High RPM slippage?

[eliteeurocars]

Took my boat out today (18' with a 88 Johnson) and under acceleration as boat starts to come up on plane motor revs excessively and boat will not pick up any more speed. If I returtn to idle and try it again I am able to get the boat to run around 4000 rpm's but any more than that and it seems like prop is slipping. Is this possible? Thanks in advance.

[boats.net]

Probably has prop hub slipping, need to have prop shop replace hub in prop.