The maneuvers are so extreme and come so fast that I would not have been able to say for certain that this wasn't just a very nasty crash in progress. But they were, in fact, completely controlled and intentional.
Incredible.
See https://en.wikipedia.org/wiki/On_Being_the_Right_Size (PDF at https://www.phys.ufl.edu/courses/phy3221/spring10/HaldaneRig...)
The tests that have shown "significant" improvements have frequently compared the Sharrow to a sub-optimal prop. Feedback from many actual users is that the gains are moderate over a narrow RPM range.
The other thing I was thinking of trying is swapping in a different "high torque" lower unit with a lower gear ratio and running a significantly larger prop.
In theory your boat in right in the sweet spot of recommended power range at 60HP. I don't know all the background on it, so all kinds of potential problems, but I would wager that "propped wrong" is unlikely to be the core culprit.
I'd start by getting it weighed and comparing your loaded weight to manufacturer specs. USCG requires positive buoyancy for hulls under 20'. This is typically achieved with using expanding foam in hull cavities, and that foam can have a tendency to absorb and hold water if the boat develops any failure of the seals around the bilge areas that are foamed. Reports of poor performance are very common for these sub 20' hulls because of waterlogging. If not a waterlogged hull, you might also just have too much stuff on-board.
To a lesser degree, a bimini can also have an adverse affect on speed/planing, if it's acting like a parachute. Not sure if you have a bimini, but if so it's worth trying a run with it up vs. down.
I'd also look at how your outboard is mounted. It's not clear if it the outboard from the factory, or if the boat has been repowered. Outboards being too high, too low, etc. are pretty common issues that can also majorly impact performance.
That's a few thoughts that comes to mind off-hand.
The boat was re-powered under my ownership. I'm pretty confident the motor height is correct based on a variety of observations and measurements, so I don't think there's really anything to adjust there.
I wouldn't say I have a complaint with the boat's performance, more like trying to get the engine to run at cruise in a more efficient range of the bsfc/hp map, which may be a tall order at 60hp. To your point, though, if I can shed a couple hundred pounds in the refit that could very well do it.
> In the centuries after Archimedes invented the Archimedes' screw, developments of propeller design led to the torus marine propeller... it was invented in the early 1890s
"the centuries" indeed. :)
A real 3D aircraft, however, has a fuselage. Similarly, a prop has a hub and the tips of each blade are spinning faster than the roots. The tl;dr of this is that real 3D lifting surfaces typically exhibit a mixture of chordwise and spanwise flow, which causes wingtip vortices to form[0], resulting in induced drag/induced power loss.
For a given amount of thrust the total amount of momentum that the prop transfers to the fluid is fixed. The tip of a conventional prop ends abruptly which causes a large pressure gradient and a strong vortex. A toroidal prop's shape causes the pressure gradient to be broader and less concentrated, therefore the wake vorticity is distributed over a larger region, reducing peak swirl velocities and lowering the kinetic energy lost to vortex formation (and to cavitation).
This page seems to have one halfway-functional photo for me: https://www.sharrowmarine.com/store/p/sharrow-by-veem-ds9sw
This URL may work: https://images.squarespace-cdn.com/content/v1/560055b1e4b017...
If I remember right what they didn't do was go exactly straight. You could see a (very modest) s-shape in the wake over distance.
ref: https://www.shipsofcalmac.co.uk/fleet-features/the-streakers
Most traditional tugs have a pair of screws for just this reason. Not so much to turn but by applying differential thrust they can pull sideways. A vector drive like this will vastly increase the envelope of possible pull conditions.
