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Hinged Bruce foiler

Posted: Sat Nov 20, 2021 6:12 pm
by Robert Biegler
Following a query during November's meeting, I will report here on my (very slow) progress on my main project, "Insert Hull Here", a frame with hinged Bruce foil. It is intended to contain all the forces generated by the rig and the underwater appendages within the frame, so that the hull only has to provide buoyancy. In theory, it should be possible to attach any hull that has not too much drag in the intended speed range, and has gunwales that are strong enough.

On the lee tack (the foil is to lee), the Bruce foil pushes up. On the weather tack, it acts like the hinged foil first proposed by Hagedoorn, and also advocated by Giles Whittaker. That is, when the foil goes over a wave on the weather tack, a rigidly connected Bruce foiler to has to overcome the roll inertia of the whole boat in order to stay in the water. The hinged foil only has to overcome the roll inertia of itself and the beam up to the hinge, and the rest of the boat can follow later. That generally gives away some righting moment, which has to be made up some other way, for the sake of the foil staying hooked in.

Note that when the boat is on the lee tack, what matters is how high the intersection between weight and foil force is relative to sail force. In the figure below, that distance is 0, meaning the boat does not heel on the lee tack.

When the boat is on the weather tack, what matters is how high the intersection between hull buoyancy and foil force is relative to the sail. Here, there is some lever arm left, though only about 40% of what it would be when using only conventional underwater appendages, But because the foil does not fully balance sail force, some righting moment has to be generated the conventional way. ... ed-public/

(I tried to post this as an image, but all that showed was the word "Image".)

The forward beam of the frame is laminated. It has some time-consuming fiddly bits, like the mast partner, and the hinge that has to fit the rear beam fairly precisely: ... ed-public/

Rear beam is partly lashed up with basalt fibre, but not yet epoxied: ... ed-public/

I have barely started on the foil. I have made some ribs out of 18mm plywood. The plan is to lay up biaxial carbon, and some unidirectional, on the inside of 2mm plywood, epoxy, screw ribs on the ends of the foil, and a nose piece. Insert a foam and biaxial carbon shear web, insert foam ribs, let the epoxy set enough that it sticks to the plywood. Then epoxy the remaining carbon to the second sheet of ply, turn over, and attach the second sheet to the rest. The ply is intended to serve only as a mold that I don't bother to remove. That is why I plan to use very thin ply.

I currently plan to use a 3 metre ladder for the beam from hinge to foil. If anyone has reason to believe that is a bad idea, please tell me. And building the hinge will involve some time-consuming fiddly bits, and I have not quite worked out how they should look. If the hinge were parallel to the longitudinal axis, and horizontal, I could just extend the ladder a bit. But I want the hinge axis to toe in a bit and be lower forward. That makes the foil toe out and pitch down when the boat heels on the weather tack. Giles Whittaker explained in Catalyst. But that means when I extend the foot of the ladder with the bits that form the hinge, those bits have to bend so that they are perpendicular to the hinge axis.