Re: prop selection advice
Props are your boat/engines contact with the water. They transfer HP into forward motion. There are sooooooo many factors that impact that interface that there is no set rule on what will work and what won't. Additionally, your boating activities may (probably will) change from one person in the boat on a fast zip around "the lake" on a cold winter's morning, to a full fledged family boatload where one or two want to ski.....couple of kiddo's on pairs of ski's, to the bro-in-law that is hitting the top of the scales and likes a slalom, or you are an off-shore enthusiast and the waves are really big and taxing everything your rig can produce.
Best bet is to start off in your 19 to 21 pitch range and experiment. Diameter is not nearly as critical as pitch, nor number of blades....lots of www info to support that. Not being a marine engineer graduated in propulsion, I can't give you the formula for pitch vs diameter for certain loads/HP marriages/prop types, pitches/# of blades.
Prop selection really needs to be such that at full rated RPM's, with the load that you normally carry usually in the summer time when the engine can't develop it's maximum HP due to thin, dry air, the engine needs to be at or near the top end of the rated rpm's.....and sometimes over....I have run Mercs for years and have run them 500 rpm's over the top end and they just sing and want more......when light loaded, not loaded "for bear", i.e. lugging.
Other thing is porting. You can buy a ported (little holes drilled in the barrel to allow exhaust gasses to blow across the blades at low speeds allowing the rpm's to develop early) SS prop that give you a great hole shot due to the engine's ability to ventilate and "rev up" on hole shot, with a heavy load (bro-in-law slalom and all) then settle down at top speed and give you the top end that you want.
Best bet is to buy cheap and experiment, or pay a little more and buy from a seller that will allow you to swap props after testing........if you don't damage them. Then you decide if one prop will do or you need several for your changing environments.
Good Luck,
Mark
