Inboard Conversion to Outboard

Discussion in 'Boat Design' started by Nordski, Aug 13, 2020.

  1. fallguy
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    fallguy Senior Member

    I'd get a small manual jackplate to be safe. Last thing you want is to need a minor mounting height adjustment on an unproven design.
     
  2. Nordski
    Joined: Feb 2019
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    Nordski Junior Member

    Yes, I agree........I will make provisions to add ballast at and forward of the engine location and adjust as needed for correct trim and make sure she sits on the water line.
     
  3. possum
    Joined: Aug 2020
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    possum Junior Member

    Looks like a lot of rocker on that design, looks like it would move better as a displacement vessel than planing. Maybe build a well for the outboard rather than hang off the transom if you really like and want to keep the lines? May need less ballast in the bow that way.
     
  4. fallguy
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    fallguy Senior Member

    I am sorry, but adding ballast to modify a design is a ridiculous proposition.

    You are essentially suggesting poorer fuel economy is preferred.
     
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  5. Nordski
    Joined: Feb 2019
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    Nordski Junior Member

    I'm not suggesting to modify the design............I suppose a builder could find a 1940's 35hp inboard engine to use in this boat......but why would you? A modern inboard will be lighter and more powerful and balast would be needed to bring the boat to the water line anyway. Here's an 18' boat of similar design that requires a 5hp outboard for power......What I'm proposing doesn't seem unreasonable. Screen Shot 2020-08-14 at 8.47.53 AM.png
     
  6. SamSam
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    SamSam Senior Member

    I'm guessing Fallguy meant the boat was designed to have the weight of an inboard centered in the hull. Eliminating that weight and putting the weight of an outboard leveraged off the transom is a design modification requiring the addition of ballast for trim, the extra ballast weight affecting fuel economy.

    The lack of flotation/displacement in the stern plus the weight of the motor leveraged off the stern, the stern will will sit lower, the center will sit higher and the bow will sit higher yet. It pivot at the center sort of like a seesaw. If you use a tiller steered outboard, that will make it worse as you have your body weight back there also. I imagine there are remote controls for the shifting, steering and throttle available for small outboards, but possibly not.

    As long as you plan to run at displacement speeds and are not particularly concerned about fuel economy, a flat bottomed boat with no rocker, run at displacement speed would seem to accomplish the same thing, plus give you more displacement at the stern. It would also give a flat inside floor and simpler construction.

    It wouldn't be the same boat, and maybe that's important to you. What do you plan to do with the boat? What are the general dimensions of the boat?
     
    Last edited: Aug 14, 2020
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  7. Nordski
    Joined: Feb 2019
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    Nordski Junior Member

    Thank you for the reply.........I'm no expert but I have built 2 dozen wood boats over the last 40 years. Whatever design I'm working with the designer has a water line that the boat should sit on, it doesn't really matter if the weight comes from the construction of the hull or a 600lb engine or 600lbs of lead, the boat will sit on its lines. The intended use for this boat is to travel my local waterways and lakes that are limited by speed. I don't need or want a planing hull so I have not looked for that style hull design. The "Barnegat" designed by Atkin looks good for my use and just trying to figure out if I can use the engine I have. Other designs that I have posted here are designed for an outboard of similar size and weight and also have a similar hull profile. Just reaching out to this group for input..........I received the same negative input from the last boat I designed and built. It's not a perfect boat but it works as intended. Maybe this group doesn't like ideas outside the box........not sure..........I appreciate the input from all and I will proceed with trusting my own experience with this build............take care.
     
  8. ondarvr
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    ondarvr Senior Member

    With that amount of rocker you would be much better off with an XL shaft OB, in the high thrust version.

    We mount smaller OBs on this size hull frequently, but it's on planing hulls. The XL shaft helps keep the OB's power head out of water and the prop in it.

    This hull design will expose the OB to potentially be underwater, or the prop out of the water, to a far greater extent.
     
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  9. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Aaron-the idea you want a displacement boat that uses a 20 hp outboard is nothing provocative. What is provocative is putting 150 pounds on the stern and 600 pounds of ballast midship for trim.

    A statement of requirements typically leads.

    The keel serves a purpose to protect the inboard prop. The hull shape is designed to deal with the midship weight.

    Find a displacement outboard design.
     
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