20hp Outboard Seems Like It Is Missing...

[HarrisonM]

Hello:

I just got a used 20hp Mercury outboard for my dinghy. Starts fine but at full throttle it seems like it is missing just a little. Someone mentioned this could be cavitation related to the prop pitch... At full throttle I am on full plane and it is kicking a little bit of a rooster tail. Could it be the prop?

Any advice appreciated...

[Tin Tin]

Adjust the tilt bar in in all the way then try it. Should be no rooster tale unless engine is sitting too high on transom. You might need hydroplane fins, they make quite a difference in some applications.

[HarrisonM]

Thanks

[rslifkin]

Get one of those small inductive tachometers. You might be under-propped and hitting the rev limiter. With our dinghy, if I select a prop for best performance when loaded, it's under-propped with 1 person. Gives up about a knot of speed solo (compared to the ideal prop for 1 person) and you have to back off the throttle a bit once it's up on plane otherwise it'll hit the rev limiter.

[Franziska]

Might be a spun prop. The Nylon bushing starts turning inside the prop housing under full load.
Bit hard to detect, as it needs a fair amount of power for this...



A spun prop happens for example sometimes if a plastic bag or rope gets caught in the prop.