Inboard to Outboard

Discussion in 'Boat Design' started by TwoShy, Aug 29, 2013.

  1. TwoShy
    Joined: Aug 2013
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    Location: Oregon

    TwoShy New Member

    Newbie question, just acquired a project boat around 24' (71 Newman) that had a inboard engine (v-8) with OMC stern-drive. My plans are to replace both with a outboard. What size motor should I be looking at for trolling and getting out of harms way (Brookings, OR). I have a full shop and skills to fabricate most anything and see light at end of tunnel with swap-out, thanks.
  2. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    Is it a planing hull? I don't know what a "71 Newman" is or looks like. The big question is how much will the finished boat weigh? Is it a 5000 pound boat(complete), 8000, or a 12000 pound boat? Is it 8' wide or 10', deep or shallow sided, heavy or lightly built? How much trolling gear are you adding and how fast do you need to go when fully loaded?
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Welcome to the forum.

    I vaguely remember a very heavy, chopper gun built, cathedral hull (or tri hull) boat of that name. I don't remember them being that long, but I don't know the extent of their line.

    If it's what I think, then yes, you can make this conversion, though a considerably amount of effort will be necessary, reinforcing things and moving weights around.

    I'll assume you had about 200 HP in your midship V8, so a similar output outboard is in order. The transom and related structure will need reinforcement, tank(s) and batteries moved forward, maybe a bracket to truly ease cockpit intrusion is an option. Can you post some pictures, so we can see what you're working with?
  4. Toooshy
    Joined: Aug 2013
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    Location: Oregon

    Toooshy New Member

    Thanks for quick response, eye balling with tape-measure 7' wide where hull leave water line, length measure 22' not counting sterndrive, title call out 1971 Newman 24', the boat has deep sides. Weight, thinking 4000lb tops when I get done with it. Using google images I google Tri-hull and Cathedral don't think so, Newman has deep v at bow with lifting/spray strakes that travel lenth of hull, vee shape flatten toward the back. Looks to be planing hull for sure, big time flair where hull meets deck at bow that disappear toward stern. Wife said it looks like a Miami Vice speed boat LOL. Search found Jarvis Newman from Maine building sail boats (35')around 1960-70 and outboard power boat (14'-20'). I'll be hanging tight with this Forum for sure during this build, thanks
  5. Easy Rider
    Joined: Oct 2009
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    Location: NW Washington State USA

    Easy Rider Senior Member

    As to speed requirements what do you mean "trolling and getting out of harms way"?

    If you're looking to go 6 knots you probably have the wrong boat.
  6. Toooshy
    Joined: Aug 2013
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    Location: Oregon

    Toooshy New Member

    Trolling 1-2 knots (Salmon) as for Harms way fishing out of Brookings Harbor Oregon and like most oregon coastal harbors things can get really ugly fast for small craft, need for speed to reach harbor for safely. I see that most of the boaters here run a small outboard kicker along side there main power plant which I already have (Honda 7hp 4-stroke). Been reading past post on inboard/outboard swap and thinking 150- 200hp for my main drive.
  7. Syncrowave
    Joined: Aug 2013
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    Location: Virginia

    Syncrowave Junior Member

    Why not just replace the existing powerplant rather than reinventing the wheel? Inboards and I/Os have many advantages over outboards (lower price and easy/cheap repairability being just two), and switching over to O/B motor will also involve other issues like needing to beef up the transom, the change in trim caused by moving weights aft in the boat, having a bunch of junk on the transom that didn't used to be there, etc.

    Not trying to be difficult here, but sometimes it's easier just to keep things the way they were...and I'd be willing to bet you could do it for a fraction of the price of a new OB engine. You can replace a 300 hp inboard for maybe $4000 if you do a lot of it yourself. Buy a new 300 hp outboard and it's going to be closer to $30,000...

  8. Toooshy
    Joined: Aug 2013
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    Location: Oregon

    Toooshy New Member

    I'm pretty new to power boats but my research show this swap-out became popular since the eighties, sheds around 300 lb plus way better fuel consumption. This project boat transom needs replacing, wood having rotted plus stringers that support engine mount, glass hull in great shape. Manufacture bolt on bracket for outboard looks pretty clean and bolts up using stern drive hole. I'll be making my own bracket with added support from floor stringers. Trim don't seem to be a problem from what I have read, I see inboard drivers hanging 100 plus quart cooler with a outboard kicker on the stern for trolling they do just fine. Looking at 200 hp 4 stroke Honda fuel-injection less than 6 thousand. This project boat came with no engine or fuel tanks, fuel tanks which can be move forward when replacing if needed for trim. I have little money in this boat&trailer about the price of a nice fishing rod. "Sit" my pants! when pulling it home when the surge brakes kick in when braking pretty smooth too! plus four new tires. This project boat will sit untill I finish up my 36" band-mill for ripping logs siting in my shop. This going to be a fun project and will be readly for next Salmon season.
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