measuring outdrive HP loss question

[d1mbu1b]

I saw mention of outdrive HP loss in the 496 build up sticky.

Is it possible to put a torque wrench on the outdrive drive shaft and measure the torque required to spin it?
Measure it while its spinning to get an indication of the sliding and not static friction.

Would the HP loss at 5250 RPM equal the torque wrench measurement in ft-lbs?

The only thing I can think of to disprove this would be the effects of fluid viscosity at temperature, and the hydraulic lock up affect of the gear oil at high RPM.



If this holds as a reasonable model, maybe by bypassing the crank pulley you can also measure the HP loss of the power accessories as well.


Since the weather finally broke here in Melbourne, I will be getting back on my motor. I am pulling the out drive and motor once again. I'll measure it and see. If I recall it seems like I could spin it by hand which is probably only 5-10 ft-lbs, so its probably not going to work. Maybe it will be a good comparison from drive to drive and not for absolute measurement.

Any thoughts?

[Morgan's Cloud]

Not too long ago there was a thread on this on that 'other' site.

If I recall correctly the loss is a percentage of the engine power that varies from engine to engine not a fixed figure for a given outdrive.

[Carl C]

I don't think that would work because you would not be turning it fast enough. The Bravo drive takes 10% of the total hp.

[VetteLT193]

I think you would have to know the exact amount of force your arm put into the wrench in order to get the ratio.

[MOP]

The forces change greatly as RPM increases so a torque wrench on the input shaft for that reason would not work. But one very important thing that very few do is to have the drives rolling torque checked this is especially important on the higher HP boats they should do it each and every season. If the rolling torque drops out of spec it is time to re-shim the drive.

Phil

[gcarter]

I worked in a lab one time where they had some "certified" electric motors, i.e., the plate HP was certified to be accurate to a very small percentage. Anyway, you could load one of these motors (at 3600 RPM), read the amperage and calculate the HP generated under those conditions.
I suppose ther's no reason you couldn't drive a drive (again at 3600 RPM) and be able to calculate the power required to do it........

[oldandtired]

No way that a static rolling torque value can give you an even accurate number. The torque required to turn the drive in a static condition like that, assuming that the input shaft was pulled from the coupler and the drive was in gear, would be around 1 ft*lb. Since hp=torque*rpm/5252 hp=1*(5250/1.5)/5252=.66hp. Drive losses are a function of gear efficiency and friction. They will be a function of torque applied through the drive in the end. The only way to really measure it is to take a direct torque reading on the engine with some kind of load cell and measure actual loaded propeller hp with a prop dyno of some kind. The difference is the drive loss. I would rather assume the 10%.

Rene