More thoughts on exhaust systems

[Barleycorn]

Thanks for all the advice about silencers that I was given in my previous thread.
I went on the boat today to replace my plastic silencer that had melted due to my impeller failing. Unfortunately I found that my rubber exhaust pipe had partially melted too, so that needs replacing.
However having got back and done a bit of googling I have more concerns. My exhaust pipe is 3 1/2" I/D, whilst the engine handbook recommends 4 or 5", although the exhaust elbow is only 3 1/2". The elbow, attached to the turbo, goes up at 45 degrees presumably to get the 12" clearance that was recommended, before turning down to where the rubber pipe joins on. This website, http://www.sbmar.com/articles/marine_exhaust_system_images/ says that this is a big no no, as in the event of prolonged cranking, ie if you have to bleed the fuel and it takes a long time to fire, water can get back into the turbo/head, causing hydraulicing which will wreck the engine. Another website suggested disconnecting the water pipe in instances such as that until the engine fires.
If I twisted the exhaust elbow down so the water runs down hill, then I will lose 6" height so the rubber pipe will be only 6" above the water line.
Whatever I do will be a compromise, but would it be better to do this and fit a flap valve, or leave alone? I am on a drystack, so there is a chance of water running back into the engine when she is lifted, but they usually lift with the bow higher than the stern.
Another option would be to get an inline injector made and fit on the end of the existing elbow, however I would have to lag the elbow and weld up the current water injection hole somehow. At least then I could get it made to 4 or 5". The engine is a Cummins 210 HP. To be honest the boat is over 20 years old, and it doesn't look as if the engine has been rebuilt, so am I worrying over nothing?
Sorry about the long winded post, but I have been thinking too much!

 

[Latestarter1]

Thanks for all the advice about silencers that I was given in my previous thread.
I went on the boat today to replace my plastic silencer that had melted due to my impeller failing. Unfortunately I found that my rubber exhaust pipe had partially melted too, so that needs replacing.
However having got back and done a bit of googling I have more concerns. My exhaust pipe is 3 1/2" I/D, whilst the engine handbook recommends 4 or 5", although the exhaust elbow is only 3 1/2". The elbow, attached to the turbo, goes up at 45 degrees presumably to get the 12" clearance that was recommended, before turning down to where the rubber pipe joins on. This website, http://www.sbmar.com/articles/marine_exhaust_system_images/ says that this is a big no no, as in the event of prolonged cranking, ie if you have to bleed the fuel and it takes a long time to fire, water can get back into the turbo/head, causing hydraulicing which will wreck the engine. Another website suggested disconnecting the water pipe in instances such as that until the engine fires.
If I twisted the exhaust elbow down so the water runs down hill, then I will lose 6" height so the rubber pipe will be only 6" above the water line.
Whatever I do will be a compromise, but would it be better to do this and fit a flap valve, or leave alone? I am on a drystack, so there is a chance of water running back into the engine when she is lifted, but they usually lift with the bow higher than the stern.
Another option would be to get an inline injector made and fit on the end of the existing elbow, however I would have to lag the elbow and weld up the current water injection hole somehow. At least then I could get it made to 4 or 5". The engine is a Cummins 210 HP. To be honest the boat is over 20 years old, and it doesn't look as if the engine has been rebuilt, so am I worrying over nothing?
Sorry about the long winded post, but I have been thinking too much!

Tony at Seaboard Marine is an old friend and writes excellent tips based on being one of the largest re-power specialists on the U.S West Coast.

Your exhaust elbow is not draining which is Tony's big beef regarding uphill set up. Even if you are OK now because the inner wall of the spray head is constantly immersed it is a ticking time bomb, it WILL eventually fail allowing small a small quantity of sea water to dribble back into the turbo at rest. This process goes on quietly for a year or two, quantity of sea water is small and no risk of hydraulicing the engine, first sign of trouble is when turbo sticks due to the sea water corroding the exducer housing.

#1 Get that exhaust elbow off and take a look at your exducer housing, should be black with soot with NO sign of corrosion.

#2 With your set up it is highly likely that exhaust elbow is already pin holed, if not paper thin. Like all manufacturers exhaust elbows are stupid ££. Once elbow is off get cleaning the inside then start prodding. I have contractor who cuts the elbow apart replaces the corroded inner tube and welds them back together. Alternatively Manuflex can make a new injection elbow, they already have Cummins drawings.

As to exhaust diameter in the Cummins installation guidelines, forget the rule book, until about 1990 the guidelines showed 3 1/2" was OK, however too many clowns where installing engines with tortuous exhaust systems leading to high exhaust back pressure so Cummins upped the anti by demanding 4 inch exhaust for the 210 to ensure the 3" of Hg back pressure limit could be easily met. From the description of your system without the muffler you will have no problems being inside back pressure limit.

If you are able to take some pictures I can take you through a rectification plan.

Sounds like you suffered a pretty severe over heat, have you recovered all traces of the failed impeller from your heat exchanger??

 

[Barleycorn]

Thanks again.
Yes, it was all a totally avoidable disaster! I checked the Vetus filter basket on the inlet of the raw water pump, and must have replaced the lid cross threaded, as about 20 minutes into our trip we heard a horrible screeching noise which was the impeller disintegrating. The filter lid was off. It didnt help that I was slap in the middle of the channel, so did creep on on tick over to pick up a mooring. On reflection this probably did much of the damage. I watched the engine temperature which didn't go up, but to be frank never thought about the plastic silencer, I should have chucked the anchor out.
I will take some pictures when I can get to the boat again. But basically the pipe goes up from the turbo at about 45 degrees for about 8 inches, then bends down again a the same angle and the rubber pipe is attached. The water inlet is just after the turbo.
I have back washed both heat exchangers, and the pipe work, so I hope I've got all the bits out.
Looking on the bright side if this episode has identified an expensive turbo fail that could have happened in the future perhaps its not that bad.

 

[oldgit]

Good idea to keep all the impellor bits you recover,reassemble all the bits you can find.. Will give you a very good idea of what is missing and either remaining in the system or went out of exhaust.
A cautionary tale.
On one of its initial voyages, first owners of my boat, cooked an impeller simply because a seacock was left shut. Boat came home on one engine.

Boat came with an extremely detailed history and this event was logged.
10 years on was replacing the impeller after two year interval, and decided to fish around in pump inlets etc as one does with the jolly old finger and out popped.... a lump of impellor.

 

[Barleycorn]

Hopefully the saga is resolved.

Checked the exhaust elbow today, and in fact the water doesn't go in at the bottom after all. It is actually a very sexy elbow made from stainless steel, with a water jacket all along its length, with the water going in the bottom, and ejecting through a ring of holes around the exhaust where the pipe joins on.

Put on a new length of hose straight through, and it didn't seem that noisey. Even measured height from spill over to waterline, and I've just about got 12 inches.

Then fitted a high temperature warning sensor in the exhaust pipe.

So all in all a happy bunny to a point, but an expensive lesson learnt!