i think my prop is cavitating but not sure. can anyone please discribe the symptums? today was the first the engine has been in the water in 25 years. i had it gone over and ran in a tank and everything was good. when i get on it, she throttles right up and really rips but all of a sudden the boat will loose headway and the engine revs high. like it's jumping out of gear. no banging or knocking sounds. i was told that if it where the clutch dogs there would be banging.
Does the gearshift handle actually go to neutral when that happens? That would be the clutch dog. However, if it is merely slipping, probably the prop hub. You can tell by a simple test. Make a mark on the prop nut and another mark on the propeller, aligned with the first one. Run the motor till the suspected slippage occurs, then stop the motor and check your marks. If they no longer align, it slipped. If they do align, repeat the test because there is a slight chance it stopped slipping in the same spot.
EDIT: If it passes the hub test, it very well may be what you suggest---cavitation. Is the motor plenty deep in the water? That motor was a 20 year old design when built and was designed for wooden boats with a keel. The cavitation plate needs to be about an inch below the boat bottom. It's rare that you can get away with jacking it up like newer motors. Also, do you have floating weeds where you boat? Those things will make it cavitate in a heartbeat.
Somebody will come along and point out that it is ventilation, not cavitation. Hey, Ole Evinrude called it cavitation, that's good enough for me.
Yes, the hub can look OK, but still slip. Use a scribe and make a line on the hub and a mark on the prop itself. Put the prop back on and run it, when it slips, remove the prop and see if the marks still line up. If they don't, take your prop to a prop repair shop and have a new hub installed.
An extremely well equipped Evinrude dealer may have the propeller testing fixture. But it is rather doubtful unless he has been in business a long time. Even then, doubtful.
Boat in the water is the best way to test it. Prop shop is second best way, but remember he wants to sell you a new one. Trying to turn it by hand or your foot is useless----no matter how strong you are, you aren't as strong as 25 horses.
I just went through the exact same thing with a 1972 Evinrude 25 HP that had been stored for 20+ years. Took prop to a shop and had a new hub pressed in. Works like a champ now. As a side note, I also wound up rebuilding the carb and fuel pump, installed new head gasket, thermostat. plugs and impeller. Each of these problems revealed themselves during several trips to the river. Finally have all the bugs worked out and she is rock solid again.