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HP vs Torque

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Old 12-17-2002, 01:13 PM
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Default HP vs Torque

OK, so now you know I don't know much, but can anyone tell me the difference between Torque and horsepower? My engine supposedly puts out 650 HP at 6200 RPM, but the torque drops off at 5800 RPM. I am getting ready to send my props to Bronson at Hill Propeller and the engine maker, HTM, says to get the RPM at WOT down to 5800 so I am at maximum torque. I guess all I have to do is write down the speed at different RPM for Hill, but am curious.

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Old 12-17-2002, 01:30 PM
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Torque gets you moving. HP keeps you there.
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Old 12-17-2002, 01:57 PM
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Torque is the physical measurable force, if you will, that any engine or lever arm producesand and is most commonly described in Ft-Lbs. Horsepower is a derived unit taking into account torque at some given rpm.

1 HP= 550 Ft-Lbs / sec which is translated into:

1HP = (Torque X RPM) / 5252

Torque provides acceleration, while horsepower nets speed.


I don't understand propping for your torque peek, though. You still have rpms left which will give you more horsepower. You should be as close to your HP peek as possible at WOT for max speed.
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Old 12-17-2002, 02:10 PM
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the angular acceleration of inertia equals torque

hp= rpm*tq/5252

actually on that extremely efficient hull I would think you should take advantage of all the hp your engine has to offer. that is something that is all part of dialing in a boat. maybe your engine builder does not want you to exceed a certain rpm for durability reasons??
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Old 12-17-2002, 02:31 PM
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Is it possible to net the same speed by using torque to spin a larger pitched prop which would lower the rpm.

But ... would that put more of a load on the engine than being able to use the HP to spin at a higher rpm.

Which is more cost effective(reliable) .... loading a BBC or RPM on a BBC?



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Old 12-17-2002, 02:38 PM
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Power out = Power in X (1 - efficiency)

Props and drivetrains vary in efficiency(slips, friction) so it's always a tough call, testing is the best way, but your best speed will be achieved if your engine is operating at its dyno-proven WOT peek HP RPM.

Anything less and your not inputting all of your available HP.
Operating at the torque peek simply means that the engine is working harder and not able to achieve it max HP rpm. (Assuming torque peek is lower than HP peek, which 99% of the time it is).

Last edited by Baja Daze; 12-17-2002 at 02:41 PM.
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Old 12-17-2002, 02:39 PM
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Think about this....the larger the prop, the higher the dynamic loads WILL be on the powertrain....most of the time the drive will loose!
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Old 12-17-2002, 02:42 PM
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"(Assuming torque peek is lower than HP peek, which 99% of the time it is)" IT IS?...better change that.
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Old 12-17-2002, 02:44 PM
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My boat saturday went 98 on GPS at 6200 RPM with no rev limiter and I have not dialed it in at all yet. The bottom needs some work and the prop has nicks in it. HTM says it would turn 6500 at peak HP, but there is not enough torque to turn the props. They say to increase the pitch or cup the props until the RPM is more like 5800 or slightly less at WOT. I have Volvo Duoprop with E4 props which is the biggest they make. Bronson says he dialed in the RainX boat and is familiar with changing the E4 props.


Let me see if I understand torque and HP:

If I have 100 ft lbs of torque at 2000 rpm that is making 200000/5252 = 38 HP

If my torque drops off to 75 ft lbs at 4000 RPM, I have less ability to pull against resistance, but more HP - 57 HP

In a car going down the highway there is not so much resistance so it doesn't take so much torque unless I am trying to accelerate, but with a boat it is against resistance from the water, so torque is keeping me from going any faster even though I have more HP available due to higher RPM even if the torque is falling off.

My engine builder says it doesn't matter as long as I don't rev past 6500 RPM, but I don't have enough torque with the current set up to get there.

I have the theoretical calculations from Ocke at home, but I think I remember it should go around 110 and I would be very happy if I can get it over 100 all the time instead of only the perfect day.

If I am going in the wrong direction please let me know. I appreciate all the advice and education I can get.

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Old 12-17-2002, 02:47 PM
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Quote:
Originally posted by audacity
Think about this....the larger the prop, the higher the dynamic loads WILL be on the powertrain....most of the time the drive will loose!
Good point

I didn't consider the whole powertrain.
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