Conversion from gas inboard to diesel

Discussion in 'Boat Design' started by rattleandbang, May 19, 2015.

  1. motorbike
    Joined: Mar 2011
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    Location: Beam Reach

    motorbike Senior Member

    Hmmm another sticky one. Put simply you cannot get something for nothing. A 60's wooden boat going to use a lot of fuel and it's going to use a resources anyway with proper maintenance and berthage.

    Fuel is the least of your worries. As they say if you can't afford the fuel you can't afford the boat. Converting to diesel is a good idea but don't do it on the cheap, 55hp is anemic, get at least triple that, the cost is worth it.

    That said just buy it and use it as you can afford it or get RIB with a 200 on the back!
  2. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    I'm not familiar with the boat, but if you are intimately acquainted with all the fuel flow/speed data, I'd like to see it, in 500 rpm increments if possible. I realised many years ago that slow trolling at 3-4 knots used much less fuel than pushing to 6-7 knots, in any boat I tried it in. But the strikes were somewhat fewer at the slower speed, so faster prevailed, even if I used three times as much fuel per hour, doing it.

    A bit like this boat, powered by a 4-stroke Yamaha 250. (info from Yamaha Performance Bulletins)

    1000 5.7 1.3 4.39
    1500 7.7 2.2 3.50
    2000 9.2 3.6 2.56
    2500 15.4 5.4 2.85
    3000 23.5 6.7 3.51
    3500 29.3 9.3 3.15
    4000 34.0 11.8 2.88
    4500 38.2 15.0 2.55
    5000 42.5 18.6 2.29
    5500 46.8 23.4 2.00
    5800 49.0 23.6 2.08

    Oh, and I said I was bemused, not amused ! :D
  3. rattleandbang
    Joined: Apr 2015
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    Location: Victoria BC

    rattleandbang Junior Member

    I did some checking on the power curve of the 327, and even at 1200 rpm it's already doing more power and torque than that diesel, and likely only doing 6 or knots at that. A 50 hp diesel would barely move the thing never mind in a chop or head sea. Maybe I can find a vw jetta turbo diesel and marinize it.
  4. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    It would get it moving OK, you have to factor in the gear ratio and propellor best suited to what you want. But I wouldn't bother doing the conversion, you will have an "orphan" that will appeal to a very limited market, if/when you go to sell. Most people who are happy to tootle around at sedate speeds will just buy a purpose-designed displacement hulled boat.

  5. goodwilltoall
    Joined: Jul 2010
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    Location: nation of Ohio

    goodwilltoall Senior Member

    This is an old thread but if I wanted better fuel efficiency on a planing boat such as a marinette, the transom would get a 4 or 5' extension with bottom upswept cleanly above waterline and tapered following plan view for easy release of water. 12-18" above WL with flush deck, railing around for easy boarding and fishing.

    Yes all benefits of planing/higher speeds would be gone, it would turn into a displacement hull with top speed reaching 10mph on the 32. Single engine would be all thats required and the 55hp diesel would be enough
    Last edited: Jun 2, 2019
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