Torque: A novice's question

Discussion in 'Propulsion' started by Carlazzomark, Oct 2, 2006.

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CarlazzomarkJunior Member

I know nothing about the mechanics of boat propulsion.

My questions have to do with the torque requirements of a human powered shaft/propeller.

Is torque less important than power? What is the optimum torque for a shaft/propeller for a common pedal boat?

Thanks,

Carlazzomark

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GuillermoIngeniero Naval

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jehardimanSenior Member

Torque is limited by the maximum amount the person can PUSH ~ 200-250 lbs for a good cycling athlete.

Power is limited by the ability of the body to oxygenate. ~0.75 hp max aerobic, ~1.25 hp max anaerobic

General long distance performance for legs only is 60-70 lbs push, 9" cranks, 60-80 rpm (i.e. .5 to .75 hp) Remember that as push goes up, cycles go down while push goes down, cycles rarely top 100-120 regardless of how low the push goes.

Go see the International Human Powered Vehicle Association website http://www.ihpva.org/

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SteamFreakUSMM

P=TxR/M
P-Power
T-Torque (ft-lbs)
R-Revolutions
M-Minute
SO normally, a power equation might look like...

150 ft-lbs X 60Rev/Min= 9000 ft-lbs/min (or to convert to horsepower)
(9000 ft-lbs/m) X (1hp/33000ft-lbs)= .27 hp

not alot...

Anyway, the fact is for a human powered machine, a high torque requirement is the last thing you want (unless you like working out and short tours). Introducing gearing to allow a build up of speed (rev/min) at the prop without requiring higher torque will give you performance without killing the legs.

So torque and power are functions of one another and gearing allows you to achieve the same output power with less torque. An average sedan might only put out 150ft-lbs of torque but achieves over 120 hp through high revs and achieves speeds in excess of 90 mph through gearing.

As far as boats are concerned, torque determines how big a prop you can turn with so much "bite" or pitch. This is why diesels are prefered for work boats. They produce high torque at low rpms to drive good size props, thus consuming less fuel oil for their output power.

A human is not an engine and as jehardiman points out, output drops alot for a human once the cells begin to run aerobic (having burned all the sugars for anaerobic exercise, the "sprint").