Boat Speed Drops but Engine Speed Does Not

[FL21WAC]

My 1998 Evinrude 200 E200TXECM998 Evinrude 200-HP engine has exhibited two unusual behaviors:

On the next-to-last trip, while the boat was on plane at 31-MPH and engine speed of 4000-RPM, the boat speed decreased but the engine speed remained steady. I reduced engine speed to idle and returning to the dock. The cause appeared to be the propeller hub was spun [that is, the coupling of the propeller shaft rotation to the propeller rotation was not being maintained]. I had the propeller hub replaced and reinstalled the propeller. I suspect the original propeller hub had been [failing] for a while.

On the last trip out the boat was running on plane at 26-MPH at an engine speed of 3,500-RPM. The boat speed decreased. I reduced engine speed to idle and returned to the dock. I made a visual inspection of the the propeller hub, and it appears to me to be undamaged.

Q1: other than a spun hub, what explains the second observed condition [that is, what could cause the boat speed to decrease while the engine speed remains constant]?

ASIDE: no local outboard engine mechanics wish to work on a two-stroke-power-cycle engine

[jimh]

Q1: other than a spun hub, what explains the second observed condition [that is, what could cause the boat speed to decrease while the engine speed remains constant]?

The engine speed is measured at the engine crankshaft. If the propeller is actually rotating at the speed of the propeller shaft, which is suggested by your visual observation that the replacement propeller hub appears to be working properly, then the only other element that could cause the propeller speed of rotation to decrease relative to the engine drive shaft speed of rotation would be a failure in the gear case, such as the a severe problem with the clutch or the gears themselves.

But before investigating the gear case, you should try a different propeller.

[Jefecinco]

It is possible your propeller picked up some marine growth while the boat was on plane. When the engine speed was reduced to idle and the boat slowed the growth could have dropped off. This has happened to me but after slowing I stopped the engine and tilted the propeller out of the water to check it for damage. I removed the remaining growth and continued without further problems.

[jimh]

It is possible your propeller picked up some marine growth while the boat was on plane.

That is an excellent suggestion.

I am sure that what is meant by "marine growth" is some form of plant or stalk, like kelp or very big aquatic weeds. In that situation the propeller shaft and the propeller are still rotating at the proper speed, but the propeller is fouled and is no longer producing much effective thrust.

[goldstem]

it is almost certainly the hub. outboard gearcases don't slip like this, you would hear and feel the dog clutch popping in and out--I have been there.

If you picked up seaweed, the drag would be reflected in [the engine speed] dropping with [the boat] speed unless the propeller started ventilating. You would hear that, too.

[Jefecinco]

goldstem - My experience is different from yours. If the RPM dropped due to the propeller being fouled it was so slight as to be unnoticeable. I believe it would take a significant mass to slow the RPM of an engine at planing speed, whereas fouling the prop would result in instant slowing and dropping off plane.

[Phil T]

Q2: how is the [boat] speed measured?

Q3: could it be you were transiting a high current zone?

Q4: how do you know the proper hub is not damaged?

Q5: did you turn the prop by hand (with the engine off and the shift in gear)