Diesel outboard

Discussion in 'Outboards' started by Hastings Harry, Apr 23, 2014.

  1. Joris
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    Joris Junior Member

    ok...:D
     
  2. Barry
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    Barry Senior Member

    I hate to correct myself. I had not considered that the original gas outboard will have its own gear reduction and for at least 3 50hp outboards that I checked the gear reduction can vary between 2.64 to one, to 1.83 to one.

    So to be more correct

    If you had a 50hp @2500 rpm diesel and ran it through a gear set that gave the same prop the same rpm as the gas engine, you would get the same performance as the 50hp 5000 rpm gas engine. ( same performance is a little arbitrary as you will get better fuel economy, but acceleration might be different)

    Both engines are putting the same horsepower to the prop at the same prop rpm
     
  3. powerabout
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    powerabout Senior Member

    Problem is its a boat not a generator
     
  4. Barry
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    Barry Senior Member


    A propeller running at 2500 rpm absorbing 50 horsepower from the water from the input shaft into the propeller does not really care what is creating the input conditions whether it is:

    A gas outboard producing 50 hp @ 5000 rpm running through a 2:1 reduction
    A diesel outboard producing 50hp @2500rpm running through a 1:1 gear case
    A gas turbine producing 50 hp @ 10,000 rpm running through a 4:1 reduction
    An electric motor producing 50hp at 7,500 rpm running through a 3:1 reduction.

    Assume all engine weights the same, and gear cases frictionless.

    The boat will run at the same speed with all of the above
     
  5. parkland
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    parkland Senior Member

    They would all run the same speed, but the hole shots would be different, as all those engines might have the same HP, but different TQ curves.
     
  6. Barry
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    Barry Senior Member

    Torque is a static force and is used in the calculation to build horsepower curves
    The hole shot would be different because the horsepower curves are different
     
  7. parkland
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    parkland Senior Member

    If this thread ends at this, it would be the first thread on the internet I've ever seen that doesn't go on for about 5 pages arguing about HP and TQ, only to get closed.

    :(
     
  8. Barry
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    Barry Senior Member

    True Parkland
    But if you put 10 mechanical engineers into a room, to discuss torque vs hp, there would not be an argument.
    Are you from AB?
     
  9. powerabout
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    powerabout Senior Member

    TQ and HP are very important items if you are trying to determine the outcome on a planning hull, displacement then all the above would probably be true.
     
  10. parkland
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    parkland Senior Member


    I'm in MB.
     
  11. parkland
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    parkland Senior Member


    Heres another thought; because the load on the engine raises so quickly versus prop speed, is TQ really important as it is on say a truck?

    If a diesel engine has peak TQ at 2000 RPM, but it runs at 3000 RPM for full speed, does that peak TQ @ 2000 really help you at all?
     
  12. DCockey
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    DCockey Senior Member

    Power = Torque * Rotational Speed
    Torque = Power / Rotational Speed
    These are definitions. (Appropriate unit conversion factors are also needed if units are not consistent)

    So the maximum torque vs speed curve and maximum power vs speed curve are linked.
     
  13. Barry
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    Barry Senior Member

    Parkland
    Torque is a static measurement that is used to develop horsepower curves. I think that because torque curves are often plotted over top of horsepower curves, many people think that you could use the Torque curve as a set of design values. But on top of a horsepower curve, you could superimpose, volumetric efficiency, thermal efficiency and even create more confusion.
    The largest area for confusion is that the horsepower curve is really a curve that shows what the MAXIMUM horsepower that the engine can produce with all the fuel flow and air flow optimized for that SPECIFIC rpm. This maximum horsepower for a specific rpm is normally at a point where fuel flow rate for the horsepower ( thermal efficiency) is excessive, Exhaust gas temperatures are excessive, and is not really a spot where you want to run the engine.
    Barring acceleration conditions, the only time that an engine produces the maximum horsepower as shown on a graph is at Wide Open Throttle when the engine just cannot produce any more horsepower. ( but this is not even 100% correct as the horsepower curves are developed in a lab where they can change fuel flow rate, and air flow rate to get a little bit more out of what an engine stuck in a bilge can produce)

    David
    I never would say that the torque and horsepower are not linked, but rather if you can determine what torque that an engine can produce, which is a static force in poundsfeet, then run it through the equation, you get the horsepower.
    Horsepower is dynamic

    Parkland
    MB eh
    113 day annual average below freezing
     
  14. DCockey
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    DCockey Senior Member

    Barry, what do you mean by when you say torque is "static" and power is "dynamic"?

    If you have the power curve you can plot the torque curve and vise-versa using the simple formulas I gave above.
     

  15. Barry
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    Barry Senior Member

    The units for Torque is Foot pounds, nothing is moving,
    Put a one foot wrench on a bolt and pull 100 pounds, and you get 100 foot pounds of Torque.

    Now let the bolt slip but the force required to move the wrench remains the same, and let the wrench move on complete revolution. The 100 pound force moves through a distance of 3.14 (pi) times the diameter, 2 feet, so you have an amount of work of 628 pounds feet of work. This could take a minute, or an hour, but the amount of work is the same. Force through distance

    But in order to get a horsepower value we need to introduce time. Power is the rate of doing work.

    One horsepower is 33,000 foot pounds per minute
    Force through distance with respect to time.

    A boat requires a certain amount of horsepower to move it at a specific speed. (not engine horsepower, but the FORCE parallel to the direction of the boats travel through a DISTANCE with respect to TIME)

    If a boat requires 200 horsepower then to move it at say 20 mph, the prop has to generate about 3,800 pounds of thrust in the direction of the boats direction to make this happen. The engine would have to produce more than this due to transmission losses, angle of the shaft with respect to the direction of the boat losses, etc


    A dyno actually measures static torque, or the amount of twist an engine could produce into a shaft.
    Then the rpm is noted, and the calculation is done to develop the maximum horsepower curve.
    Of course, a dynos computor just does the calculations and the read out can be horsepower.

    So you just look at the units, Torque, foot pounds
    which is just static and then Horsepower, foot pounds per minute which is dynamic

    I did not mean to over simplify the explanation but just to create a basic progression from foot pounds of torque through to horsepower.

    Torque is used more for the design of the specific drive components, ie size of the shafts, size of the transmissions, load on the gear teeth, engine bed mounting requirements etc.
     
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