can wet exhaust be reduced

[Paul Klein]

Good morning to all,
I recently bought a Herreshoff H36 Nereia built in steel and in need of finishing including engine installation. I notice now that the engine, Yanmar 3JH4E is fitted with a 3 inch mixing elbow whereas the rest of the fittings already welded in are 2 inch!! To fit a 3 inch waterlock and hose is difficult because of limited space. It is possible but would require addition of a riser and cut out the stern fitting and weld in a new one, all extra costs. Can I safely reduce the exhaust from 3 inch to 2 inch?? The mechanic at Yanmar says no but also said that the 3JH2E model came with a 2 inch exhaust. Warranty is not a consideration as even though the engine is new and still in the crate, it is 10 years old and I am the second owner. Any advice would be appreciated,
regards Paul

[Sailmonkey]

My instinct would be yes.

How many horsepower is the engine?

[Paul Klein]

Thanks for the quick reply. The motor is 38 horsepower 3 cylinder and the mixing elbow is probably a standard unit for the 3 and 4 cylinder models. The exhaust run is about 3 meters

[IslandHopper]

Generally speaking, reducing exhaust diameter = increasing backpressure, which will effect the engines performance, I would stick with the manufactures recommendations....

Here's a good article on why...

http://www.gomog.com/allmorgan/exhaustbackpressure.html

BTW, just recently been through the same process installing a new 4JH4-TE to replace a worn out Isuzu, had to go from 2" to 3"....

[Captsteve53]

Your mechanic is sort of correct, the standard rule of thumb for a 40 hp DIESEL engine is actually 2.5" (recommended), your also correct that Yanmar risers cover several different models (hence the 3") , the 2.5" tends be a bit of a bastard size with some manufacturers again why many jump from 2" to 3 ".

CONSIDERATION:most yacht engines are not running wide open throttle for long periods or any where like it, so in fact if only using say 25-30 Hp as a norm the restrictions may not seem to be a true issue

Since your fittings are there already in place and your engine is not turbo charged i would think you will get away with it also, your run is not to long, I have seen numerous systems run the larger hoses with reducers made at the exit point to go down to the 2" skin fitting(yes it means more clamps) and messing about , comes down to(which is more cost effective or ease to complete

Cheers Steve

[Terra Nova]

Originally Posted by Paul Klein

...Can I safely reduce the exhaust from 3 inch to 2 inch?? The mechanic at Yanmar says no...

But that's not the answer you wanted?

[Sailmonkey]

Originally Posted by Paul Klein

Thanks for the quick reply. The motor is 38 horsepower 3 cylinder and the mixing elbow is probably a standard unit for the 3 and 4 cylinder models. The exhaust run is about 3 meters

I would have said, sure, no problem.....you'll lose some power near the top end. Then you threw the 3meters in there. Is this the straight line measure? Or the measure of the path including the gooseneck after the waterlift?

The longer run may require larger hose to reduce back pressure.

[boden36]

We have a 3JH2 in a comparable boat. (Boden Daru) The exhaust is 50mm. Length 3+ metres.
No problems that I know of. Maybe the bigger diameter is meant for the turbo engine?

Regards,
Richard

[jimbunyard]

This thread is pretty much the opposite of the one currently running about the Westerbeke. A look at a picture and the drawings of the 3JH4E, at 1.6 liter displacement, and the 4JHE4, at 2.2 liter displacement, show the same size exhaust outlet dimension, 76.3 mm.

Given that the relevant criteria here is backpressure, and one of the most important inputs defining backpressure is exhaust gas volume, I'd say that the financial interests at Yanmar (costs them a lot less to use the same elbow for a variety of engines than to produce a different one for each engine) should not trump the more important considerations of 1) correct backpressure and 2) your financial interests.

It is important to note that having an exhaust tubing size that is too large can also lead to problems, in this application more likely engine flooding rather than exhaust velocity issues...

I'd match the ID of the exhaust manifold outlet to the ID of the muffler inlet ID, and measure the backpressure. If that is in spec (as is more likely than not) then you should have no problems.

[jkishel]

You could install the 2 inch system then measure the back pressure, if it's less than 2 psi you should be OK. If it's greater than 2 psi you'll need to upsize.

[Snore]

Ignoring what the factory trained Yanmar tech said.

By reducing the exhaust from 3" to 2" you are halving the surface area. This will significantly increase back pressure. Suggest you do the surface area calculation to see for yourself.

[jimbunyard]

Another way of looking at it, using a 3" exhaust system tubing versus a 2" (if that's what the actual ID of the exhaust manifold outlet is [not the bell, but the actual ID exiting the manifold]) in this application could actually increase backpressure, as the smaller swept volume of the 1.6 liter engine works harder to push the larger volume of water, induced by having an oversized tubing, out of the system.

The things that matter here are the configuration of the system and the numbers they produce, not the (supposed) qualifications of a 'factory- trained technician'...

[S/V Alchemy]

Originally Posted by Paul Klein

Good morning to all,
I recently bought a Herreshoff H36 Nereia built in steel and in need of finishing including engine installation. I notice now that the engine, Yanmar 3JH4E is fitted with a 3 inch mixing elbow whereas the rest of the fittings already welded in are 2 inch!! To fit a 3 inch waterlock and hose is difficult because of limited space. It is possible but would require addition of a riser and cut out the stern fitting and weld in a new one, all extra costs. Can I safely reduce the exhaust from 3 inch to 2 inch?? The mechanic at Yanmar says no but also said that the 3JH2E model came with a 2 inch exhaust. Warranty is not a consideration as even though the engine is new and still in the crate, it is 10 years old and I am the second owner. Any advice would be appreciated,
regards Paul

Go to a transverse exhaust. Two two-inch pipes welded in either side of the engine bay. The bonus is that you can lose your anti-siphon set up.

[JOHNMARDALL]

I suggest that good starting assumptions would be that Yanmar know what they're doing, know what they're talking about and are not installing improperly sized mixing elbows to save money. (Full disclosure- Yanmar owns our companies, Vetus Maxwell Group). From your remarks, I assume that the fitting already installed is the transom exhaust. You could go ahead and size everything to connect to that fitting and then check the back pressure at the mixing elbow/exhaust manifold connection zone, but if the back pressure is too high, you're then looking at junking the new exhaust hose, waterlock, goose-neck etc and having to buy it all over again at the size dictated by the mixing elbow (our recommendation). For me that would be too much of a financial risk. I would size everything to the mixing elbow, with an adaptor at the transom, then if the back pressure is too high, all that has to be done is to replace the transom exhaust port - not easy but not the end of the world.

Good luck
John Mardall
Vetus Maxwell Group

[jimbunyard]

Originally Posted by JOHNMARDALL

I suggest that good starting assumptions would be that Yanmar know what they're doing, know what they're talking about and are not installing improperly sized mixing elbows to save money. (Full disclosure- Yanmar owns our companies, Vetus Maxwell Group). From your remarks, I assume that the fitting already installed is the transom exhaust. You could go ahead and size everything to connect to that fitting and then check the back pressure at the mixing elbow/exhaust manifold connection zone, but if the back pressure is too high, you're then looking at junking the new exhaust hose, waterlock, goose-neck etc and having to buy it all over again at the size dictated by the mixing elbow (our recommendation). For me that would be too much of a financial risk. I would size everything to the mixing elbow, with an adaptor at the transom, then if the back pressure is too high, all that has to be done is to replace the transom exhaust port - not easy but not the end of the world.

Good luck
John Mardall
Vetus Maxwell Group

By all means, follow this advice. Yanmar certainly has looked at your specific boat, application and financial situation, all the time keeping in mind that your personal satisfaction is paramount to their business model.

That's precisely why they're parts are so difficult to ascertain the correct application for, expensive, and easy to get ahold of...

Seems I remember specifically Mr. Mardall recommending fairly recently that a fellow CF member throw his Universal (Kubota) engine away because it was 'too old' and in 'his experience' the trouble of repairing it was not worth the effort.

And, since we don't know specifics of your application, by all means, buy a 3" muffler, 10 feet of 3" exhaust hose, the attendant fittings, and when your engine floods, or overheats, or burns a valve because of the excessive back pressure created by the weight of the water the little 1.6 liter engine has to push out through the oversized system, Yanmar, or maybe Vetus, or maybe even Mr.Mardall himself, will gladly pick up the bill to rectify the results of his proposed 'experiment' for you.

Just went out to verify the exhaust size on my TAMD41B, 200 hp , 3.6 liter, turbocharged, exiting-through-an-outdrive Volvo engine...3.5" OD...


Leaving the theatrics aside, as before, in all likelihood (Mr.Mardall's opinion aside), the 2" outlet will be perfectly adequate. It would probably be best to get ahold of an experienced mechanic and/or shop, with verified references, to assist you with an onsite analysis of your specific application...