Correct prop for boat

Discussion in 'Props' started by Greifnation, Mar 31, 2024.

  1. Greifnation
    Joined: Nov 2023
    Posts: 4
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    Location: Australia

    Greifnation New Member

    Hello lads,

    we have just built ourselves a new boat. We have already tested a propeller for the new motor and we’re extremelyhappy with how it handled and the speed it got.

    however it only revved out to 3650 RPM WOT. We have built an identical vessel to our 10 year old boat except for putting a bigger motor in and some extra weight (250KG)

    our old boat had a 315BHP motor that also only revved out to 3650RPM and achieved 31 knots. The new motor is 370HP and with the prop that revved out to 3650RPM it was doing 33 knots (not ideal but will do)

    the problem is our engine dealer. He has stated the engine must rev out to 3815 with 95% load. We have taken the initiative to try and do this, so we got the prop re pitched to achieve this but we lost 4 knots of speed and low ended performance was terrible.

    our old boat doesn’t have a modern electronic motor and has been an extremely reliably workhorse with over 6500 hours on it. That motor is now starting to show its age and will need some extensive work to get it back to tip top shape before we semi retire it.

    With the new motor, we aren’t fully understanding of the was all the electronics role in running that motor. Ultimately the boat performed best for our use case with the “overpropped” propeller but for “warranty” we aren’t suppose to be doing that.

    Will this cause any major issues with how the electronics manage the motor or is it best to have the vessel perform worse than expected to keep it all happy?
  2. seasquirt
    Joined: Dec 2015
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    Location: South Australia

    seasquirt Senior Member

    Hi Greifnation, might be safest to run it as standard until the warranty period is over, meanwhile gathering more info on what to do. The electronics would already be optimised, and changing anything there could cause or exacerbate problems. Adding weight wouldn't have helped either. 250 Kg is a quarter of a ton. See if you can trim some of that. What exact engine are you running, someone here may know a thing or two about it.
  3. Greifnation
    Joined: Nov 2023
    Posts: 4
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    Location: Australia

    Greifnation New Member

    The new boat has a yanmar 8lv 370.

    We have ran our 315 6lpa over propped its whole life due to us liking better performance with lower RPM.

    The new boat with over propped propeller was perfect for our needs. Yanmar supplier told us he needs it to run at 3815WOT with 95% load for warranty otherwise yanmar will be hesitant to accept any warranty claims.
  4. C. Dog
    Joined: May 2022
    Posts: 183
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    Location: Coffs Harbour NSW Australia

    C. Dog Senior Member

    I don't know if this applies to Yanmar, but some manufacturers have sought to gain a perceived advantage over rivals by wringing the maximum from smaller displacement engines by the use of turbo chargers and super chargers, sometimes adding both to the one engine. This makes engines more liable to failure.
    DogCavalry likes this.
  5. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    What is the data plate maximum on the engine rpm? Usually companies shoot for a percentage over max rated rpm. Often times it makes them under prop, helps safe guard against the inevitable weight creep. Kinda wondering if they might be going a bit overboard on the saftey factor.

    If you're weight is accurate and your prop numbers are good, only other option is the data plate horse power numbers are off. There is a fellow up here who chased his tale trying to figure out a performance issue, eventually he had a dyno done on the main... and it didn't make rated power. Not entirely sure what or why as it degenerated to a lawsuit. Suffice to say it's out of the real of possibility that the engine is under performing.
  6. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    The owners manual says this engine is designed according to a specific leisure boat standard and to operate at full throttle between 3600 and 3800 rpms WHEN FULLY RUN-IN, within the limits of this standard. I recommend reading this manual.

    According to the performance documentation, it will deliver 370 hp over the rpm range mentioned. However, it is recommended that during the 50 hr run-in period the engine is "lightly" propped, ie with a prop requiring 352 hp @ 3800 rpms, and with the vessel in "leaving condition", in order to reduce the risk for thermal overloading during this period. The supplyers' prop recommendation is not quite correct mathematically, but fair enough.

    The reason for this is that if the engine is installed in a short and heavy vessel ("low slenderness ratio"), the boat's hump resistance will force the engine to work very hard close to the mid-range torque limit in order to get over the hump. That puts high thermal loads on the engine parts, with possible deformations. If this happens before the various parts have been "rubbed in" to each other, there is a risk for damage. You'd be surprised by the amount of twisting that can occur in an engine block during varying loads and temperatures!

    Now, that said, when the run-in period is through, you should be able to use a propeller that gives full-thottle rpms in the 3600+ region. Remember that the engine is often the one and only component in your boat that has passed an individual performance test before delivery!! If you don't get the expected performance from your new build, there are other suspects that you should check first.
    Last edited: Mar 31, 2024
    philSweet likes this.

  7. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Please tell us the specs on the prop on the old boat (and tranny ratio), and the prop that was repitched (and the new boat's tranny ratio). And a photo of the two boats would be helpful. I'm wondering about the cup on the new prop and how the prop shop went about computing the pitch adjustment. What was the top end rpm with the repitched prop?
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