Prop stalling engine?

RobbieW

RobbieW

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On passage the other day in a quartering sea of about 1m with a period of 3-4 seconds we experienced something that felt as if the engine was being stalled. This happened perhaps a dozen times in about 6 hours until the sea subsided a little. As the boat rolled with a wave going under us, the engine note sounded as if it stopped for a second or so and then carried on - most disconcerting.

I'm trying to understand what was going on and what I should now be having a close look at. We have a Brunton Autoprop so one of the few theories I have is that the speed of the wave passing under was sufficient to try to reverse the prop blades and try to stall the engine - feasible?? The roll when this occurred was away from the exhaust so I dont think that back pressure was involved.

I will certainly be having a close look at the plastic drive coupling on the shaft and the engine mounts, I'll also have a feel of the prop blades next time I dive. Anything else ?
 
Waves more or less go up and down rather than along so should not have much impact on the prop. Depth may have an impact on cavitation, if you were getting any.

My guess is that the exhaust outlet went underwater, making it sound very quiet as the wave came through.
 
We have an Autoprop and although we don't get the symptoms you do, in similar conditions it does do odd things. Despite having an Aquadrive we think that the shaft is displaced sufficiently for it to hit the stern tube occasionally, causing a machine-gun type rattle. The Autoprop is a heavy propeller, on a fairly slender shaft, and I think that at speeds close to its 1st critical the 'tail can wag the dog', causing the shaft to bend briefly. It has been like it for over ten years now, so no damage seems to be occurring. I may fit a small steady bearing at the forward end of the stern tube to eliminate the noise.
 
It could have just been dirt in the fuel system. Worth a check of the filters to make sure they are all clear. Rough weather can cause sediment to get stired up.

Yoda
 
Thanks for the thoughts so far...

Apart from the engine sounding as if it had stopped, for about 1 sec tho it felt like 30, there was no change in revs. One instance was certainly with the boat rolled, we were rolling through something like 60 degrees, away from the exhaust - which is on the waterline anyway. It could have been cavitation, its almost impossible to know whether the prop was clearing the water; it was a short sharp chop, kind of an exaggerated Solent chop. I dont think its going to be dirt because the engine ran about another 30hrs without missing a beat, the bowl is clean too.

Its also possible that we ran over some debris and the symptoms we got were the Stripper doing its job, no sign of that on visual inspection though.

The good news is that a visual inspection of the drive couplings, engine mounts and physically waggling the prop blades shows nothing of obvious concern. Just called Bruntons who had no quick answer but the product manager wasnt available, the chap I spoke to said he'd have a chat and get back to me.
 
The reason I asked about the rev Robbie is because we had a very similar problem on the way to Tunisia , it sounder as if the engine was cutting out but the rev wasn't changing ,but for sure a change in the engine sounds only in rough seas , it turned out to be dirty on the pick up line in the tank . This is not the say you have he same problem but if it isn't too much of a problem it might be worth just checking it out .

www.bluewatersailorcroatia.webs.com
 
Mine does the same in a heavy following sea (who is so naive as to think waves only go up and down).
Sequence as follows:- Wave surfs boat, Autoprop coarsens, wave leaves boat and Autoprop cavitates, governor shuts revs down. Next wave comes along, prop more submerged, engine struggles, finally recovering - then it's all repeated.
Answer is to find the right rpm for the sea state or (best of all, sail). Problem is exacerbated if the exhaust outlet is on the stern and covered by the top of the wave
I doubt you have a mechanical problem, suggest observing the phenomenon, for longer and find many of the theories outlined as amusing rather than probable. However there can be no doubt that a boisterous sea stirs up your tank detritus and, especially with a low level of fuel will lead to fuel starvation - the best way to correct that is to have the tank out and steam-cleaned. It's happened to me with 3 squeaky-clean in-line filters.
I've had my Autoprop for 23 years, done about 10K hours on 2 engines and, whilst uncommon in N Eiuropean waters have found it happens fairly frequently in the Med with its shorter, sharper seas.
 
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Thanks Charles; the passage was in the Med, Corfu to Siracusa, and the seas definitely short and sharp! Not quite enough wind to be useful with the rolling induced by the waves, the sails were mostly banging - probably should have changed destination but one does get fixated about getting where you set out to go :)

Vic, I will take the top off that tank again and have a look and feel - I know it was clear at the start of the season.
 
RobbieW said:
Thanks Charles; the passage was in the Med, Corfu to Siracusa, and the seas definitely short and sharp! Not quite enough wind to be useful with the rolling induced by the waves, the sails were mostly banging - probably should have changed destination but one does get fixated about getting where you set out to go :)

Vic, I will take the top off that tank again and have a look and feel - I know it was clear at the start of the season.
Click to expand...

It's the little bits of tar, rolling about in the bottom of the tank, that get into the up-pipe and jam solid. The only answer is to take the inlet right out of the tank and put sufficient air pressure to blow it out again (I use a big professional bike pump which is good for 8 bar). A get-you-home work round is to move the inlet farther away from the bottom of the tank by sliding the olive down at the top connection.
I reckon that having diesel tanks out and steam cleaned is a 5 year/ 5000 hour preventative maintenance task, all the other suggested procedures being mere palliatives. Even cleaning the tank through a large inspection hatch (the only recourse for those with large, built in tanks), can be hit and miss. The Pela, in my experience, cannot get out the last 5mm of water, leaving quite enough to start another colony on the water/diesel interface.
On that trip I'd have popped into Crotone or (preferably for me) into le Castella and waited for the seas to die down. LeCastella has two world-class ristorante, a most amiable English-speaking parish priest (he's an Oxford graduate) and a very friendly Lega Navale club.
 
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