Diesel engine rust

My wife and I live on a 1978 44' Gulf Star trawler with two Perkins 120's in San Diego Harbor. I try to run the engines 3 or 4 times each month for 30 - 40 minutes. Each time I fire up the engines they blow lots of smoke -- which doesn't worry me -- but a good deal of brown stuff which I suspect is rust -- into the water for the first few minutes. This I do worry about. Is this normal? Should I be doing this a different way? Any suggestions or information appreciated.
Jim H.

I'd be worried. My exhaust water runs clear (seen here near the waterline slightly astern of the pilothouse door.) Undoubtedly your raw-water cooling system is dissolving. Sounds like your engines and their systems are tired.

I suspect it is likely the exhaust system and not the engine itself. My engine has less than 400 hrs and I see rust coloured water for the first few seconds after start up. The discoloured water is coming from salt water sitting in a low lying 90 degree steel elbow in the exhaust line. I will eventually pull that metal elbow and install a fiberglass replacement.

But the OP is talking about 36-year-old systems.

(My practice is to run the engine three to four hours at a time at cruising speed, several times a month. Running engines at the berth doesn't count.)

Mark is correct in that it could certainly be something more serious. I generally hope for the best and expect the worst when it comes to the boat. I always take a look at the easy stuff first and move on from there. The exhaust will be easy to rule out. Do both engines exhibit the same symptoms?

Could the muffler be rusted out on inside ?

Sounds like lack of use and maintenance has led to a leak in the HX with salt water having replaced coolant. I'm with not so fond memories of many raw water cooled vessels where rusty exhaust was normal and of no concern, until the engine finally died.

Along with the OP we all are just tossing up wild a$$ guesses.

Some Perkins have sea water cooled exhaust manifolds. When shut down, they will corrode, and spit out rust on start. Not a big deal, but due to this manifolds will not last forever.

Ski in NC said:

Some Perkins have sea water cooled exhaust manifolds. When shut down, they will corrode, and spit out rust on start. Not a big deal, but due to this manifolds will not last forever.

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Ditto. Observed the same behaviour on my previous boat. Perkins 6.354 with sea water cooled exhaust manifold. Engines were 40 - 50 years old. One with original Perkins manifold, one with custom made manifold.

Unless your entire exhaust system is stainless steel it is rust. Eventually it will eat through leak. How long is that? Who knows? On most generators the life of an exhaust manifold is 4-5 years. Probably a little more on mains.

good info

Thanks for all the good input. Appreciate it.

markpierce said:

........... (My practice is to run the engine three to four hours at a time at cruising speed, several times a month. Running engines at the berth doesn't count.)

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I questioned the value of running engines that weren't otherwise being run (the off season in particular) with a well respected diesel mechanic. I couldn't see the benefit of it but he pointed out that certain exhaust valves are open to the atmosphere when the engine isn't running and it would be a good idea to run the engine so different cylinders would be in that situation during the off season.

Of course you can't do that on boats left on the hard all winter so I'm not sure his reasoning is sound.

Many of us can't or don't want to take the boat out on a cruise during the winter months.

Starting an engine just to see it run ,not operate to get to normal temperature under load, is not to my knowledge recommended by any engine mfg.

I don't think that running them is a bad thing. It may not be in the service manual but as almost no one that I know follows the procedure that is in most service manuals for long term engine storage, I think given the two alternatives of running or not, I would rather have the engine started on a regular basis. Also, I would point out that over time the oil film on metal parts tends to thin out leaving the possibility that moisture may make contact with otherwise shiny metal parts. I am thinking camshaft and followers as an example. Just my thinking. YMMV.