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Custom Outboard: Gearing Prop RPM HELP

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  • Custom Outboard: Gearing Prop RPM HELP

    Good morning All:

    I am currently new to the whole hydroplane scene. I inherited a project from my father building a 10.5' 3-point hydroplane. When he was 16 (1976) he copied his best friends hydroplane with some wood scraps and fiberglass. I plan to finish this unit and make a mold from it so that I can reproduce 1 or 2 of these.

    Originally he was saying that they powered these with a 15HP outboard. My plan is to take a reed case injected 600cc twin snowmobile engine(99hp stock) and adapt it to a yamato lower unit that I picked up on ebay. Below are some specs:

    600cc twin
    reed case injected
    carbureted: twin 38mm diaphragm
    max RPM (in snowmobile) 8200 with single pipe 99 hp

    My questions are as follows:

    How much HP will the yamato drive take?
    What gearing should I run?
    Where can I find gears for this unit?
    What size/pitch prop should I start with?
    Do I need an anti cavitation plate?
    What height should the center line of the prop be from the bottom of the hull?

    Any help is greatly appreciated.


  • #2

    Re: Custom Outboard: Gearing Prop RPM HELP

    Lots of speculation at this early level.
    What are the hull design specifics (round nose, pickle fork) and dimensions of the boat: rear bottom planning surface width, afterplane, sponson depth, balance, weight of total package under way, etc.?
    Gearing: depends on how you will use it, lake runner, kilo runs, -----?
    Height of prop: depends on the use intended, usually higher for lower drag on gear case but that has limits.
    Prop size: pitch, diameter, # blades, blade shape, rake, etc: depends on above
    What Yamato model lower gear case: 102, 302?
    For Yamato lower what are you plans to cool the engine: prop blast feed or force feed from another source like a transom mounted pitot?
    Will Yamato gear case handle 99 hp?
    bradleyrichardms and bill hoctor like this.
    "Keep Move'n" life is catching up!
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    • #3

      Re: Custom Outboard: Gearing Prop RPM HELP

      Brad: You have an interesting project. Would suggest you be very careful because a 10.5 foot hydro is the size we run with 25 to 30 hp in the MOD category. 99hp would be equivalent to our 750 and 850 classes which run 12 to 13 foot hydros. Your propeller will be dependent on which Yamato gear ratio you have. You will probably want something in the 13 or 14 pitch range similar to the 850 MOD propellers. Be very careful.

      john sharp
      bradleyrichardms and bill hoctor like this.


      • #4

        Re: Custom Outboard: Gearing Prop RPM HELP

        Interesting project. My thoughts are that 99HP is too much for the Yamato gear case. A 102 or 302 gear ratio is 14:15... 1 tooth differential. A Y80 gear ratio is 14:16... or 2 teeth differential. Basically 14 dividend by 15 = 93% or 7% gear reduction from 1:1. And 14 divided by 16 = 87.5 % or a 12.5% gear reduction from 1:1.

        If you want to use a racing gear case you will need a Rossi, VRP or Konny unit that will handle 100 HP. Or you might be able to find an older Yamato Racing gear case that was used on one of their Pro Alky motors of the past. Or of course you could use a gear case from a 100HP Service Outboard.

        Maybe find a Yamato powerhead to go on that lower unit.

        A 10.5 Hydro is not big enough for 99HP..... more like a 13.

        You might want to contact Boat Racers in your area to help you and give some advice.

        Good luck on your project.
        bradleyrichardms and bill hoctor like this.

        Dean F. Hobart


        • #5

          Re: Custom Outboard: Gearing Prop RPM HELP

          It's not just the horsepower that will stress the lower unit but the fact that the power is produced by only two cylinders, by our standards two BIG cylinders, hitting those little bevel gears and that little sheer pin WHAM, WHAM!! A big parallel twin (again, 600cc is a big twin for us) shakes pretty hard as well. Great for sleds may not be so good for outboards.

          And as others are suggesting, 99hp is at least twice what any 10 1/2' hydro would ordinarily take, AND this is specifying a boat used for closed course racing on smooth protected water, and emphatically NOT a playboat for a non-racer who is going to be out on unprotected water with other pleasure craft leaving their wakes and rollers. Hydros will take some chop, but not cruiser wakes and rollers. And non-racers using hydros as playboats tend to not wear helmets, tend to have light-duty and inadequate life jackets, and tend not to have "deadman" throttles or ignition cut-offs, all of which they surely should have when running over wakes and rollers. Hydros are wonderful raceboats, but not very good playboats unless you are sure of having the lake to yourself.

          This is NOT to discourage you from carrying on with your dad's project (and we'd all be interested to see a photo), but you say you're new to this stuff, so you might consider what everybody has said.
          Last edited by Smitty; 01-25-2018, 01:45 PM.
          bradleyrichardms likes this.


          • #6

            Re: Custom Outboard: Gearing Prop RPM HELP

            I am new to all the racing stuff also, but here is my 2 cents.
            Finish the boat, get a Yamato 80 they are very cost effective.
            Throw it on and hold on, about 60 mph on a little boat is a great adrenaline Rush.
            Get safety gear, a lot of great people on this forum sell used equipment all the time.
            When you finally get on the water and squeeze that throttle, if you don't believe in God, you will when you get back to shore safely.
            If you put a turning fin on
            Or you fly out of the boat like a cartoon character.


            • #7

              Re: Custom Outboard: Gearing Prop RPM HELP

              I agree with everyone else in that boat is way too small for that motor. For example, my daughter runs a 175cc motor (ten cubic inch) on a twelve foot hydro. My suggestion would be to finish building the boat and look for a proper size motor for it; you could still use the gearcase. Then send the snowmobile engine to Craigslist.
              bradleyrichardms likes this.


              • hshawwpba
                hshawwpba commented
                Editing a comment
                I can tell you the Yamato unit will not hold up, we tried them under 44 and 6 mercs both racing units and 302 gears and the failure rate was 100% in about 3 laps! The issue as noted above is the torque of the engine on the gears way to much even on non 1-1 ratios.
                I would suggest a bass gear case and tower it will hold up under your engine without failure as long as you keep gear oil filled with good oil.
                As for the engine you will find that an engine designed to run on a snow mobile is ported wrong for an outboard engine. There is no slip in water and demand on the engine is high vs snow mobile application. Good luck on the project, will be fun to see finish project and as said above on a 10.5 boat hang on if you don't sink sitting in the water or go over backwards on take off the first left turn at speed needs to be filmed for "funniest home video show!!