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View Full Version : Gearing questions in general



cfkingfish
05-18-2004, 11:10 PM
I was looking at outdrives, notably the Alpha and Bravo One, and I just wanted to ask how I would find what gearing is best for my application. Is the gearing similar to car differentials, where the higher ratio, the faster it will be off the line but it will top out at a lesser speed, and vice versa? Will it also determine what size prop you need? Thanks again.

-Christian

Ranman
05-19-2004, 09:24 AM
An outdrive is geared to match the HP and weight of a boat to an available prop selection.

Available props range from around 17 inches of pitch to about 32 inches of pitch with the most common being 21 - 25.

MOP
05-19-2004, 10:00 AM
Randy is 100% right! But something I have found out, the higher the ratio (say 1.36) running closer to engine speed lesser pitched props can be used. This is important to guys that run a lot of power as the run out of options and prices get crazy. Nate just changed his A drive to give him more prop options. The lower ratio (1.50 and above) need to run steep props when powered up or go to 4 or more blades. I know some will argue the point but the high 1.36 ratio drives are a bit stronger due to gear contact area. Another point lesser pitched do not paddle wheel as bad giving less steering toque.

3 cents

Phil

Rootsy
05-19-2004, 12:40 PM
HP in equals HP out (minus frictional, thermal and inertial losses of course)... by changing the gear ratio you decrease or increase the torque at the prop but in order to mathematically satisfy the equation shaft speed must increase or decrease since HP is a function of torque and rpm.

takes torque to spin a prop... less torque you have at the prop shaft the less pitch yuo can spin at a given rpm. by changing gear ratios you change the torque output by altering the multiplication by the gearing...

this is why drives with higher gear ratios can handle larger hp before they die... i'll take more rpm in a good gear train than torque.. torque is the nasty little carrier of death... once she makes the turn from the input shaft and gear it is decreasing from there in a "higher" ratio drive such a 1.36:1 vs say a 1.5:1... as shaft, bearing and gear speed increase... so make sure you can keep her all COOL and use GOOD gear lube.

then there is the whole prop efficiency theory of small pitch vs big pitch and slip and climbing and yadda yadda yadda...

*DISCLAIMER* as Rabid Poodle so politely pointed out to my alzheimer ridden arse once upon a time... TRS' have oil pumps inside taht don't like high rpm... so don't try this at home kids...