Woodnaut
Chief Petty Officer
- Joined
- Jul 4, 2007
- Messages
- 634
OK guys, here's the story - 16' 7" Deep Vee runabout, homebuilt (stout and heavy) with a 2006 Yamaha 90 HP four stroke. Currently has a 12 7/8" Dia x 19" Pitch 3 blade aluminum prop (with no cup) that gives a WOT of 5,800 RPM. I?m totally satisfied with this setup - it's really just about perfect.
Now comes the problem. For a spare, I decided to try a 12 3/4" Dia. x 18" 4 blade aluminum. This particular prop had a double cup. It provided a WOT of 5,200 RPM, and the engine soon started "making oil". Not good at all, so that prop came off of the boat pretty quick.
I took the prop it into a local (and reputable) prop shop here in Houston and had it pitched to 17" with no cup. Now I get a WOT of 6,100 RPM and a very strange start-up phenomena. When I get on the throttle I can feel and hear what sounds almost like "slippage" occurring. After its up on a plane and speed starts to increase, it feels and sounds like the prop suddenly bites into the water better - I can hear the engine load down a bit and the speed continues to rise. It is a sharp and very distinct change. In other words, it?s almost like there is a small amount of cavitation occurring when starting up, but then it suddenly stops. I certainly did not expect this out of a four blade prop - I was expecting less slippage if anything.
At 6,100 RPM the WOT is a little too high and, as I mentioned, the startup characteristics are squirrelly. I am debating whether to keep the prop at 17" pitch and have it "double cupped" or bring it back to 18" pitch with no cup. Either one would lower the WOT RPM and hopefully resolve the goofy startup phenomena. What do you guys think?
Now comes the problem. For a spare, I decided to try a 12 3/4" Dia. x 18" 4 blade aluminum. This particular prop had a double cup. It provided a WOT of 5,200 RPM, and the engine soon started "making oil". Not good at all, so that prop came off of the boat pretty quick.
I took the prop it into a local (and reputable) prop shop here in Houston and had it pitched to 17" with no cup. Now I get a WOT of 6,100 RPM and a very strange start-up phenomena. When I get on the throttle I can feel and hear what sounds almost like "slippage" occurring. After its up on a plane and speed starts to increase, it feels and sounds like the prop suddenly bites into the water better - I can hear the engine load down a bit and the speed continues to rise. It is a sharp and very distinct change. In other words, it?s almost like there is a small amount of cavitation occurring when starting up, but then it suddenly stops. I certainly did not expect this out of a four blade prop - I was expecting less slippage if anything.
At 6,100 RPM the WOT is a little too high and, as I mentioned, the startup characteristics are squirrelly. I am debating whether to keep the prop at 17" pitch and have it "double cupped" or bring it back to 18" pitch with no cup. Either one would lower the WOT RPM and hopefully resolve the goofy startup phenomena. What do you guys think?
