Hi Bassman,

What you describe is called cavitation, could have a number of reasons, a too small prop, if the prop is metal, is it sharpened and balanced?
Is the motor geared down or direct drive?
Geared down you could try a larger prop or a three-bladed one, if a two-bladed is stock.

A prop that's too big will choke the motor and won't allow it to get revved up.

How fast is the boat at present?
The shape of the boat will allow air to get under the boat when run significantly over scale-speed, this in turn will result in cavitation and loss of rudder control.

Just some thoughts...

Regards, Jan.
(drummerboy)