4.3 Mercruiser to Diesel

Discussion in 'DIY Marinizing' started by TomBradbury, May 25, 2013.

  1. TomBradbury
    Joined: May 2013
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    Location: Ireland

    TomBradbury New Member

    Hi All,

    I have a Searay approx 20 foot long, currently powered by a 4.3 Mercuiser(approx 170 hp) with an Alpha 1 leg, top speed is about 32 kts flat out, 20 knts at 3500 RPM , prop is currently 14.5 x 21.

    The problem is with petrol been so expensive is it almost prohibitive to use as it does about 4 mpg cruising!!

    The advice I'm looking for is what would be a suitable diesel to put in it, I've done some looking and Ford have a 2.2 TDCi with 200 hp. I'm fairly competent when it comes to diy, and have access to necessary equipment for a conversion.

    Any comments / suggestions would be appreciated

    Tom
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Gears have two critical ratings: RPMs and HP inputs. At what RPMs is the rated HP of the new engine?
     
  3. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    My advice is to download the Lancing marine catalog. It lists what conversion parts you need and what it costs, so all you have to do is divide the grand total by the price difference between gasoline and (red) diesel to find your break even point.

    If you really want to lower the running costs, install a small diesel that is unable to get your boat planing, although I admit tugging along at 6 kts. in a Searay looks a bit silly.
     
  4. TomBradbury
    Joined: May 2013
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    TomBradbury New Member

    The engine I'm looking at produces 200hp at 3500 rpm, I was hoping to compensate loss of rpm with a bigger pitch prop.

    I've also looked at the lancing marine catalogue and don't really see a high enough hp engine, the top end speed is important to me for short bursts, and I agree 6kts is a bit silly looking in a searay.....
     
  5. 7228sedan
    Joined: Nov 2009
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    Location: New Jersey USA

    7228sedan Senior Member

    Tom do you know what the torque specs are of the Ford diesel? The alpha drive is the lightest duty drive Mercruiser makes, I personally wouldn't trust one behind a big block gas engine never mind a diesel. A 200 hp diesel is more comparable to a healthy v8 then a v6. I looked into repowering my Fathers boat to diesel. He currently has twin 4.3's with the Bravo 2 drives. We looked at the small 2.8 d tronic motors at the time. The problem is unless the range increase is your primary goal due to the lower consumption, you will need to run the boat practically for a living in order to see the savings. Assuming that the existing drive will hold up and you do all of the labor and fabrication yourself, have you worked out an estimated cost of this project? The cost might very well fund enough petrol for the next 10 years. Of course the more hours you run the boat, the more feasible it becomes. Not for nothing but 4 mpg at cruise isn't all that bad at 20 knots :)
     
  6. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    200hp @3500 = 300ftlbs
    but you need to see what peak torque of that engine is as it may be more at a lower rpm which will give a 1 drive a hard time.
     
  7. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    If there was a clear advantage to such a set-up (diesel sterndrive) in a smallish boat, the market would be awash with them. It isn't. You have to be concerned about how the drive will stand up to the diesel torque, and i doubt the gains in fuel economy will be worth all the drama. And if you go trolling.......the smell of diesel fumes.
     
  8. 7228sedan
    Joined: Nov 2009
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    Location: New Jersey USA

    7228sedan Senior Member

    Exactly! I was referring to the entire torque curve of the motor in question.
     

  9. TomBradbury
    Joined: May 2013
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    Location: Ireland

    TomBradbury New Member

    Hi All,

    Thank you for your advice, having done some more research I think I might get away with a lower hp, about the 130 or 150 range, producing about 250 ftlb.

    I realise the Alpha 1 is not the strongest leg, and I wont be slamming the throttle wide open so I;m hoping it will handle that amount of torque.

    I'm based in Ireland where gasoline costs $ 9.50 a gallon, hence the reason I want to get some more efficiency.
     
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