M35BC Engine Vibration

[wingman]

Was out for a shake down sail this week. Forecast was for 5mph but we got a nice 15-20kts instead and had a chance to knock the rust off boat and crew and try out all the new running rigging. First "fun" sail in about a year.

Motoring back in the marina channel at 3 kts, we were surprised by a brief (2-3 sec) but loud vibration from the M35BC or propulsion system and throttled back immediately.

It appeared to be momentary, there was a longish puff of white smoke or steam, and then engine ran normally back to the slip.

I'm not going to over-react to a burp, but on the other hand I'm always listening for potential issues.

Leading theories:
1. The prop picked up something but quickly freed itself
2. An air bubble or other temporary contamination in the fuel system. Boat recently splashed after a winter on the hard and we did bounce around a bit that day.
3. Something related to work over the winter including PSS dripless shaft seal maintenance and damper plate replacement (3 hours of motoring before this, so less likely)
4. Someone suggested cavitation caused by following seas (way down on the list)
5. ?

[waughoo]

I immediately went to the prop picking something up and then discarding it as you throttled down.  Next would likely be the fuel having a small contaminate in it perhaps at an individual injecter.  The white smoke is curious though.  Might lead more towards a single injecter issue more than the prop theory.

[Ron Hill]

Wing : A LOUD? vibration.  Bang? clang? and puff of white smoke.

White smoke is unburnt fuel usual cause 1. Poor atomization (injector) 2. Poor compression (yours was short time 2-3 sec) 3. Water in fuel (yours was NOT continuous)

I'll guess that you had a sticky injector because it was a short time only a second or two.  Could have been compounded by a short slug of water? all at the same time.

I'd probably change your Racor for the new season and fill it with some PS diesel injector cleaner (best price WalMart).  That should clean out and lube your injection system.  Also might consider some Water Absorber for your fuel. 

A few thoughts 

[wingman]

Ron, I wasn't at the helm but loud enough to get my attention, more like a vibration.

Was out again yesterday, motored for about an hour, nothing unusual.

[Robert Mann]

My bet is it was a steam puff caused by water in the fuel.  I had the same issue when an Ericson I owned had water enter the tank because the o ring on the deck fill perished and we had snow in Georgia, it found it's way into the tank.  If you see it again you might think about cleaning the tank out.  (my tank had a really good slug of water and that was the path I took)