Exhaust system design

[djmarchand]

I am an avid follower of the boatdiesel.com forum and a sometimes poster. I was recently struck by the following scenario that plays out time after time on that forum, often once a week:

1. Someone posts that his engine seems down on power, isn't getting up to wot rpm, blows black smoke, etc.

2. Tony Athens, the Cummins/Yanmar moderator, posts a response asking for pics of the engine installation.

3. The OP hems and haws but finally posts the pics requested.

4. Tony then asks the OP to remove the exhaust elbow and take pictures of the exhaust turbo outlet.

5. The OP posts the pictures.

6. Tony and others tell him that he has been getting seawater in his turbo, it is trashed and he needs to replace it, but first he needs to redesign his exhaust system so it doesn't keep happening.

It seems that our DE boats are prone to this happening, particularly those without an engine box. A flat deck almost assures that there is not enough height for a properly designed system. A minimum of 12" is recommended; see the attached drawing, courtesy of Tony.

But I don't hear too much about this problem on this site. So what happens. Do owners simply replace the turbo every 2-3 years and consider that a price of doing business. Do owners just live with low turbo performance and low power in between replacements. Or is there something about every day commercial use that keeps it from happening. Almost all of the cases on boatdiesel are recreational owners. Or do we just not talk about this problem.

I wonder?

David

 

[hntrss]

Another good topic. One thing missing in this diagram is a surge tube, which most DE seem to have, another is a muffler, which problably 50% or better have. Both help to stop water getting up in the elbow. Another thing to take into account is the fact that most DE boats motors are running all day, which in itself stops water from traveling uphill. I am no expert, but smoked turbo's do not seem to be a major problem in the NE. I used to really worry when backing down on tuna at high rates of speed, but having a muffler and surge tube must have done the trick. Only did one turbo in 15k hours. that was a result of not changing air filter soon enough (or so I was told). I lost track of it while fishing and imploded it. Definitely operator error on that one.

 

[jwalka51]

Thats why serious DE owners that want flat decks have dry exhausts. GO DRY!!!! And never worry about these kind of problems.

 

[jwalka51]

Or have a huge stupid Engine box that is only there because you needed room for your riser.

 

[hntrss]

I hate my huge stupid engine box.

 

[F/V First Team]

No engine box, wet exhaust and still 12" of height between the exhaust and the static waterline

 

[F/V First Team]

It works pretty good

 

[hntrss]

I would guess the RP was a foot over too, but you don't see much of this water ingestion in your customers boats, do you? A friend hydro locked his 3208 in a 31 bhm, but he was gillnetting and most likely had a lot of weight on the boat. That was way more then swapping a turbo too.

 

[plowin]

Good discussion. I am willing to bet that most commercial operations rarely see the engine shut down which makes water ingestion pretty difficult. The boat I am currently in has a step in the deck giving me 13" from waterline to turbo but my BHM had a 63P volvo with a flush deck and never drank a drop. Not sure of the height from turbo to water on that boat but it made over 5000 hours without issue or having the need to replace the turbo.

 

[hntrss]

The one I am talking about Billy's. The old Never Satisfied.

 

[plowin]

I think Billys BHM had two speeds, shut off and wide open. I believe he put a couple of motors in the boat while he had it. Never heard that it was hydro locked but I believe you.

 

[Toolate]

Surge tube is just this correct? Inlet is on the high side and outlet is to the left and right/low side is capped?

Cant say I have ever seen one in person. The idea is that water will slosh in and hit the dead end and have less of a chance of getting to the motor?

Also, how about the average muffler on wet exhaust. Don they hold a certain amount of water at all times? Inlet is usually in the middle of the muffler and outlet as well leaving the lower portion of the fat part full to the invert of the exhaust outlet. I would think that marine mufflers would have inlet and outlet offset to be flush with one side of them so they would drain.

 

[hntrss]

That's the tube, simple but effective.

 

[hntrss]

I think Billys BHM had two speeds, shut off and wide open. I believe he put a couple of motors in the boat while he had it. Never heard that it was hydro locked but I believe you.

He locked up the one in it. Guy had a rebuilt in stock, so they swapped it out. Something along those lines. There was a reason it happened too, I do think the water box was too close to the water line. Anyway, that's the only one I know of personally. Just saying it doesn't seem like a common problem.

 

[plowin]

I agree, not real common. I guess it does happen occasionally though.

 

[petrel]

Toolate- you're right- mufflers help w/ a surge,but not everyone has a muffler. The muffler companies also make check valves to keep water out when the engine is off.

 

[captainlarry84]

A serge tube is not the answer. The problem occurs with the engines timing chains. This is much more common on gas motors that diesels. Tara had a major recall. On certain motors at idle the cam chain has slack. This allows both valves to hang open a little to long. In doing so the down stroke of the piston can pull water into the motor. A above idle RPMs the chain become tight and the valves function correctly. I have never heard of this problem on a diesel but if the chain is worn I would assume the same problem can happen. Most gas motors are automotive design so when this happen to a car it is not a problem, but in boats it is the kiss of death. Tara did some resigning on their exhaust systems to prevent this in later models. A serge tube only prevents the motor from taking a gulp of water at rest.

 

[hntrss]

That really doesn't support the statement that surge tubes aren't the answer. Like I said, more commercial DE boats seem to have them than not have them. Must be a reason. Just logically, they make perfect sense, the water will take the path of least resistance every time.

 

[captainlarry84]

That really doesn't support the statement that surge tubes aren't the answer. Like I said, more commercial DE boats seem to have them than not have them. Must be a reason. Just logically, they make perfect sense, the water will take the path of least resistance every time.

Correct it will, and when a glup of water comes flowing thru the tube will do it job and save the motor. However if the water ingestion is related to the motor cam chain then the water will follow the suction draw of the motor.

 

[hntrss]

Ok, gotcha now. I was thinking more along the lines of the posed question, about the elbow being less than a foot above the water line. This is where I feel a surge tube and or muffler would be a good idea. Anything to break the wave, so to speak.