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Thread: Newbie from Aussie land!

  1. #1
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    Jun 2019
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    Default Newbie from Aussie land!

    Hi Guys,
    I'm new to here, and finally joined up so I could write some posts to get even more info than I have already found in others posts.
    I don't have a top dollar, super high speed racing boat, and a budget to suit!
    However I do have a nice solid 14' wide body aluminium fishing boat with a 3mm plate bottom and a hull designed for stability at high speeds (very deep nose).
    I have recently got the bug to get as much speed as I can out of the setup as I use the boat for fishing at night time on my local salt water lake, and have many favorite spots from one end to the other....
    However its not a small lake!!
    Lake Macquarie in NSW Australia is the largest tidal salt water body in the southern hemisphere.

    I have trialed a few different motor/prop combinations on the boat now and feel that I'm on the right path finally with a 1998 Evinrude 40hp 2 stroke (OMC 50 cui 2 cylinder).
    I have changed the carbies to a pair removed from a johnson 75hp that are big bore and butterfly, with adjustable mixture screws and have adjusted the timing to 23 degrees at WOT while running 98 octane fuel.
    I have also fitter sports fibreglass reed valves from USA, and have ported the reed cages and manifold for smoother unobstructed flow.

    But there are a couple of things I cant find enough info about, that I feel can still help get the most out of the engine.
    *Spark plugs and gaps?? I have found so many different settings online (.030-.045'') that I would prefer to just ask someone with experience, rather than trial and measure the speed.

    *And compression levels? I am currently getting 160psi cranking pressure in both cylinders.
    Will I benefit much from increasing to around 180psi? I only ever use 98 octane fuel and it has electric start.

    I have 2 different props to choose from, a 15'' and a 17''.
    The 15''is almost enough to flip the boat, and even with a full load and 2 passengers tops out at the same speed as it does with just me (55km/h)
    But when the 17'' is fitted, the top speed is over 65km/h when empty, but with my usual passenger the top speed is limited to 56km/h.

    My aim is to improve torque and power enough to be able to use the 17'' prop, and increase the loaded speed to as close to the empty speed as possible.
    A 16'' prop would be ideal, but there is no such size!

    Any thoughts or suggestions would be most appreciated.
    Cheers Ken

  2. #2
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    Sep 2008
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    is the hull smooth underneath in the middle or does it have the join seam protruding?
    Thats an issue when raising the engine height

  3. #3
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    Jun 2019
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    Quote Originally Posted by Powerabout View Post
    is the hull smooth underneath in the middle or does it have the join seam protruding?
    Thats an issue when raising the engine height
    The hull has a small keel type join down the center, about 3/4'' raised from the boat.
    I don't think I could raise the motor any more as it begins to cavitate and "spin" the prop under power once I begin to trim the motor up.
    It has elec tilt, so I adjust the trim to achieve max speed while driving, but there is only 2-3 km/h to be gained before it cavatates

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