Need help, 15HP Evinrude, cavitation??

I have a mid 90's 15HP Evinrude on a 1648 Jon. It has been on my boat for a year now. I have put about 15 gallons of fuel through it so far with no problems. It pushes it about 15 MPH and I have been very happy. Yesterday, going across the lake in 40 foot of water, full throttle, it suddenly came a quick drop in speed as the RPM stayed steady. I was able to navigate it to the boat ramp at low speed but each time I would try to full throttle it would seemed to "cavitate" on me. Since I know that the motor cavitation plate is set correctly for this boat, what is going on?? I have read once that a prop could spin out?? My question is what is the best way to check for this, or anything else I can check, thanks in advance

A quick way to check is to put your motor in gear (while off of course) and try and spin the prop by hand. Usually if the prop is spun then you will be able to spin it by hand without much force. I would get the prop hub wet before hand also, this will act as a lube just like when in the water. When it is dried out the rubber may grip harder making it seem like the prop is not spun.

Holding one hand on the flywheel so the motor does not turn over while trying to turn the prop can also help. But you don't want to get carried away cranking on the prop and holding the flywheel.

Thanks, I tried that once I got out of the lake for 1-2 turns and it seemed to turn the motor. I will try again and pay more attention.

A second way to check for this is to simply mark the hub and propeller with a marker or paint and run it. The marks will tell you if the hub is turning.
By chance when the boat slowed and the RPMs stayed the same did you come off plane, go to reverse, and clean the weeds off the prop and then see if that cured the issue? May be all that was wrong.

I held the fly wheel and can not turn the prop when its in foward or reverse. Question, when in fowrd or reverse I can turn the prop about one inch ( play ) in clockwise or counter clockwise before I feel it engage or have resistance, Is this normal? Pappy, this lake has no weeds, very clean lake but not to say i could of hit some trash.

If this was a mechanical problem what could I be experiencing? I had no unusual noise or vibration. It shifts fine from forward through neutral to reverse.

It still could be a spun prop.......... And I really hope that is what it is. If it is not the prop and the engine is revving up and not going anywhere then gears could be striped but you would have heard and felt that for sure. I would take the prop to a prop shop first and have it tested. Even though it did not spin by hand it could still be spun. Try Pappy's idea by marking the hub and running it.

another vote for spn prop. mark and see what happens.

Check the gearcase oil and see what it looks like then just go run it again and see. If it was just weeds it will be fine and if not the marks will have moved. Nothing ventured nothing gained!

Thanks guys, I will mark the hub and prop and look for the simple first. I will post as soon as I try it.

Marked the hub and prop , went to the lake and it runs fine. I can only assume I hit some trash like a plastic bag. I needed some good news! Thanks who helped