Prop slipping?

[Tom62801]

1983 Evinrude 140hp, 24' Party Barge. Motor runs fine, but boat speed seems to be limited to about 20mph. Motor accelerates up to 4000rpm and boat speed increases proportionately. If motor is pushed beyond 4000 rpm, rpms increase, but boat speed does not. I have tried a 14 X 22 stainless prop and a 13.25 X 17 aluminum prop. Both show same symptoms (motor revs past 4000, but speed does not increase). I don't think the props are slipping on the shaft, but rather loosing their grip on the water due to boat weight and motor power.

Any suggestions?

Thanks, Tom

[Daviet]

Scratch a line across the prop hub to the outside of the prop itself, run the engine until the problem happens and check the lines, if they do not line up the hub is slipping in the prop.
Seems funny that you have two props doing the same thing, but stranger things have happened.