Prop Dia vs Torque vs....5-6 ton sailing cat

Discussion in 'Props' started by mark424x, Jul 25, 2016.

  1. mark424x
    Joined: Feb 2006
    Posts: 33
    Likes: 2, Points: 8, Legacy Rep: 28
    Location: Seattle, WA

    mark424x Junior Member

    I need help making a tradeoff between larger diameter w/ less torque (12” prop on Yamaha T9.9), higher rev, higher torque, (10” diameter on a Suzuki DF20a), or the monster Yamaha T25, 12” prop, lots more torque, medium revs, a lot more weight.

    Boat, Maine Cat 38, sailing cat, lightship 8200lbs, design water line 12,400lbs. Twin outboards just inside the hulls in bridgedeck nacelle. water line 36.5’, length to hull beam ~11.4

    In a fantasy world I'd like 7knots at 75% throttle on one engine, 10-11knots both engines WOT. I'd like to go with the Suzuki as it's EFI and a number of friends just love the EFI and the efficiency the computer dials in for long range. The local dealer who handles Honda, Suzuki, and Yamaha said the Suzuki EFI has been the most reliable engine in this range by far.

    Yamaha T9.9
    • RPM 5500
    • 2.92 gear
    • 114 lb weight
    • Prop RPM WOT 1884
    • Torque @ max RPM 27.6 lb ft
    • 12" max prop diameter
    Suzuki DF20a (EFI)
    • RPM 5800
    • 2.08 gear
    • 108 lb weight
    • Prop RPM WOT 2788
    • Torque @ max RPM 37.8 lb ft
    • 10" max prop diameter
    Yamaha T25
    • RPM 5500
    • 2.42 gear
    • 201 lb weight
    • Prop RPM WOT 2273
    • Torque @ max RPM 57.8 lb ft
    • 12" max prop diameter
    Not sure how to compensate for slender hull. If I put one hull into half the weight, one engine, it seems to say that the suzuki should work, it's recommending 9-10" diameter x 6-7" pitch.

    A number of articles talk about slip in the 50% range being normal for displacement hulls, so don't think I'll create a milkshake.

    On the other hand, some calcs seem to show that thrust is a function of diameter^3, so every extra inch of diameter should offer huge benefits. I wonder if the T9.9 would get close the 20hp in real world due to the low speed big prop. Don't really want to weigh the thing down with the T25.

    Lots out there that talks about adjusting pitch given a fixed diameter prop. But haven't been able to find the math that helps be compare these scenarios.

    Any guidance on the analysis much appreciated.


    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Calculations are always based on speculation , rules of thumb and similar boats.

    Why not launch the boat , find a pal with a sport fish and tow rope and measure the forces required with the boat at different speeds and weights?

    Guesstimates are fun , reality is better.
  3. Barry
    Joined: Mar 2002
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    Barry Senior Member
    This is the maincat website
    They use 15hp, high thrust engines that produce an 8 mph motor cruise
    Why not just email them to find out what performance they would expect with bigger engines?
    One phone call, email and you get advice from people who build the boat

  4. mark424x
    Joined: Feb 2006
    Posts: 33
    Likes: 2, Points: 8, Legacy Rep: 28
    Location: Seattle, WA

    mark424x Junior Member

    The first boat w/ twin 15s hasn't been launched yet (#1 had a single 50 w/ very low slung center nacelle). Will certainly test when it hits the water next month. Just trying to understand the theory of prop diameter vs hp/torque and how that effects thrust w/ various slip ratios and pitches. Thought there would be some formulas to help sort through that.

    The choice of Honda 15s is largely driven by the availability of the XL leg off the shelf. I'd prefer the suzuki due to the EFI, the local suzuki mechanic say you can swap the air restrictor plate and computer and get a suzuki 20hp w/ XL leg for <$400 in parts/engine for the upgrade, all other parts are identical. Supports the same 10" prop diameter as the honda, but not the 12" diameter of the yamaha 9.9 or 25.
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