Re: how to pick prop size?
Do some reading by googling "Propeller Selection", "Propeller Design", etc. Every boat, every engine, every load, every water sport, has a unique set of circumstances. Unlike a car or truck for example, a boat has no transmission so the transfer of power into pushing power for the boat is done through the prop. In general, high pitch translates to speed but less power for water sports. Low pitch translates to less speed but more towing power for water sports. If you do both with great regularity, you would likely end up with two props, one for each. The other option is to prop for the activity you do most and compromise on the other. Since we and you have no idea what you have, we can't possibly help. Go to Yamaha's web site and check Performance Tests. There you can select your engine, your boat (or one similar to it) and then see what prop they used to achieve optimun performance. Whatever you choose, the engine must reach the upper end of its maximum recommended rpm at wide open throttle with normal load. Whatever speed that achieves is what you get.
