Choosing the HP of diesel engine required

Discussion in 'DIY Marinizing' started by DaEdster, Jan 25, 2012.

  1. DaEdster
    Joined: Jan 2012
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    DaEdster Boat builder

    Hi all, ship, January is near over!

    Looking to marinise A diesel truck/tractor motor and need to find that elusive web that shows mostly how....
    I get that Google 1,578,987 found in split second stuff and I'm really over it by page 3! - note to self - use different search words.

    I found an interesting start to what size motor to get by way of the quoted part reply to a site some place on earth - although I'm never sure it was earth these last few days, lol

    "Cheapest is to pick an engine that will operate at or near the torque peak , producing the hp you need at that rpm."

    Question is, can any one explain this some more to me please?
    Much much appreciated.
     
  2. DaEdster
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    DaEdster Boat builder

    Maybe the question could be:
    Having worked out HP needed to propel the motor boat, how does one go about picking an engine that fulfills this?
    Keeping n mind the quoted text above needs to be allowed for.

    The idea is to get an engine that will produce needed HP at peak torque - for those times we need to 'get the heck outta here' otherwise powering down to get a more comfortable ride and economy too.
     
    Last edited: Jan 25, 2012
  3. powerabout
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    powerabout Senior Member

    the torque curve for a diesel ( especially a truck govener) is the reverse of the prop curve usually
     
  4. DaEdster
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    DaEdster Boat builder

    Mmmmmm..... wot?:(

    Out of my depth here it seems, I like to know though, so, please do go on...

    Edit. So an upwards curve for torque wood generally = the downwards curve of the prop.... if one drew both on the same bit of graph paper...
     
    Last edited: Jan 25, 2012
  5. powerabout
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    powerabout Senior Member

    take a look at a truck engine torque curve ( peak way low)then take a look at prop curve for a boat and it will be low where the diesel is high then as the diesel torque curve drops away the prop load goes up.
    Lay the 2 graphs over each other you you will see it all
    ( assuming you are not powering a tug boat and are using a fixed pitch prop)
     
  6. DaEdster
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    DaEdster Boat builder

    Ah ha, yes, now to get those printed or write a list of favorite engines with those RPM/HP figures to get comparisons.
    The prop, would I get this curve from the supplier given he has my boat spec's (not yet done) or will he need the engine type and details?
     
  7. powerabout
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    powerabout Senior Member

    well the boat is fixed so generally the prop diameter has a maximum so your prop guy can then calculate the best prop after your boat designer tells him how fast it goes then you get the engine and gearbox that best fits that
     
  8. gonzo
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    gonzo Senior Member

    You calculate the power you need for the boat at the target speed. Depending on the use, you want to get that power as continuos (commercial) or intermittent. Then you have to calculate a propeller that will absorb that power at the rated RPMs. It may be necessary or advantageous to use a reduction gear, depending on the design and use.
    You will have to find a flywheel that will accept the coupler or plate to fit a marine transmission to it. Also, you will need mounts, probably a wet exhaust and maybe a different injection pump mapped for the application. Overall, it is usually more expensive than getting a used engine with gears.
     
  9. DaEdster
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    DaEdster Boat builder

    Thank you powerabout & gonzo, both answers fxed me up, now to do the math with professional prop guys help.
    And yes, I do believe getting an already marinised donk with gearbox is cheaper, prob second hand, if I can see it running....
     
  10. CDK
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    CDK retired engineer

    About the original question:

    "Cheapest is to pick an engine that will operate at or near the torque peak , producing the hp you need at that rpm."

    The reasoning behind this is fairly simple. At peak torque the engine performs best, so you get the most power output for your money. Fortunately diesels have a fairly large rpm range where the torque curve is flat, so picking the right gearbox and prop isn't as difficult as you might think.
    In a well calculated combination, full throttle rpm should coincide with the point where the torque curve just starts going downhill.
     
  11. DaEdster
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    DaEdster Boat builder

    Yes got it, great stuff CDK, that'll give me further options on engine choice.... if have any in the 2nd hand market.... I have time, sort of.

    Any one have any input on fitting the prop into it's own tunnel?

    As I get it here, I need to get prop size for given hull size, weight & design/wanted speed, to work back to engine size.
    So, this will give me the tunnel size to build, + 2" tip clearance for the prop.
     
  12. CDK
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    CDK retired engineer

  13. Yellowjacket
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    Yellowjacket Senior Member

    Your selection of the prop pitch also involves what kind of hull you are powering.

    If it is a planing hull, you need to have sufficient power to get over the hump and onto a plane. This means setting the prop pitch lower, and running the engine at a higher rpm at high speed. The last thing you want to do is have too much pitch and not be able to get the boat on a plane.

    For a displacement hull, picking the torque peak as the point where you are running the engine is the most efficient, only if you are matched in terms of power delivered, and that you want to cruise at that high power point all the time (highly unlikely since you will suck fuel and beat up your engine). If you set your cruise point at the torque peak, that does not necessarily result in the most efficient cruise fuel consumption. You need to look at the power maps (Caterpillar has them on their web site) and compare speed versus fuel flow for a given power. You need to draw your power consumption (prop demand) curve and then look at the fuel consumption and then determine what the speed difference give you in positive or negative fuel flow.

    Trying to use the torque peak is just a rule of thumb, and it does not necessarily result in the best answer since you aren't actually going to run the engine at that power, at cruise you will be at a lower power.
     
  14. DaEdster
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    DaEdster Boat builder

    Yes yellowjacket, as an apprentice boat builder, I had the pleasure of being crew for the racing tunnel boats our firm raced, one of my jobs was to crawl forward to get the hull over that hump.... the crawl back was trickier!

    My hull is a planing 32 ft'er. with 13 deg deadrise @ transom
     

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  15. DaEdster
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    DaEdster Boat builder

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