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Torque with relation to Acceleration

  1. Oct 22, 2009 #1
    I have a question having to do with the affect of torque on a vehicle. I have 2 situations to make it easier to respond.

    Case 1

    There are two boats. Both are identical, identical in weight, drag, etc. Boat A and boat B have single 300hp outboard engines. Although boat A's engine makes 200lb-ft of torque and boat B's engine makes 300lb-ft of torque. Both engines are otherwise identical(weight, gearing, drive-train loss, etc.).

    How will the increased torque in boat B affect it compared to boat A? (acceleration? top speed?).

    Case 2

    Same situation. Identical boats and horsepower, boat A has 200lb-ft and boat B has 300lb-ft. But the variable this time is gearing. The engine in boat A has lower gearing than boat B.

    By lowering gearing does torque increase? How does the gearing affect boat A compared to boat B. (acceleration? top speed?).
  2. jcsd
  3. Oct 23, 2009 #2

    Since both boats are making the same horsepower, boat A's engine will be at higher RPM than boat B's engine. This means that the screw on boat A will be turning faster, and should therefore be designed to be efficient while doing so. The screw on boat B will probably be bigger, since it will be turning slower. Assuming the screws have the same efficiencies, the boats should accelerate at the same rate and have the same top speed (although efficencies of screws probably change with speed, so I doubt we will have identical accelaration curves).

    If you wish to insist that EVERYTHING is identical, including the screws, then I suppose it's a matter of which boat's screw is more properly matched to its engine. If you have a bigger screw, then most likely the boat with more torque will accelerate/ go faster. If a smaller screw, then boat b will probably have better performance.

    As for Case 2...it's the same thing. If you have lower gearing then you have more torque but less RPM, which would call for a larger screw. Higher gearing would call for a smaller screw.

    If you put a screw too small on a high-torque drivetrain, then you'll end up with a situation where the screw can't take advantage of all the power of the engine, and you'll have to let off the gas so as not to bounce off the rev limiter. If the screw is too large and the torque of the engine is too low, then you might not be able to turn it fast enough to keep the engine at its power peak, and again you wouldn't be taking full advantage of the engine.
    Last edited: Oct 23, 2009
  4. Oct 23, 2009 #3


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    Homework Helper

    Boat propellers have 3 key factors, number of blades, pitch, and diameter. This acts as gearing. There also may be actual gearing. As mentioned in previous post, if the prop and gearing are optimized for boat A, then boat B won't run as well and vice versa. If the propellers are optmized for both boats, then the acceleration and top speed should be the same.
  5. Oct 23, 2009 #4
    This is a cross post, its also on the engineering forum.
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