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Old 08-03-2021, 12:16   #1
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engine issue? prop? nothing? help!

Hi everyone, we are looking for some guidance on an issue with the performance of our 1985 Catalina 36.


She has a Yanmar 23 hp on which we recently changed filters, did full maintenance, and also replaced the exhaust elbows. She has been running great, much better than in recent memory, since the exhaust repairs which we completed 2 1/2 weeks ago. Before she never got above 2600 rpm, or well below; since then consistently 3000 or above when needed. Until yesterday...

My sailing partner had her out in 20-22 kt winds. No issues going out of the channel off our mooring, but later motoring back directly into the teeth of the wind, the engine would lose rpm rapidly to 1000 or less when put in forward. He'd put it in neutral, then put it back in gear, it would go to normal forward rpm but then quickly drop off rapidly again. (the first time he did this, it operated normally for ten minutes before dropping down near idle, then the time spans became much shorter). He had to call a tow eventually given the conditions and the boat's location near shallows.

We are having a hard time assessing the potential issue - if any. Prop is clean (at least as of this morning, we thought last night that it was fouled perhaps but no signs of anything when I dove below and it turns by hand). Goes to 3000 rpm in neutral on the ball with no apparent issues. Could not test her off the ball yet meaningfully due to weather today (other than 10 seconds in forward and reverse on the mooring), but we are perplexed as to what issue, if any, there may be. Is it possible my buddy pushed her too hard into the wind? It was dead against the tide and the wind at a narrow channel entrance, and I'm wondering if it was simply too much load for our 10 year old 23. Or could it have been bad fuel? (we were in the bottom 1/3 of the tank, but he saw, and I saw today, no signs of smoke or discharge or anything to suggest poor combustion). I'm not aware of any prop issue, and the bottom is fairly clean. We only use the motor for 30-45 minutes max normally just to get in and out of our channel, and I assume that was true yesterday although I don't know when my buddy fired up the engine to head home.

Any guidance would be appreciated! What could have happened and what should we check? Or do we just give it a run and see if in fact there is no ongoing issue. Thank you!
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Old 08-03-2021, 12:21   #2
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Re: engine issue? prop? nothing? help!

Always try the easy and cheap solutions first.

My first guess would be fuel. If the tank hasn't been inspected or cleaned in a long time, sailing in gusty conditions could easily stir up sludge in the tank and clog the filters and give exactly the symptoms you describe. Also could have stirred up something in the tank that was partially blocking the pickup.

First thing, check the fuel filter for debris, water, etc. If it looks dirty, time for a change and see if that fixes the problem.
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Old 08-03-2021, 12:59   #3
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Re: engine issue? prop? nothing? help!

Thanks, you may well be right, we haven't had a tank polish in a long time. and it was very choppy apparently. Will check the filters either way. But if crud/debris was stirred up, why would that only impact her when in forward? In neutral, or during her (very!) brief time in gear this morning, there was no problem getting up to a high rpm and no telltale signs of bad fuel which trust me we've seen before? Thanks again - just trying to educate ourselves so your thoughts are welcome!!
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Old 08-03-2021, 18:14   #4
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Re: engine issue? prop? nothing? help!

When in gear their is a much larger load on the engine which requires a much higher rate of fuel flow. A very common symptom of a fuel supply problem is reving well in neutral but not in gear.

As far as running better after a period of shutdown it is possible that the blockage caused a partial vacuum to start building during prolonged running. Shut down for a while could allow fuel to trickle into the system reducing the vacuum giving you a bit of reserve in the lines to run better until starvation returns.

These are possible explanations but until you start trouble shooting it's just my semi educated guess.
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Old 08-03-2021, 18:33   #5
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Re: engine issue? prop? nothing? help!

Hi, I would also check the tank vent line for any obstruction.
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Old 08-03-2021, 18:33   #6
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Re: engine issue? prop? nothing? help!

I agree with skipmac. Sounds like a fuel delivery issue. Had that happen to me once at an inconvenient time. Turned out to be a partially activated manual emergency fuel shutoff. Once opened fully, all was well again.

A vacuum gauge on (or downstream) of your fuel filter provides a great diagnostic tool.
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Old 08-03-2021, 18:45   #7
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Re: engine issue? prop? nothing? help!

I also agree with Skipmac. It sounds very much like a fuel delivery problem. As he said, the demand when running hard against a sea/wind is much, much greater than at idle. You might possibly be able to replicate the issue by running hard astern for a prolonged period (say 5-10 minutes) while on the mooring ball.

Other, less likely causes that spring to mind are an exhaust restriction (unlikely given your recent work, but did you check the manifold for carbon buildup?) or over-heating. When an engine gets really, really hot the piston can start to stick to the cylinder wall and rpms drop. But I think this is unlikely. A seizure usually follows shortly after that and you'd also most likely see white smoke (steam) from the exhaust beforehand.

My money is on fuel.
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Old 08-03-2021, 19:10   #8
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Re: engine issue? prop? nothing? help!

Quote:
Originally Posted by Uncle Bob View Post
Hi, I would also check the tank vent line for any obstruction.
Also a good idea.
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Old 08-03-2021, 20:00   #9
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Re: engine issue? prop? nothing? help!

That happened to me on a windy day. Turned out to be partially clogged primary filter bracket.
Visit Sail La Vie | 1999 Catalina 36 MK II to learn how I solved the problem.
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Old 08-03-2021, 20:47   #10
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Re: engine issue? prop? nothing? help!

Quote:
Originally Posted by Uncle Bob View Post
Hi, I would also check the tank vent line for any obstruction.
Another good possible explanation.
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Old 09-03-2021, 09:55   #11
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Re: engine issue? prop? nothing? help!

"You might possibly be able to replicate the issue by running hard astern for a prolonged period (say 5-10 minutes) while on the mooring ball."

Good idea - be ready for your prop kick to keep trying to move you to port or starboard - maybe do it when a tide is running or in strongish wind.

Other than fuel - or maybe engine air starvation - perhaps you did pick up something on your prop which then dropped off.
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Old 09-03-2021, 10:07   #12
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Re: engine issue? prop? nothing? help!

I think you can probably rule out dirty fuel as a cause. New filter plus I assumed you saw no water or crud when you put a light in back of the filter container. Furthermore I can envision no way a partly blocked fuel filter or water would go away as you describe. A partly clogged vent might fit your symptoms best. As others have noted, watching your temps, running at full RPMs while moored for 10 min or so (after warming up) would be a useful step.
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Old 09-03-2021, 10:28   #13
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Re: engine issue? prop? nothing? help!

Intermitand fuel starvation under load. Check first : fuel lines specially the flexible ones with rubber inside, just change them as precaution. Than filters, gascolator, tank vents, and fuel contamination in the tank. Test if problem disappears after first item.

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Old 09-03-2021, 10:31   #14
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Re: engine issue? prop? nothing? help!

When you find the problem let us know, we can always use your experience to help others !
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Old 09-03-2021, 10:32   #15
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Re: engine issue? prop? nothing? help!

Another vote for fuel issue: Vent line, dirty filter, etc.
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