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I noticed that the large custom sportfish builders are making the exhaust exit through the bottom of the hull. Not only for noise but also for scavenging. Any one have any experience/comments on this?

How much scavenging takes place from this? think its possible to have negative back pressure with this system?

I have not looked at the underwater exhaust on any of the new boats, but I would think that they have an exhaust bypass so that the engines will start. They need to breath to start, and I might be wrong, but I would think they would be hard starting if the only exhaust was to push water out of the exhaust system.

Once the engines are up to speed then I think the exhaust pressure would keep water out of the exhaust and allow the engine to breath underway.

Tony- I think the idea in question has to do with the vacuum created as water flows past the underwater exhaust outlet. Something I never considered, but quite an interesting thought.

That is what I was thinking also, but isn't that the Venturi effect?

In any event I would still think they need a bypass or bleed off of the Exhaust gas until it kicked in. So not sure it really accomplishes anything other than quiet down the boat and reduce or eliminate exhaust in the cockpit due to the station wagon effect.

I kinda wondered about the increase in back pressure being a problem at idle and low speed...Eddie Murphy in 48 Hours with the old Banana in the Tailpipe Trick stalling out the detective's car...funny stuff.

Back on track...I think it's 3 psi max back pressure for my Cummins 6BT...I have seen some boats with the exhaust ports completely covered when loaded heavy. I'm not sure how much as I haven't gone that heavy to sink my ports completely for any length of time. Some motors don't mind back pressure as much. Yanmar comes to mind as they seem to run small-diameter exhaust comparatively. I think I heard some motors kinda need a certain degree of back pressure. I may be wrong on that, I asked a good mechanic friend about my exhaust temp being high/back pressure and he went off on a tangent. Don't you hate people like that???
So...maybe a bleed-off is needed or maybe once the pressure builds up enough to start flowing the pressure subsides.

I'm not sure if it is the Venturi effect or the Bernouliis principle. I think the Venturi effect deals with the lowering of pressure through constrictions and Bernouliis has to do with faster-moving fluids having less pressure than slower-moving fluids. As it concerns boat exhaust, the faster-moving water going past the slower-moving exhaust creates low pressure inside pulling the exhaust out.

I'm going back to work to give my brain a rest while we wait for someone smarter to answer this...

Carl

I took a look around the internet and so it is true. They do have a bleed off of the exhaust until the boat gets up to speed.

Notice they say it is the Venturi effect. LOL

Finally I get to use a word that is not in my vocabulary.

Based on this, the flow of water under the boat does not seem to be helping to reduce the back pressure, its the venturi effect that is working in favor of the exhaust flow.




WRITTEN BY
Jorge Lang
Jorge Lang is the brand ambassador for DeAngelo Marine Exhaust with 21 years of experience in the marine industry. +1 954 763 3005; sales@deangelomarine.com; www.deangelomarine.com

An underwater exhaust is known as a clamshell, an eductor, and a cowl, among other names. If you visit any yard with larger vessels hauled out, you'll see them. They can be round, square, short, long, or barely noticeable. They vary in design as much as the vessels that have them, but what's their purpose?

An underwater exhaust is just that — the exhaust exits the bottom of the boat when the vessel reaches cruise speed. At idle or at low-speed operation, the exhaust gases exit via a bypass above the waterline. This way, you won't have gurgling and a need to push a column of water with exhaust before the gases can escape.

Engineer Creates Software to Help with Vessel Operations

Some of the advantages of an underwater exhaust include:

The system is quiet as the exhaust exits below the waterline and will be far from the vessel as it surfaces and the boat moves at cruise speed.
No main muffler is needed. In comparison, many European builders use a large, dry muffler mounted over the engine. While this will be a quiet system, it poses some major issues - that steel muffler will expand as exhaust temperatures increase and will then reduce in size at ambient temperature, which can cause cracks. This also generates heat in the engine room, even with insulation, and makes it look crowded.
A properly designed underwater system will have a bypass about one-third the size of the main underwater outlet. For example, if you have a 14-inch (35.6 cm) main exhaust outlet, the bypass should be five inches (12.7 cm). For attenuation, a muffler is used in line on the bypass.

Issues that would affect underwater exhaust performance mostly involve proper design. The clamshell needs to be sized properly and should cover about 50 percent of the outlet, which will produce the Venturi effect and help the exhaust or column of water out from under the boat. The cowl's shape will vary also, depending on the vessel's cruise speed. Another important design consideration is the outlet's depth - too shallow and the exhaust will be heard if the vessel is rolling in rough seas as the outlet moves above the waterline, and too deep and the engine may not be able to "blow" the column of water out the stalk to enable the gases to be released.

Why Yacht Engineers Should Be Onboard Sampling
Some of the stalk connections have hoses and clamps; some have flanges with or without butterfly valves, depending if the vessel is classed and by whom. While most vessels are candidates for an underwater exhaust system, it's mostly for vessels larger than 24 meters.

Thanks for doing the homework on this Tony. Interesting read.

LOL, Yes I did notice it said "Venturi" effect...hard to miss that in bright orange, I stand corrected.

Carl

The interesting thing is that there is a bleed off line, which is always there to release pressure. From the designs I saw online I don't see that it closes off at speed. That means it is hard to tell if the flow of water helps reduce back pressure. I am assuming that the flow of water helps a little, but it looks like the design is more for reducing the station wagon effect you get when running.

I am not sure what it does to the exhaust sound. I would also like to hear from the offshore guys that have them if they think it helps or hinders raising fish.

Blowing the exhaust underwater is not all that new, I've seen older boats with a thing-a ma-bob casting going from where you'd expect to see the transom exhaust port that went down below waterline to release the exhaust.

Also I/O's vent from the prop as do outboards

Carl

It is an old technology. I don't think it ever really caught on or was such an innovation every boat had to do it.