of effort below. The resultant force points from the hinge down and to windward.
Is it a bruce foil to counter the leeward drift and heeling moment of the boat
Yes.
or is it just to get the ama out of the water?
That would only work if you you limited rotation around the hinge axis to turn the foil into a fixed foil for angles of heel from zero to whatever hull flying height you want. Then, if the boat heels more, the foil’s rolling around the hinge axis hooks the windward side into the water over a wider range of heeling angles than a fixed, curved foil could.
Sailrocket’s foil is fixed and curved so that the upper portion of the foil lifts the stern out of the water. The more that portion of the foil emerges from the water, the more the total force is dominated by the lower portion that hooks into the water. It works much like Fritz Roth’s non-hinged vector foil proa. (I have a feeling that “unhinged” is grammatically correct, but that has acquired a different meaning. If anyone knows Malcolm Barnsley or Paul Larsen, you could ask whether they mind Sailrocket being called an unhinged vector foil proa.) Sailrocket has no problem with that fixed foil working only over a limited range of heel angles because her leeward lifting rig makes her heel to windward anyway.
Regards
Robert Biegler