Puzzled
08-10-2004, 01:32 PM
Friend has a Mr. Pike 17 with a Yamaha 115HP 4 stroke, runs about 40 mph at 5900 rpm with stock 13x19 aluminum prop. Has trouble getting the bow up. He bought a used 13x19 SS prop, no name on the prop, looks to have some cupping and rake. Anyhow, he put it on, the bow came up like expected but...rpms dropped down to 5200. Top end dropped slightly to 38-39. What gives? Why would the rpms drop so much but the top end was close to original? Very confusing. Any thoughts, ideas?
Das Boot 3
08-10-2004, 01:45 PM
When you switch from aluminum to stainless it is normal to drop in pitch. The stainless is more efficient in converting HP to thrust so a lower pitch (17 or 18) would net more speed than a aluminum 19. Switch to a lower pitch to bring the RPM back and keep the lift while adding holeshot and speed.
DB3
T Mac Mt
08-10-2004, 02:12 PM
Much cupping makes a prop harder to turn. Here's an example: We might run a 19 Yamaha black SS prop on a boat and get correct rpms, but this prop leaves some to be desired in the bow-lift department. But....then we can put a 17 Michigan Rapture on the same boat and get fairly close to the same rpms but more lift and more speed because it is cupped more... despite having less pitch.
Also...with a cupped prop we can raise the engine, which also helps bow lift and speed.