cooltoys61

This should be easy. Where do you want your rpm for top speed? I would think right at peak torque. I read somewhere it should be after that but considerably before peak HP. TQ gains the MPH right? Another way to put it,,can you go any faster with 1 Ft/lb less torque but more hp?

SeaRay Jim

Top speed? At peak HP. Top speed is generlaly a short term thing and you have a motor that makes a certain amount of energy and you want to use it for peak performance.

Ideal criuise speed, peak torque. The motor shouldnt' be laboring as much and you should be able to cruise all day.

Somethiing like that.

Sean H

well, we all prop to turn the motors has hard as we can for top speed, so i guess we ain't propping for torque.... cruise speed and best fuel mpg you would want to travel at peak torque or peak effiency.... but speed comes from rpm's.....

articfriends

iTrader: (1)

Prop for hp peak,your torque peak will probably be close to your cruising speed anyways. If you had a motor that hp peaked at 5400 and pulled a 24 pitch prop and you tryed to turn a 30 pitch at 4200 your motor would be under a tremendous load,detonating,loading up etc. Plus if you were truly propped for "peak torque",you'd be at wot at that peak which again would make your motor act sick plus you'd have little acceleration and a real hard time getting boat on plane,Smitty

Harper220

Top speed will come when propped for top HP, I do know that. But, how would you know if you are propped best for peak torque and gain more mid-range speed while maintaining top end?

throttleup

Peak HP for top end, HP is a measure of the amount of work the motor can produce. TQ is the twisting force, acceleration. HP is speed and TQ is acceleration.

Propping for peak TQ will cause a variety of problems not only for the motor and drive train but will cause ill handling of the boat as well.

Matt

Edward R. Cozzi

If you look at the power curve the "sweet spot" or ideal cruising RPM is lower than the WOT RPM and will give better economy and less wear-and-tear for extended running periods.
Prop the boat to run at WOT in the recommended RPM range for that engine and throttle back to that ideal cruising RPM close to where the torque and horsepower lines cross. You'll feel the boat "settle-in" to its correct mode.

44MTI

It is all about compromise. If you want top end you want the tallest prop your motors can pull up to your max rpm. However this usually takes a while to get to and you will lose accelertion in the mid range. Most peopl will prop just slightly short of this 1-2 pitches which will give of a couple of mph on the top end but give you much better acceleration, which is what we use most IMO.
How many times have you really stretched you boat out, I mean way out like 10 plus miles WTO. Then how many times have you been running 50 -70 beside someone and hit the gas for a mile or so.

vette131

try to prop it at least 1000rpm's above peak torque.

mcollinstn

Top speed is found when propped at peak HP (as installed with drive losses and marine exhausts).

Best and most efficient cruise is at torque peak, although a particular hull/power/setup combination may actually prefer a little more or less rpm for best cruise.

Torque and hp relationship is NOT nearly as confusing as many make it out to be.

Torque is twisting force. Simple enough. No more no less. Think of twisting force as pulling on a rope tied to a box sitting on the floor.. It may or may not be enough to drag the box (or turn the shaft) but it is still a measure of FORCE generated. No work is being done, however, until the box begins to move.

Power (horsepower, or other measure), is the amount of twisting force that is delivered in a certain period of time (or the distance you drag the box across the floor in a certain amunt of time).

Since torque is the twisting force on a prop, isn't more better? Yep. If all else remains the same.

But which of these is better?

400 ft-lbs at 100 rpm?
or
400 ft-lbs at 1000 rpm?

Obviously, the second one.

Okay, now a little twist.

How bout which one of these is "better"?

400 ft-lbs @ 5,000 rpm?
or
200 ft-lbs @ 10,000 rpm?

Answer? They are the same. You may have a reason for choosing one over the other, but they will both produce exactly the same amount of power at the listed rpm for each one.

You have to examine the torque curve of a motor to get feel for how to prop it for a particular boating application. Depends on usage and needs.