25ft midcabin boat. From I/O to twin outboard conversion

Discussion in 'Boat Design' started by Forsmanni, Aug 31, 2020.

  1. Forsmanni
    Joined: Aug 2020
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    Forsmanni New Member

    Hello,

    I just bought a 25ft midcabin cruiser with broken 250hp v8 inboard/sterndrive engine setup. Im thinking that I will do a little bit bigger swimming platform and put one or twin outboards there instead of fixing the old v8.

    So my guestiong to those who have made similar conversion is. How much power do I need to get the boat roughly into same cruising speed? And is the fuel economy going up or down with old 2-stroke engines? So Im thinking eather twin Yamaha 130hp v4 outboards or one 250hp v6 Yamaha. Will the power be enough to get this old lady nicely to plane.

    Boat is 25ft long and weights 5500lbs (2000kg) without engines.
    Old motor is 250hp inboard. cruising speed is 22-26kn and topspeed 33kn.
     
  2. TANSL
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    TANSL Senior Member

    Welcome to the forum.
    In a quick response, without studying anything or knowing your boat, I think you should take into account several things. Most importantly, in my opinion, the overall weight of the boat with the new engines is going to change, as well as the position of the CoG. This will affect the planing conditions, which must be re-studied. The power required will be very similar since the power calculations usually do not take into account the type of engine or its position on board (since, in addition, the direction of the propeller thrust will not change), although it must be considered the overall efficiency of the engine-drive unit, which will undoubtedly change.
    If the boat is designed to achieve its optimum point with current engines, it can be assumed that any other configuration will be less efficient in terms of speed and consumption.
    The I / O motors leave much more headroom for the swimming platform.
    It would be wise for a technician to study your current boat and the new configuration you want for it.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Forsmanni, I would estimate your cruise speed won't change that much, but the fuel burn could be up as much as 50% with the twin 2S 130's, assuming that your old engine is carburetted. A single 250 won't be quite as bad, but still noticeably more. I doubt the balance of the boat will be greatly affected, but if it has more "vee" than most, it could become a little more tender at rest.
     
  4. fallguy
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    fallguy Senior Member

    How you gonna swim platform with twin ob?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    He could have a small platform either side, I guess.
     
  6. Forsmanni
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    Forsmanni New Member

    So the platform main idea would be that it is a bracket for the engines/engine. Not build for swimming :) 50% more fuel burn sounds bad... Mercruiser v8 carb. takes 1,6Liters/mpk.
     
  7. Barry
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    Barry Senior Member

    1.6 liter/mpk mpk?
    Re the hull Many manufacturer's are using the same hull with an inboard configuration as well as single or twin outboards with good success.
    You can check out the fuel curves for the Cutwater 30 available with a single DD Volvo diesel compared to the 300 twin Yamaha. And there are others. You are replacing an old carbed V8, which would not have given you good numbers
    and switching up to EFI outboards, four stroke. The separation between these set ups MAY not be far apart. A single larger outboard will be a little better for a lower fuel consumption. As the old boat was a single, it appears that you are ok with
    not having a get home redundant engine.

    Removing the old iron V8 and heavy leg, you may see a drop in weight of maybe 400 pounds,

    If your make of boat has been around awhile, you can search forums and often find people who have done the same thing. Or even other makes of similar hull deadrise, beam and weight. Ie 25 foot Bayliner or XYZ make, inboard to outboard conversion to see what the results were.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    So what is the story with the sterndrive, that renders repair impracticable ?
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Barry, he did say going to "old" 2S engines. The old carbed sterndrives gave reasonably good mileage figures if treated with a light hand on the throttle, sitting just on plane, the temptation to throttle up, if surrendered to, quickly puts an end to that.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Even switching to twin fuel injected 4S outboards, a costly exercise, would not yield improvement in mpg figures, in my opinion, over an old carbed V8 sterndrive. What you picked up with the fuel injection, you would lose with having two engines.
     
  11. Barry
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    Barry Senior Member

    Thanks, I missed the "old 2S" engines,
     
  12. kapnD
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    kapnD Senior Member

    I’d think it more prudent to repair existing motor, possibly upgrading to EFI to improve economy.
    The costs of rebuilding the boats interior combined with the cost of the bracket for the outboards, plus rot Repairs in stringers and transom that may surface will get out of hand quickly.
    The transom will need reinforcement at least, and outdrive holes covered up.
    Old two stroke motors are notoriously thirsty and environmentally offensive, and of course, being used, they may require rebuilding as well!

    Kashif, that’s about the most off-topic post I’ve ever seen!
    You should be a politician!
     
    brendan gardam likes this.
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    It depends what "broken" means.
     
  14. brendan gardam
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    brendan gardam Senior Member

    i have done several conversions over the years and honestly the amount of work is huge and can be very successful but i would not recomend it. if you can replace the v8 with a recon or new v8 it will be a lot nicer boat than having a pair of yammy 130s hanging off the back.
     

  15. brendan gardam
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    brendan gardam Senior Member

    you can't count stringer and transom rot repair because that still needs to be fixed no matter what powers it.
     
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