Sizing of exhaust lines and water lift mufflers

[afrayedknot]

Greetings -
I have a universal M3-20B that’s about 15 years old. When a local shop installed it, they left the old steel muffler and most of the exhaust line in place, including everything after the muffler, most of which is made of ABS drain pipe. I suppose it has been Ok but the whole set up is on the loud side, and I think the muffler may be undersized, at about 190 cubic inches. The line is 1-5/8 ID and the internal volume of the line to the spill-over point is about 170 cubic inches. I have already removed my fuel tank (and replacing it; that’s another story) so I currently have good access to the whole system. I’m considering replacing the muffler with either a Centek Vernalift or a Vetus Aqualift, as well as much of the exhaust line. I’m hoping to get a little advice on sizing of both the muffler and the line. And on these mufflers if anyone has experience with them.
I checked the sizing of the engine’s exhaust flange, thinking that would be a good clue. However it appears that all of the Universal engines use the same 1-1/4 NPT, from their 10 - 30 hp. I’ve also read what Westerbeke says in their installation manual but it doesn’t say much about absolute sizing of lines. So, what say ye, sailors?

 

[JoeWhite]

I would remove ABS drain pipe and install exhaust hose made for this purpose. I do not believe the size of it is important as long as it is more than 1 1/2" ID. What is the size of the exhaust throuhull.
You can start there. If the muffler is not leaking and it is in a good shape, no need to replace it.

 

[dlochner]

The specs say the exhaust outlet is 1.25". The exhaust hose should be 1.25" or 1.5". The engine needs some back pressure from the exhaust system, if the exhaust hose is too large there won't be enough back pressure or pressure to force water out of the muffler. If the hose is too small there will be too much back pressure.

There are several water lift manufacturers, Vetus, Soundown, and Centek. The muffler needs to be large enough to hold the volume of water in the hose above the muffler. If it isn't large enough water can back fill into the exhaust manifold.

 

[Stu Jackson]

You do not have a unique engine. Call the manufacturers of those mufflers, tell them the lengths between the engine, muffler and muffler to transom exhaust hose length, and let them do all the work for you. They will know your engine's requirements better than anyone on this forum and will tell you exactly what you need. Good luck but in this case luck should and will have nothing to do with it.

 

[afrayedknot]

Thank you - I appreciate each and every nugget of advice! Tagging up with the muffler makers seems like the best next step.

 

[Project_Mayhem]

My boat has the same size hose. It's a bit hard to find. I can look up where I bought mine from for a reasonable price if you'd like

 

[afrayedknot]

Someone on another thread suggested Trident Marine, but if you have another good source, I'd appreciate that.

 

[Project_Mayhem]

It was the corrugated Trident Extra Flex hose. Defender had the best price/shipping at the time. I found my hose had failed at a bend so I figured this would be an improvement

 

[jssailem]

You're in Seattle?
Fisheries Supply would be a good source. Lots of brands. I have also used Seattle Marine & Fishing Supply on W Commodore Way, when I am down near Fishermen's Terminal

I think when I did my rebuild, I used VETUS Exhaust Hose.

 

[NYSail]

Centek are great products and customer service is also great. They helped me spec my genset. I love the Centek as it will never melt if there were ever an issue with water flow.

good luck!
Greg