Cavitation Plate and Clunking Noise

[undertaker]

I have a Mercury 15HP outboard with a long shaft. I have a 16' G3 jonboat with a 16" transom to which I've added a 4" mounting plate to raise the motor height. The cavitation plate now sits about 2-3" below the bottom(flat)of the boat. Will this cause any problems since it's not level with the bottom of the hull? I asked the marina where<br />I bought the motor and they said it should be fine and to try it out to see the performance. What are the consequences if the cavitation plate is too high or low? Also, when I first start the motor, it runs fine and then I hear a "clunk", like it's going into gear but I know it's not in gear( water in tank not churning). Any help is appreciated!!!

 

[12Footer]

Re: Cavitation Plate and Clunking Noise

If the cavitation plate is too high, the prop will cavitate,(sound like a hydroplane just a-geeting it). This condition is bad for the prop hub, and puts undue strain on the lower unit.<br />If the cav plate is too low, the boat wont plane properly,and drink mass quantities (of fuel). <br />Best to be too low than too high. Being low wont hurt,unless you hit a sandbar

<br />2-3" below the keel is fine.<br />As for the clunk...Clunks are not good. It could have been a chunk of metal picked up by the gear train. Or maybee something hard (rock,rust chunk or shell) going thru the impeller. This would've led to an overheating problem tho. <br />You might have a bad bearing on the driveshaft or at the powerhead spline.<br />First thing to check is the condition of the lower unit lube. Drain it into a pan,run your finger thru the old lube, and feel for chunks larger than a grain of sand. And the color. If it's gray or streaky,or pure water runs out, you have a bad seal(s), and possable gear damage.

 

[undertaker]

Re: Cavitation Plate and Clunking Noise

Thanks for the help 12-footer......