Inboard to OB conversion questions

Discussion in 'Boat Design' started by zrr, Apr 8, 2021.

  1. zrr
    Joined: Apr 2021
    Posts: 3
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    Location: North Carolina

    zrr New Member

    What I am considering may be blasphemy to some, but usability is more important to me than keeping her as she is.
    The boat is a 1953 cruiser built by my grandfather. She's all mahogany wood and in mostly amazing shape, except for the engine and transmission which I have been fighting for over a year now to work consistently. Recently, the shifting post on the transmission snapped so I'm done with the whole unit. My original thought was to just stick a brand new inboard/trans on but due to the age I can't find one with even close power/size/weight numbers. So my new thought is to do away with the inboard entirely and mount a bracket with an ob on. I realize that this will somewhat kill the look of the old classic, but as I said I want something that's reliably useable. However, because of the way the boat is laid out I'm not 100% sure its doable without massively altering the original framework. Or even if its possible on a boat so old. Any thoughts, suggestions, or comments would be much appreciated!

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  2. rocco611
    Joined: Jan 2019
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    Location: Madera Ca

    rocco611 Junior Member

    I would think a modern inboard would likely be more power , smaller and lighter than what you had. overall I wouldn't think that to be a problem. yes you would likely have to make new mounts, adapt the exhaust, controls and drive shaft. By looks of the transom it would take a lot of work to make the transom and stringers strong enough to support an outboard or a pod. I would suggest if you want to go with an outboard send pics to company that builds outboard pods, they know about conversions and could give a good Idea how much modification it would take. I considered this very thing with a 1950's cabin cruiser I inherited, in my case the boat just wasn't strong enough to support and outboard without a lot of modification.
  3. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    What did it have in it, and what were you looking at to replace it?
  4. zrr
    Joined: Apr 2021
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    Location: North Carolina

    zrr New Member

    Currently has a Gray Marine 625, I was thinking of replacing it with a 50hp. Just something light to cruse around with.
  5. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    I'm thinking any retrofit with a current inboard system will be far easier, quicker, and less costly than going the outboard route.
  6. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    I don’t understand the problem with switching to a newer inboard.
    It would actually be a lot less work, even if the strut and stern tube had to be changed.
    The existing transom/stringers would require a lot of beefing up, and then there’s the gaping hole left in the middle of the boat.
    I’d be looking at small diesels that are usually found in sailboats.
  7. zrr
    Joined: Apr 2021
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    Location: North Carolina

    zrr New Member

  8. Barry
    Joined: Mar 2002
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    Barry Senior Member

    There would be considerable advantages to converting to the outboard

    The Yanmar weighs in at about 500 pounds dry , add in a transmission etc and you could get to 600 pounds
    I would anticipate some work regarding stringers, exhaust penetrations through the transom and perhaps the engine cover
    You did not elude to the cost of the engine and transmission

    A 50 hp Merc runs 250 ish, pounds and costs about $8,000. You will gain an immense amount of storage space. You will gain vectored thrust which provides for better control at slow speeds and certainly backing up.
    You will have to plug the existing shaft, rudder and exhaust holes. Obviously there will be work to strengthen the transom but there are millions of wood boats with 50 hp engines in a wood engine well.

    I would expect due to your location, that there are shops able to take this work on and if you DIY it, google similar sites
    Last edited: Apr 10, 2021
    DogCavalry likes this.
  9. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    That's an amazingly neat little engine. Guessing engine plus gear will equal a bit of sticker shock, but the package should last a long time.

    Mercury used to produce a gm 4 cyl that was 365 pounds and under 7 grand. A little zf gear, adapter shaft and prop would tip you over 10k. Guessing that will still be half the cost of the yanmar block. It's a higher horsepower motor, but that's easily remedied with a throttle...

    About 5 years ago I needed a small diesel propulsion engine for a hydraulic pump drive. Ended up buying a complete boat some old retired navy guy had built with a retro outboard boat. I payed 8% on the dollar for the boat what he payed for 40 hp engine alone the year before. Yarded that beautiful little engine with 7 hrs out, chainsawed the boat in 2 and off to the landfill it went. Didn't have the heart to tell his widow what became of it.....

    Moral of the story, inboard diesels are cool, but they are shockingly expensive. A modest bracket and some aluminum stringers plus an ob will be much much much cheaper than diesel anything.
    DogCavalry likes this.

  10. Slowboating
    Joined: May 2021
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    Location: Cortes Bay

    Slowboating Junior Member

    I don’t think it’s that far fetched to put an outboard bracket or even a pod on that transom. I have one on mine and it works very well.
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