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523 Posts
Would bet you a beer that your prop has a lot of growth on it. When it starts cavitating, you are effectively losing your 'bite' on the water and spinning the prop in air bubbles as a result of the growth, which is why the engine is accelerating but your speed isn't. The cavitation is also the source of the vibration you're reporting. Usually, reducing throttle speed will help, but you really just need to clean your prop and she'll be good as new. Ignore it and you'll soon feel as though your prop is missing. This time of year, growth is quick to accumulate. Do it yourself or hire a diver -- git 'er done and get back out sailing. 
