Re: Gear Ratio Info.
Gear reduction multiplies the torque of a propshaft but makes it rotate slower than the motor, it does not "make" horsepower as some folks think. Actually, friction losses thru gearing lowers the engine horsepower at the shaft. According to Mercury Marine, the higher torque allows for a larger diameter, slower turning prop which is more efficient than a smaller diameter faster turning prop. They say you will have better acceleration and top speed, up to the point where the drag of a larger gearcase exceeds the benefit of prop efficiency. They are talking about popular recreational boats, not tug boats, off shore racers, or unlimited hydroplanes which have their own unique applications and designs.<br /><br />An example is a 1968 85hp Johnson/Evinrude that used a small diameter bullet nosed gearcase with a very small reduction, close to 1:1. (I don't remember the exact ratio) It turned a 10 X 9 1/4 prop on most runabouts for a speed of around 30 mph. The next year, 1969, Johnson/Evinrude came out with the thru hub exhaust, geared down, larger diameter gearcase that used a 13 1/4 X 17 prop and ran about 10mph faster with much better acceleration and skiing ability on the same boat. Both motors put out 85hp at 5000 rpm but performed completely different due to prop size and gearing. <br /><br />Merc may still publish a good primer on propellers called "Everything You Wanted to Know about Propellers". I have early copies from 1982 and 1993. The part number is 90-86144. Ask a dealer if it is still available.<br /><br />OMC, years back, published the R.P.M. System booklet and also gave out prop theory books when a tech attended factory training.<br /><br />There are plenty of gear design and propeller design books available thru college bookstores, used book stores, and maybe even on Ebay, but that may be a bit of overkill, unless you are really into the esoteric areas of propulsion.
