Slipping prop ?

[Bob Kimber]

Hi everyone, just looking for some opinions.<br /><br />2000 model 90hp Merc runs perfectly, but last few trips maximum indicated RPM at WOT (only run for very short periods) is gradually increasing (now up by approx 700 RPM). Maximum boat speed is unchanged, verified by GPS but now occurs at the increased RPM, initialy suspected faulty tachometer but now thinking about prop.<br /><br />* prop is 20 yr old Mercury S/S 13-3/4" x 15" pitch from previous motor, 90hp inline six merc, has never been rebushed. <br /><br />* performance from standing start to maximum speed seems normal (no slippage obvious)<br /><br />* I have cleaned prop hub and center bush and marked alignment marks to see if there is any obvious slippage on next trip (may be a while)<br /><br />* I can get tach calibration checked, and I have the original aluminium prop (same size) from old motor, which I can try.<br /><br /> Is it possible for some slippage to occur only at high speed causing motor to over rev? Any suggestions would be welcome.<br /><br />Bob

 

[DJ]

Re: Slipping prop ?

Bob,<br /><br />You're on the right trail.

 

[Bob Kimber]

Re: Slipping prop ?

Hi djohns19<br />Thanks for your reply, I hope to run motor in a test tank in the next few days (too windy to sea test), with a bit of luck should be able to check tach with test gear and maybe see if prop shows any slippage in tank.<br /><br />Bob

 

[Bob Kimber]

Re: Slipping prop ?

^ bump

 

[Bob_VT]

Re: Slipping prop ?

I doubt a "water tank" test will build adaquate resistance to produce slippage. I think it will really have to be tested in real conditions.<br /><br />Bob