I have been thinking about Alan Craig's oscillating fin drive, and what I can infer from Ken Kingsbury's Foil Rower:
As far as I can tell from 0:55 - 0:58, those foils are rigid. There seems to be no attempt to make them twist, like the fins of the Hobie Mirage drive. I think the difference is that the axis around which the Mirage drive fins turn is in the water, and so to get a reasonable angle of attack, the fins must twist. The Foil Rower's axis seems to be about twice as far from the water as the draft of the foils. Therefore even a rigid foil will experience much less variation in the angle of attack from where it breaks the surface to its tip. There may even be some drag reduction in that the smaller angle of attack and lower wing loading near the surface creates less surface waves.
So might it help to do the following:
1) Move the horizontal rotation axes as far up as practical, given structural constraints
2) Use rigid foils with a good profile, perhaps commercially available rudder blades. It is difficult to judge the size of the foils on the Foil Rower. They seem perhaps 50% longer in span than Alan's fin, and have quite small area. I don't know how important that high aspect ratio is. On the one hand, it suggests high wing loading, but on the other hand, the short chord should have a smaller dead zone while reversing each stroke. Model helicopter blades are available in lengths up to 70 cm, but would probably need reinforcement if required to operate in water at high load. I read that the tail rotor blades of full size helicopters get swapped out at regular intervals, but have not been able to find out where to get hold of the discards. Those should be strong enough.
3) Limit the angle of attack of each foil by a string attached to the end of a tiller above and as close as possible to the horizontal rotation axes. Attach the other end of the string not to a fixed point, but to another set of tillers connected by a parallel linkage. Steering that second set of tillers steers the boat by changing the midpoint of the foils' flapping range. Then no separate rudder is needed, which gets rid of some drag.
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