Comment by javiramos

6 days ago

The coolest recent development in marine propellers is toroidal propellers which are now commercially available and seem to perform significantly better than standard propellers: https://www.sharrowmarine.com/

19 comments

javiramos

Reply
brk  6 days ago

For the most part the Sharrow props have not proven to be much of an improvement, particularly for the high price.

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.

  • jcgrillo  5 days ago

    Do you have any more information/sources to share on this? I have an Eastern 18 powered with a Yamaha 60hp 4 stroke and I've been struggling to dial the prop right. I don't know off the top of my head what the specs of my current prop are but basically I feel like I'm not taking advantage of the engine's torque at less than WOT, so I basically just run it flat out. If I could extract just a little more thrust out of the prop at lower RPMs it feels like the engine would have enough grunt to make the boat plane in the mid-high 4000s instead of 5000-5200rpm where I currently run it. Ideally, given the bsfc/hp curves, I'd like to run the engine at a bit lower RPM, but the way it's currently set up at ~4600rpm it's not fully up on the step. I was (perhaps wishfully) thinking a little more efficient prop design might help.

    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.

    • brk  4 days ago

      Sources are primarily boating forums, dockside conversations, etc.

      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.

      1 reply →

  • trillic  4 days ago

    Most boats operate 95%+ of their engine hours in a very narrow RPM range.

    • brk  4 days ago

      Yes and that range is at higher RPMs where the Sharrow props have reduced benefit.

LoganDark  6 days ago

That website seems to no useful information; only marketing speak about how great it is... Do you know of a good source on how toroidal propellers work and the engineering behind them?

  • stackghost  6 days ago

    Propellers (both marine and aero) are just spinning wings. If you picture a 2D airfoil like you might see in one of those "intro to lift" diagrams, all the flow is in what we call the chordwise direction, that is the flow is entirely along the axis of the wing's chord (leading edge to trailing edge).

    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).

    [0] https://www.youtube.com/watch?v=duSZ1hyK7sY