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Old 16-02-2020, 16:40   #1
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Need exhaust advice

In the course of some other maintenance, I decided to remove the exhaust riser and waterlock on our 1984 Universal M50 diesel. It crossed my mind that in our previous boat the exhaust riser rusted through, and sure enough in the course of removing this one it too fell apart where one of the pipes was nearly rusted through. The riser is a series of 1 1/4" black iron pipe coming right off the engine exhaust manifold with one section containing a 5/8" hose nipple for water injection. The waterlock is a Vetus LP 40. The M50 is a 44hp diesel, our previous boat had a Universal M25 21hp diesel, however the exhaust systems are identical.

I would like to replace the waterlock, and had considered using a Vetus NLP 40 which is supposed to provide more muffler function. In looking at the specifications for each, I was a little alarmed to see that both are recommended for engines up to 24hp. Most retail sites use the diameter of the exhaust hose to determine the waterlock - whether it is Vetus, or Centek (Vernalift). Our exhaust hose is 1 5/8", and the exhaust manifold on the engine only has a 1 1/4" square opening transitioning to 1 1/4" pipe. Anectdotally, the system has worked fine for 35 years, and upsizing the hose to use a bigger waterlock doesn't seem practical since the current system is based on the outlet size of the engine exhaust manifold.

Ultimately, I'm inclined to go with a Vetus NLP40 for the additional sound reduction unless there is compelling reason not to, in which case I will probably just replace what is there with same model. For what it's worth, both our previous boat and current boat have always been freshwater boats.

--
Scott
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Old 16-02-2020, 17:12   #2
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Re: Need exhaust advice

AFAIK there is no compelling reason not to do what you want but it's probably not ideal. I'd fabricate the exhaust out of schedule 10 316L ss if you want longer life ( maybe lifetime in fresh water) & you can buy weld on reducers to size up to your desired exhaust hose to match the muffler.
I'd be a bit concerned too that you are putting on a muffler only rated for 24hp on a 44hp engine but as you say it's worked for a long time like that. Better to go oversize than under on exhausts as a general rule.
You have to remember some water flashes off to steam after it's injected so making a sudden volume increase, thats why usually exhaust hose is bigger than manifold outlet.
Google marine exhaust design if you want a better explanation.
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Old 16-02-2020, 17:52   #3
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Re: Need exhaust advice

Before I did anything, I’d find out what size exhaust the engine manufacturer recommends.
I bet yours is undersized or to sounds like it is. If it is, then it’s excessive backpressure.
If it’s not then I wouldn’t go to a larger hose than the exhaust, reason is you may need the exhaust velocity to flush the water out, if the hose is too big then it may not be enough to blow out the water.

Most likely your engine manufacturer has recommendations / requirements for exhaust. Find out what those are.
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Old 16-02-2020, 23:27   #4
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Re: Need exhaust advice

Quote:
In the course of some other maintenance, I decided to remove the exhaust riser and waterlock on our 1984 Universal M50 diesel. It crossed my mind that in our previous boat the exhaust riser rusted through, and sure enough in the course of removing this one it too fell apart where one of the pipes was nearly rusted through. The riser is a series of 1 1/4" black iron pipe coming right off the engine exhaust manifold with one section containing a 5/8" hose nipple for water injection. The waterlock is a Vetus LP 40. The M50 is a 44hp diesel, our previous boat had a Universal M25 21hp diesel, however the exhaust systems are identical.

I would like to replace the waterlock, and had considered using a Vetus NLP 40 which is supposed to provide more muffler function. In looking at the specifications for each, I was a little alarmed to see that both are recommended for engines up to 24hp. Most retail sites use the diameter of the exhaust hose to determine the waterlock - whether it is Vetus, or Centek (Vernalift). Our exhaust hose is 1 5/8", and the exhaust manifold on the engine only has a 1 1/4" square opening transitioning to 1 1/4" pipe. Anectdotally, the system has worked fine for 35 years, and upsizing the hose to use a bigger waterlock doesn't seem practical since the current system is based on the outlet size of the engine exhaust manifold.

Ultimately, I'm inclined to go with a Vetus NLP40 for the additional sound reduction unless there is compelling reason not to, in which case I will probably just replace what is there with same model. For what it's worth, both our previous boat and current boat have always been freshwater boats.

--
Scott
The important thing is how much back pressure you have.

35 years seems to me to suggest that for your application things are sized correctly.

1 1/4" seems small for a 44 hp engine, but 44 hp seems large for a 34' boat (if it's in the 39' one, not so much). Since CFM is directly related to engine RPM, you may never (or rarely) exceed the capacity of the 1 1/4" exhaust.

Since you have to replace the iron pipe section of the manifold anyway, I'd do that and then measure the backpressure of the existing system. If it's in spec, you can eliminate your concern and either replace with the larger (how noisy is it now?), adapting up to the bigger muffler inlet after the riser, or stick with the original.

If you want to change the riser size, iron pipe is cheap, and could be used as a test before changing to fabrication in stainless.

If long runs of hose are involved and the engine is below the water line, remember that the volume of the hose and the muffler, as well as the height of the engine in respect to the waterline need to be kept in mind.
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Old 17-02-2020, 10:41   #5
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Re: Need exhaust advice

40 mm / 1 5/8" exhaust hose size is too small for that engine.

Use a NLP 50 and run 2" hose from engine to the transom thru hull. Which should also be 2".

It may be the same muffler and you may be able to buy new 50 mm fittings for your current muffler.
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Old 17-02-2020, 10:56   #6
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Re: Need exhaust advice

Have you considered asking Vetus?


Exhaust hose sizing is sometimes an oddball. Why? A few years ago I had to replace my muffler (M25 engine - 21 hp). I was fortunate to have a contact who was replacing his M25 with a Beta engine of the same HP, BUT...BUT...BUT, Beta insisted that the exhaust hose and muffler in & out be 2 inches instead of the 1 1/4" that was on the boat with the M25. He made the muffler available to me for just the cost of shipping it to me (great skipper).


Point being, exhaust sizes vary, most likely for back pressure reasons.


Talk to the guys that know: engine manufacturer & muffler specialists. And if it's been working fine for all these years, factor that in, too.


Good luck.
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Old 17-02-2020, 14:53   #7
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Re: Need exhaust advice

Just checked our 8hp yanmar for an example:
Exhaust outlet in head is 1 &1/8" Exhaust riser is 1&1/2" Those I know but I THINK it goes to 1 &3/4" where the hose attaches to go to the waterlock muffler & then is 2" after the waterlock to the stern outlet.
It's been that way for 40 years AFAIK except I made an ss riser to replace Yanmars cast-iron engine destroyer that dumps salt water on the exhaust valve if it fails. ( Ask me how I know) But my fabrication is sized the same as Yanmars elbow
Better to make sure the injection is on the downhill side of the elbow so if it fails the salt water doesn't end up in the bore. Boon for Yanmar their design, they can sell more parts.
If you make an ss riser do not make it out of tube, make it out of schedule 10 ss pipe & it will last waaay longer.

Good thoughts in other posts
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Old 19-02-2020, 15:16   #8
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Re: Need exhaust advice

Correction: the exhaust hose is all 2" on our 8hp diesel. Engine is not below the waterline.
I'm certain you could put 2" hose on your 44hp engine if you wished to.
Please post back what you do Scott & any difference if you change muffler & exhaust hoses.
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