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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 22, 2009 #2


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    Staff: Mentor

    Case 1 is physically impossible. Since torque and horsepower are directly proportional to each other via rpm, Boat A's propeller spins at 3/2 the rpm of Boat B's at that operating point. But if the boats are physically identical, driving the boat can't require two different horsepowers or torques at the same time. Ie, if driving Boat A at 20 mph requires 300 hp (the operating point you specified), then Boat B will never reach 20 mph and never output 300 hp or 300 ft-lb of torque. Why? Because Boat B's propeller is only spinning at 2/3 the speed of boat A's!

    Case 2 corrects the problem of case 1 by making the horsepower and torque at the propeller identical for both. As far as the the end result is concerned, the boats are now completely identical in final speed. However, since Boat A's engine and beginning of the drivetrain operate at 150% higher rpm than boat B's, rotational inertia will likely cause it to take longer to accelerate up to speed.
  4. Oct 22, 2009 #3
    I don't understand that explanation. Why is it physically impossible?

    Where did you come up with,
    "Boat A's propeller spins at 3/2 the rpm of Boat B's at that operating point"?

    I stated that they have the same gearing. Include same prop. Therefore both propellers must be spinning at the same speed per mph.

    Start at a standstill and accelerate. 300hp & 300lb-ft vs 300hp & 200lb-ft.
  5. Oct 22, 2009 #4
    Power is torque * angular velocity.

    You cant have the same power and different torques ar the same speed.

    Just try popping it into the equation to see why.

  6. Oct 23, 2009 #5
    So I understand an engine with 200lb-ft of torque may have 300hp at 7878 RPM and the 300lb-ft engine will have 300hp at 5252 RPM.

    This means with the same gearing the boat B will hit its power sooner and accelerate faster but top out sooner. Correct?

    Maybe I am wrong about torque in this case. There is a test like this I am trying to confirm why the results are so but the torque figure is not given for either engine.

    So maybe the case is this:
    Boat A and Boat B have equivalent Horsepower AND Torque. But Boat B has more low end torque and a much broader power-band. What will be the result of that?
  7. Oct 23, 2009 #6


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    Staff: Mentor

    That depends on the particulars of the engine. If they have similar rotational inertias, the one with more torque may get to its maximum rpm faster, yes.

    Also, remember that when the propeller spins at a certain rpm, the water provides a certain resistance: a torque. The torque that the water provides may not match the torque capability of the engine.

    Think of it this way: if you put your car in neutral, step on the gas pedal and rev the engine up to 3000 rpm, does it put out more, less, or the same horsepower as if it is in gear and at 3000 rpm? Less, right? Because there is no resistance on it.

    Well if your high torque, low rpm engine can't rev up to a high enough rpm to get resistance from the water without redlining, it'll never generate the torque it is capable of generating. So what does that mean? It means it is geared too low. In your case 2, the boats may be geared appropriately to match the engine to the drivetrain (you aren't specific enough to know for sure...), providing nearly identical performance.
    Much better acceleration for boat B, but the same top speed.
  8. Oct 24, 2009 #7
    In ideal case for an engine, this this shows why peak figures are useless to determine performance.

    As Russ said it will have the same top speed but better acceleration low end.

    However you simply cannot attain this scenario with two real similar engines. If you tuned for low end torque, you would loose high end torque and vice versa.

    An engine of a certain size and configuration will have a 'maximum potential' based on its BMEP. This means that the area under a power/torque curve will generally be quite similar regardles of tune with only smaller overall gains to be had. So if you raise power/torque in in one location it must fall in another.

    The only way to extract more from an engine (ie increase the area under the power curve) is to raise its BMEP, generally by forced induction.
  9. Oct 24, 2009 #8
    There are several ways to increase lower end torque.

    Increasing displacement and tuning it for lower end power.
    A simple exhaust pipe mod.
    Using fuel injection instead of a carburetor.
    A 2-stroke engine vs a 4-stroke engine.
  10. Oct 24, 2009 #9
    The reason for these questions is from a video I saw. Two engines were mounted on a boat. Both were 225hp engines. One was a 4-stroke and the other was a DI 2-stroke. The 2-stroke has a lot more low end torque.

    The boat was a heavy boat. While running the 2-stroke the boat accelerates and gets on plane decently and then keeps going to 30mph. The 4-stroke never gets the boat on plane and reaches max plowing speed at 13mph.

    I was just wondering if it was the extra low end torque that would allow the 2-stroke to push the heavy boat onto plane.
  11. Oct 24, 2009 #10
    Apart from increasing displacement and using a 2 stroke, which is changing the engine configuration. You said having two identical engines, with the same peak power and torque output figures but more low end torque, which isn't possible.

    It is possible if you have two non identical/similar engies.

    If you tune for low end you lose high end.

    If you want to move something heavy more torque = better.
  12. Oct 24, 2009 #11
    So by having more low end torque that could be the reason why that engine was able to get the boat on plane?
  13. Oct 24, 2009 #12
    Possibly, without more information I wouldn't like to commit either way as there are too many factors.

    But it's possible that the extra torque allowed the boa to be pushed that little bit extra, so it gets on the plane at a lower speed than the other engine. Meaning it coul;d make full use of the power.
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