Re: changing transmission gearing
You did not indicate what performance issue you are trying to solve but I suspect whatever money you spend will be a waste. You are changing two elements of the drive system to put you back to where you were in the first place. When your boat was built, the manufacturer selected a gear set that matched the power output of the engine. There is logic in that to swing a larger diameter prop with more pitch you need to increase the gear ratio. This speeds up the engine but it does not automatically translate into a faster or more economical boat. With the 10P prop you currently have, each revolution of the engine "theoretically" moves the boat forward 10 inches. So at 4500 engine rpm (ignoring prop slippage) the boat would move forward 3750 feet. That translates to about 42 miles/hour with the 1:1 drive ratio. At 4500 RPM, a 15P prop at a 1.5:1 ratio would mean that at 4500 RPM the actual prop speed is 3425 RPM. 3425 prop rpm translates to 48 miles/hour. So theoretically you would pick up eight MPH at 4500 RPM. While that may seem a logical thing to do, there is the little caviat called available power and I suspect the engine would not be able to twist a 16 inch 15P prop. If you actually did this and did pick up eight MPH, I would wager hole shot would be so terrible that you would beg someone to put it back the way it was. There is also the situation that while speed may be slightly better than you had before, the engine would be lugging and not able to get to it's recommended WOT range and therefore economy would be worse than before. So you see, there is no free lunch when it comes to gearing and props. You need to trust that the manufacturer knew what they were doing when they matched the gearbox, engine and prop.
