loud knocking once boat gets in water

[scorpionboat]

i have a 87 bayliner with a 2.3 ltr ford and an omc lower unit. when the boat is on the trailer is starts and runs great (no noises) but when i put the boat in the water it starts making a loud tapping or knocking and looses all power. th boat on the trailer revs up good and the rpms are strong and it sounds great but in the water it will not get up to 3000 rpm, knocks or taps actually with no power. any ideas what this could be or where to start. all input would be greatly appreciated. thank you all!!!!

 

[Outsider]

Re: loud knocking once boat gets in water

You have a valve problem that reacts adversely to the back-pressure gained when you put it in the water, maybe ...