Temporary Drop in engine revs

[cueball]

A few weeks back I was returning to my home marina in some let's say "lumpy conditions" wind was on the nose and I had some tidal gates I needed to make. Stuck the engine on and motorsailed for a couple of hours, then out of nowhere the revs dropped from 2200rpm to 2000rpm this lasted maybe 10 seconds then back to 2200, she did this over the next 40 mins or so maybe 3 times. I opened up the throttle but she wouldn't go above 2500rpm so I backed off and shut the engine off. My thoughts were to sail in and change fuel filter when moored and in calm water, as I sailed up to my home marina I started the engine and no problem. Next day I changed the fuel filter, after I changed the filter I noticed the diesel looked cloudy, emulsified water I thought! But I now believe it's the fuel filter that made the diesel look cloudy, As the filters feel very dusty. I have run the engine under load for around 45 mins and it hasn't missed a beat. I also added a shock treatment of marine 16 to the fuel tank just in case I have the dreaded bug, but no deposits in the old filter. I have also checked the engine hoses for air leaking in but they are all tight. Please enlighten me if anyone can shed any light on my issue. It seems fixed but I would like to know what happened. Engine is a yanmar 3JH 3E, primary filter a wasp (similar to racor)
Thank you

 

[bignick]

How full was your fuel tank? A bit of air getting drawn in to the suction line can cause the issues you describe. Only tends to happen in lumpy conditions. it can clear itself Like you say as well.

 

[Daydream believer]

Polythene bag around the prop. In rough water you might not notice any vibration but it might take a while to free itself from the blades and cause them to resist rotation for awhile intermittently. I have had one picked up between dieppe and Le Havre and not fully clear it until reversing against the fuel pontoon in Cherbourg when lots of bits of white polythene suddenly floated to the surface. It caused some uneven revs in the interim

 

[cueball]

How full was your fuel tank? A bit of air getting drawn in to the suction line can cause the issues you describe. Only tends to happen in lumpy conditions. it can clear itself Like you say as well.

Probably about 1/2 a tank at that point

 

[cueball]

Polythene bag around the prop. In rough water you might not notice any vibration but it might take a while to free itself from the blades and cause them to resist rotation for awhile intermittently. I have had one picked up between dieppe and Le Havre and not fully clear it until reversing against the fuel pontoon in Cherbourg when lots of bits of white polythene suddenly floated to the surface. It caused some uneven revs in the interim

It did feel like the prop was fouled, but as I didn’t see anything so dismissed it, maybe took a while for the ambassador to eat it.

 

[bignick]

Could be air, could equally be a fouled prop. Fuel starvation results in reduced power, so they can feel similar, as you’ve got reduced thrust.

We brought a friends Dehler 32 back to Poole after the RTIR and it did exactly the same thing both this year and last. In 2021 we didnt realise what the problem was quickly enough and kept trying to run the engine, until we must have got air in the fuel pump, at which point we had no engine at all.
This year we turned the engine off as soon as it started spluttering (right in the middle of the north channel at Hurst!!) and sailed back. We restarted the engine once we got back into flat water, ran it on a fast idle for a few mins and it cleared itself, after which it was happy again

 

[DangerousPirate]

Okay, I had similar problems a few weeks ago, although I had the issue when I ran the engine in idle (for power mostly), it would drop a bit and go back up, no issues.

Long story short: I now have very severe Diesel Bug and got to work on that.

See, your tank was half full in bad conditions, maybe water got in and you fed the vampire. Shakey boats also allow the sediments of the bug to swirl up from the bottom of the tank into the "fresh" Diesel and end up in the system. I would drain the tank and see how much water you get out and also would check the filters again for water and "slime" in the bowl. And also have another look at the colour of the diesel.

I am not 100% or anywhere near an expert, but it does sound similar and it's worth checking out.