diameter of wet exhaust pipe

can anyone give me the internal diameter of exhaust hose as it leaves the mixing u pipe. I need to buy a new connector.

Stewart: Hi!

Zygote has 1.75 inch (ie 1 3/4") ID hose (Shields Flex II marine wet exhaust hose, 250-1340) that has an OD of 2 1/8" and burst pressure of 300 psi.

The water injection hose on Zygote is Goodyear Horizon 5/8" rated at 200 psi.



BTW Stewart, I am having trouble with my exhaust hose being toooo stiff.

I noticed when I was checking the prop shaft coupling alignment, that if I leaned on the engine it created an unacceptable mis-alignment.

Seems that the v soft engine mounts on my 3GM30F are the cause.

Ok, so I stoped leaning on it and secured the alignment nuts.

I was replacing the exhaust hose from engine to water lift and noticed that the hose was stiff enough to push the prop shaft coupling out of alignment as well.

I have been looking for a silicone bellows that I could use, but only found them manufactured for 3" ID and larger.

Like Bil , this too stiff exhaust hose is a Shields wire reinforced wet exhaust hose, I purchased from West Marine.

Has anyone found a remedy for a problem like this ?

even more interesting, my stiff exhaust hose ran thru a wood holder which further stopped flex, and of course as you may have recalled the massive exhaust riser fractured at the elbow. Fitted the new ss pipe etc but when connected to the vetus exhaust it was v tight so have to work in a small radius connector to relieve the strain. Ya have to think about some of this stuff. I’m off to the Bahamas again in 3 weeks, the exumas this year will maybe try to do the blog again, I have been v slack.

My present solution to a turn between the exhaust elbow and the waterlift muffler that’s too tight for the wire-reinforced hose, is a stainless right angle piece. Yeah, I know, stainless??? OK, I will keep my eyes on it! Since my new welded-up elbow is also stainless, and the middle piece of the Yanmar exhaust riser is stainless…yeah, I’ll keep my eye on all of it. Come to think of it, that could be the best policy for just about EVERYTHING on a boat, even a BCC! Especially a 30 yr old BCC like Shaula.

Fisheries Supply sells 45 and 90 degree elbows made of non-corroding GRP, which may be a better solution than SS elbows?

Stewart, how about an update on your exhaust system, and maybe a picture? Is your mixing elbow the standard Yanmar casting?
Dan Shaula

I think vetus makes some angles same material as the muffler so one would think that it would be at least as good. Got pix of fixed system alas getting them onto the forum may be a challenge to a guy of my limited computer skill. I will see…


Hi Bil,

I see from this thread that you also have a 1.75 inch shields wet exhaust hose installed, which I assume is set-up the same as mine starting with a 2 inch hose coming off the engine followed by a somewhat radical bend before connecting to the water mufflers 1.75 inch bib. According to my mechanic, the 3GM30F is supposed to have a 2 inch wet exhaust hose from engine to 2 inch thru-hull. Do you have any idea why SLM stepped the hose down from 2 inch to 1.75 inch at the water muffler? Not a very good set-up in my book…

Ahoy Traveler , I can not even attempt to answer for Bil , but does the hose size of these differences have a bad effect on exhaust back pressure ?

I followed Nigel Caulder’s book , on exhaust installation . He explains how to make a manometer (sp-?) to check your back pressure .

My 3GM30F has the same set up as Bil’s , and my mamometer tested readings are fine !

Hi Douglas & Bil,

I’m just wondering why Yanmar specs a 2" exhaust hose and SLM installed a 1.75"? There must be a reason! I already had the yard replace my 1.75" thru-hull with a 2" thru-hull, but I would still like to do the check and balance analysis regarding my mechanics recommendation. We are also going to replace the original wet muffler with a Vetus that has a 2" bib, this will eliminate having to compress a 2" hose onto a 1.75" bib with a hose clamp. All new 2" Vetus wet exhaust hose will round out this project. As for the back pressure, that will require some research.

I’m pretty sure that Zygot was built with a 2" exhaust hose clamped to a 1.75" bib on the original W. H. Salisbury & Co Hydro-Silencer wet muffler like mine was, so I was curious whether any modifications were made to make her wet exhaust uniform in size from engine to thru- hull? Hopefully I haven’t made an error in judgement up-sizing to the 2".

On the issue of the exhaust, my wet exhaust is situated to the side of the engine (port) which is a pain to accses all the back parts of the space, where does everyone else have their wet exhaust placed. It would be nice to have it well out of the way.

Hey Stewart, Our muffler is mounted directly behind and below the drop of the injection elbow. Makes for an almost straight shot from the elbow to the muffler. THis is the best pic that’s handy. You can see the muffler next to the shaft coupling in the engine compartment. Curious how we got the motor out? See the other pic.

thanks for the prompt reply

Hi Stewart,
I glassed small triangular wood pieces to the hull and screwed a plywood shelf to them so the muffler sits a bit to starboard, above the prop shaft. It’s made access to the stuffing box less than ideal, but I’ve lived with it since 1988. Crawling past the engine into the area under the port cockpit seat isn’t a problem. I should have attached the inboard side of the shelf to the wood beam that the fuel tank sits on, with a vertical, fore and aft piece of plywood,. It’s on the job list and I hope to get it done in the next week. Then I’ll remove the port side of the shelf. This will make access to the (traditional) stuffing box a lot easier. I’ll post a pic when I finish it.

What does your exhaust system look like now? Maybe a pic on your blog if not here on the forum? I think I now understand that you have a Vetus silencer-type muffler in your lazarette, but that your waterlift muffler is next to the engine,s exhaust elbow. Is this right?

On the exhaust back pressure issue, I would be surprised if Yanmar did anything as simple as specifying 2" ID exhaust hose.

Let me explain:

  1. Overly high back pressure can be a problem, leading to lower engine efficiency and also prematurely breaking the vanes of the rubber impeller in the raw water cooling pump.

  2. Back pressure relates to at least all of: (a) ID of the exhaust hose; (b) length of the exhaust hose; and (c) the water lift height.

  3. From memory, Yanmar specifies a max exhaust back pressure in the order of 2 psi. I’ve never bothered buying the Yanmar installation manual (so I’d need to commute into downtown to browse the one copy that sits unsold in a bookstore to get the exact figure). 2 PSI is about 4 inches of Hg or 56 inches of H2O.

  4. The most common and most serious source of excessive exhaust back pressure is from carbon/lime deposits, formed when hot exhaust gases meet seawater in the mixing elbow, which reduce the effective ID of the exhaust. Gurus such as Nigel Calder recommend a yearly examination/clean of that elbow.

  5. As Douglas noted, Calder gives a quick and inexpensive technique to measure exhaust back pressure. Zygote (and BCC 111 Nereus) have in-line pressure gauges in the raw water line (just after the impeller pump) which measure raw water pressure and so indirectly measure exhaust back pressure (or at least indicate when back pressure is unusually high due to deposits reducing the ID of the exhaust elbow).

  6. If I get enthusiastic in the next day or three, I might measure the water lift height and the length of run of exhaust hose in Zygote. I suspect that Sam L Morse Co did its usual fair job. By that I mean (a) the water lift height is not excessive (each foot of water lift height would theoretically add about 0.4 psi to back pressure; 1 psi = 27.7 inches of freshwater, adjust for seawater) and (b) the length of exhaust hose is not excessive (a longer exhaust hose should theoretically add to back pressure).

  7. If you’ve a longer than normal run of exhaust hose, increasing the size of the exhaust hose should reduce back pressure.

  8. In anything but a brand new engine, the biggest source of excess back pressure is the coke & lime crud that deposits in the mixing elbow. Installing a bigger ID hose will do nothing for that. By which I mean the experience of Douglas and other BCC owners, including me, is that the mixing elbow is the weak point, not the difference between 1.75" and 2" ID hose.



Referring to item 4 above, I had one day last year, a complete absence of exhaust water on starting the engine (Volvo Penta 2002).
The raw water impeller was intact and working, as was the raw water strainer.

Eventually I took the step of disassembling the mixing elbow, and found three of the four ports very badly blocked.

I unblocked the ports, had the contact surfaces machined, and reassembled it. Problem solved, but as Bill says, this is in my opinion an annual maintenance chore worth doing.

All the best,

I just looked at your hull 126 blog again and noticed that your 3YM30 has its exhaust riser on the port side of the engine. The 3GM30’s is on the starboard side, which makes it easy to route the exhaust to the starboard side of the fuel tank. I guess it would be possible for you to shift your waterlift muffler to the starboard side and reroute the exhaust hose, but it doesn’t sound like a fun project.

By the way, the wood work on your BCC must be the finest of all BCC’s! Gorgeous.

Web address for Stravaigin blog?


Never mind, I found it.

Thanks to one and all for your responses. I now have a new 2" thru-hull, new ball valve, new Vetus waterlift muffler, new 2" Vetus Flex Hose (I hope this flexible hose will prevent any prop shaft coupling alignment issues since its not nearly as stiff as the Shields hose I removed) and new mixing elbow. All of the hoses on the engine were also replaced using AWAB clamps to secure them. Next I intend to replace the water injection hoses with 5/8" Trident Tru-Flex Wet Exhaust hose. Its slow but steady progress getting the mechanical work accomplished; however, one day soon I shall have the wind in my sails once again…