Grady White Outboard to inboard conversion

Discussion in 'Powerboats' started by rjqn, Sep 17, 2008.

  1. rjqn
    Joined: Sep 2008
    Posts: 1
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    Location: NC

    rjqn New Member

    I have a 1989 Grady white Sailfish 252G that I am considering converting from twin outboards to a single SBC Inboard. I believe I have most of the major parts I would need to complete the conversion. I have an SBC engine, A direct drive BW gear, a 18/19 prop, and various other assorted parts.
    My plan is to mount the engine as far aft as possible and have the prop shaft exit the hull at the extreme bottom of the transom in the area of the drain plug hole. The center of this hole is about 1.5" above the bottom of the hull. If the prop shaft extends about 12" past the transom between 1/2 and 2/3 of the prop should be below the bottom of the hull.
    There is an extension on the transom on which the outboards mount which is about 27" long and the bottom is about 6" below the waterline. I plan to use this to mount the prop shaft strut and rudder.
    I have been a mechanic and fabricator my entire 47 years and as for whether or not physically it can be done I have no doubt it can.
    I have however a few questions in my mind about the project.

    1) Has anyone out there accomplished this or a similar project.
    2) With this amount of the prop below the hull would backing be a problem.
    3) Would a SBC (300 HP) with a direct drive gear be enough power to turn a
    18/19 prop and push a 6000 or 7000 lb boat

    I would appreciate any input and if anyone is interested I would post pics tracking the projects progress. As I have access to any equipment and machinery I would need to complete the project, I figure it should take 1 to 2 months to complete
  2. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 148, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    To answer your questions:

    1. Read my post in the surface drive section.
    2. Backing is always a problem with just 1 prop and a rudder. You will experience quite a difference compared to an outboard or stern drive, but you'll get used to it.
    3. That is enough power.

    With that prop shaft angle there is bound to be a large vertical force vector, so you will need trim flaps.
    Have you considered the fact that the prop is near the surface and will draw in a lot of air?
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