Here's one that works for me. During the winter, I pack plastic drop cloths around the motor well to keep the cats out, but during warm weather, I don't want to mess with it. Our barn cat climbs in under the cover and leaves hair everywhere, and clawed one of my seats a couple of years ago. I got a motion detector light fixture for 6 or 7 dollars and an adapter to convert the light socket to a standard AC plug. Put a cord on the light fixture and plug a radio into the light fixture/plug adapter. Anything tries to crawl under the cover, or climb on the cover and the radio comes on. LOUD http://www.bassboatcentral.com/smileys/Laugh.gif The cat thinks that boat is after him and he keeps looking at it like it's going to pounce on him any second. http://xs-s.com/zf/images/smile/emthup.gif
