This is actually a serious query, but I hope I've got your attention! The problem is my outboard propeller is behaving in ways I don't understand, and not giving the results I should get from the engine.
The basic problem is that the propeller is not "gripping" the water, or in other terms I'm getting massive prop slip - 50% or more.
The engine is a Tohatsu 30HP, optimum rev range at WOT is 5250 - 6250, gear ratio is 2.17, and is mounted on a Mac 19. I've fitted a 9" pitch prop, which should be good for 15-18 kts without ballast. At 18 the slip would be 10%. In practice the max is 11 kts, with the engine spinning at 5750. That's the ideal revs, but the prop slip is around 45%. That makes no sense to me.
Something else that seems odd is that if you open up the engine to WOT with the boat stationary, the revs hit 5750 immediately, before the boat's even started to move. It's as if the prop's just not getting any traction in the water.
Some things I've read suggest that I need a prop with a bigger blade area, either larger blades, and/or a 4-blade. I wouldn't have thought increasing the pitch would do much, it would either overload the engine or lead to even more slip.
I think the prop height is OK, I've attached a photo of the motor fully down, taken at the level of the anti-cavitation plate. It's definitely well into the water. The plate is below the bottom of the hull - can a prop be too deep?
I'd appreciate any thoughts from the technically minded, and those with experience of messing with props. I'm tempted to go for a stainless 4-blade, but it would be expensive, and a lot of wasted money if it's not the answer!
Thanks in advance.
Rick Jones (Treasurer) 1994 Mac 19 "White Lightning", Isle of Wight