[C320-list] Engine noise

[mike hunter]

Does prop cavitation cause a “knocking” sound?  While motor-sailing out of the bay last weekend I heard an occasional knock when the boat rocked going over other boat’s wakes (@ 2800 RPMs).  At the end of the day, and in very little wind, we dropped our sails and cranked the motor back up to around 2800 and noticed a constant knocking.  We dropped down to 2200 and the knocking went away.  There was no vibration, just the knock.  What’s strange is there was no noise / vibration when the boat is heeled over slightly with the main up and motoring.
   
  I read this on another site: “Tip noise comes mostly from having insufficient blade tip clearance. A propeller actually works just like a sail, creating lift on the leading edge, which "pulls" the propeller forward. The blade tip has a tip vortice just like any airfoil or hydrofoil - and this excess energy is swept away by the passing water. However, if the tip is too close to the hull, the energy will "slap" the hull and make a real "knock."  The distance from the tip of the blade to the hull should be 0.15 to 0.2 times the blade diameter. If it is less than that, you will almost always experience knock and vibration. Have your teeth stopped chattering now? Good.”
  I believe it is the original 2-bladed prop???  
   
  Has anyone else experienced this?
   
  Thanks!
Mike        

jonvez at comcast.net wrote:  Jim,

Could be cavitation from the prop, which can happen in rough seas or if the boat is outrunning the rpms. If you have a PSS, it can chatter when bouncing around sometimes. I've had this happen and have done what you did...ease back the throttle, make sure everything else is okay and continue....

Regards,

Jon Vez

-------------- Original message -------------- 
From: "Jim DeLong" 

> Wondered if anyone else has experienced this engine noise? 
> 
> While in the ocean a couple of days ago, trying to outrun a squall, a crew 
> member cranked up the RPM's to 3000. More that I'm normally confortabe with. 
> 
> After 15 minutes or so, the normally smooth engine (Yanmar 27 HP) began to 
> sound like metal grinding. Pulled the RPM's bact to 2200 and all was well. 
> Cranked it up again and after 15 minutes it started again. No other symptoms 
> at all. Ran well but just sounded funky. In port yeaterday we started the 
> engine and ran it up to 3000. Could not get it to do it again. 
> 
> Thought it may be from the transmission or a slipping shaft, but the prop 
> seemed to turn just fine. 
> 
> Anyone experience this one? 
> 
> Thanks, 
> Jim DeLong #453 
> 
> 


 
---------------------------------
Don't get soaked.  Take a quick peak at the forecast 
 with theYahoo! Search weather shortcut.