Re: prop for a mercury 90
Thanks Walleye.<br /><br />Agree that this is a 2 cycle. The only things that move, can move, are the inlet reed valves which is nothing more than a thin sheet of metal, vibrating back and forth like when the piston sucks on it during the intake half stroke then turns it loose for for the compression phase, feeding fuel to the engine, and the rings expanding in the cylinders, on loopers with wedge shaped rings. <br /><br />Enerything else is tied tight on needle bearings which don't require much oil to stay slick. Not like 4 cycles with cams and push rods, and/or valve train drive chains and all.<br /><br />Back when I was a kid, we used to race the Kiekhaeffer Merc 25 hp 4 cyl -had a green clamshell cowling- and later the Mark 55 rated at 40 hp. We'd take a stock powerhead, put on a Merc racing lower unit with a tiny little high rake/pitch 2 blade prop, put it on a 13' hydroplane and go, baby go. To get on plane we squeezed the throttle and just shot water 50 feet behind the boat. Finally the prop would eat enough water to get the boat moving and next thing you know it was white spray mist and bye bye blackbird.<br /><br />You don't want to know how fast these engines were revving. <br /><br />I have a thought about engine ratings and rpm's are a part of the horsepower equation. So if a mfgr has a lineup of engines, he has to rate them at some hp, so that has to be done at a certain rpm. Otherwise the delivered/developed hp is more or less. <br /><br />Since he may make a 75hp and 90 from the same power head, all he has to do is increase the fuel supply which allows for more rpm's under the same load and baby you now have a larger engine.<br /><br />Take my 90. Same powerhead as the 75, with 85 cu in, but the 75 is rated at 5250 max and mine at 5500. They just put bigger jets in the carb to develop more power under the same load and get more rpm's.<br /><br />When's the last time you saw/heard of a 2 cycle outboard blowing up as a result of overrevving?<br /><br />I know they used to put overrev kill switches on engines, but that was based upon the lower unit kicking up and an anticipated overrev condition, not by actually measuring a certain rpm and shutting her down......had no way to do it back then before the high-tech electronics age. The last 2 engines I bought didn't have them so they must have figured it a waste of time.<br /><br />Oh well, enough boring you with my ramblings.<br /><br />Have a great holiday.<br /><br />Mark
<br />BTW, I have an 18ft superfisherman Starcraft with a 200 Rude...I can spin in the upper 6k range if I want.5750 Recommended but about 90% of it's 400+ hours this year were right at, or real close to 6k..these are 2-strokes, not cars...