Correct prop for an E31 with a Yanmar 3
All, Greg makes a good point as to prop calculators being only a good ball park tool. For years now, two E31 owners single hand their respective boats each summer by buddy boating off the coast of Washington and into Canada. Hull #06 is fitted with a Yanmar 3GM30F with three blade prop while hull #19 has a Yanmar 2GM20F with 3-blade prop and it was a great frustration of the owner of hull #06 to have hull #19 consistently walk away from him when under power. I happen to have close to the same set up in my E31 powered by a Yanmar 3GMF with 3-blade prop as does hull #05. Mine is 22 1/2 HP with a 30 amp alternator while his is 27 HP with a 50 or 60 amp alternator, thus the major reason for the difference in power which at the end of the day, is about the same at the prop for his boat and mine. The point of all this is that years ago I went through three iterations of my prop to zero it in at 3,700 RPM max, the factory specified RPM and his couldn't achieve that. After talking together, I gave him my prop measurements and suggested that he consult with one or two prop shops up there, Tacoma Propellor being one and I forget the other. It so happens that mine ended up at 13 X 12 1/2 but that's not important to this discussion. Armed with my numbers, he spoke to the two places. One flat out said that they couldn't or wouldn't bend a prop to 1/2" increments (I don't know enough about this science and art to know why) and the other told him that my numbers were way off and that he'd never zero in on 3,700 RPM. I asked him to trust me on this one and go with my numbers. He did and hit 3,700 smack on. I should say here before going on too much further that our engines are not designed to operate at that speed for more than 60 minutes before overheating (which they will do, don't ask) but that maximum RPM is an indicator of proper pitch and allows the safe operation for sustained periods between 3,000 and 3,400 RPM and no soot on the transom or being under pitched either. Now he can motor cruise with hull #19 and not only stay up with him but pull ahead with the increased performance of his larger engine. The point her isn't to see who can go faster, but to solve the puzzling frustration of being wrongly pitched and correcting it despite being told otherwise by the "pros" that it wouldn't work based on their calculator. As an aside, can anyone explain what's with the one shop refusing to correct in 1/2" increments? Bending is bending and I imagine if it were me, I'd just stop bending at the half way point between one inch and the next. But there must be more that this dullard doesn't get. Oh and for anyone interested, hull #19 is pitched at 13 X 12 and he can tit 3,700 RPM too. Cheers, Glyn Judson, E31 hull #55, Marina del Rey, CA