Seaboard Marine - Tony's Tips | Building a Custom Lift Muffler & Exhaust System

Here are some pics to show you how some "out of the typical box" thinking can help in making the exhaust as it should be - Safe, practical, and politically correct for the vessel to allow an easy "get around engine room" and one that truly fits the installation.


Starting with a "factory" inline muffler shape applicable to the install.	Gathering the necessary "puzzle" pieces.


Outlet for muffler "dip tube" and side exhaust outlet.	Building the custom mitered exhaust outlet tube to "fit" the boat.


Custom "dryriser" and muffler inlet design.


Getting the "hull" set up for an exhaust outlet.	Side exhaust outlet tube mock-up - You need to protect the boat's LP paint job!!


Outlet flange building process on "mold-release" tape.	COMPLETE !!! - notice the water bypass-this system showed under 25" H2O pressure at WOT and was "super" quiet...

We are here to help, so if you need anything, just drop us a note or post on the forums.

Dry Exhaust Designs and Ideas

Dry exhaust systems are not very common in the typical marine vessel used in recreational service. There are many reasons why you do not see them although, in many cases, they do have desirable features when designed and installed correctly. A dry exhaust system can add considerable costs, is usually more complex as to proper design, fabrication and installation, and can be a dangerous fire hazard if the design and installation is not 100%. What many operators and builders DO NOT understand is the approx 1/3 of the total energy (heat) that comes from the fuel that is consumed by a diesel engine is expelled as heat thru the exhaust system, and this heat must be contained and then exit the vessel safely. With a water cooled exhaust (wet systems), controlling and dissipating this heat is much simpler.



Commercial Fishing Vessel "Ocean Pearl" - Images above show shop construction, layout, and pre-assembly is time consuming but it's the only way. This "dry system" took about 60 man hours to design and fabricate. Images below show support brackets and installation. The under deck installaion took about 2 hours.

When looking at the pictures of dry exhaust systems, a few of the common design & installation features that you may notice are:

1) The use of stainless steel through-out the system when practical. 304L & 316L series stainless allow the use of lighter weight sections because it is not subject to the corrosion effects of diesel exhaust on the inside or corrosion on the outside from salt laden air. SS can operate at the continuous temperatures in the 700F to 1300 F range that diesel engines exhaust produce without losing strength or scaling. SS can tolerate these high temperatures without being cooled by large external air flows that trucks and other non-marine applications have.



Commercial Fishing Vessel "Saint Peter" - 28 Ft lobster vessel powered w/ a Cummins QSB. Images above - we had limited space but worked around what we had. We only had to "wrap" under the deck because of proper "drafting" of the muffler compartment. Images below show lower muffler support and our "unique" type of muffler supports that not only give a lot of adjustability but transfer close to zero heat to the nearby attachment surfaces. "Top hat" and upper stack that employs the same features. Expensive and time consuming to design and fabricate, but this is one place you DO NOT want to cut any corners.

2) Un-cooled exhaust piping "moves" or expands and the use of flexible "bellows" is mandatory. We use SS bellow made of 321 SS at it has shown to give long term reliable service in demanding high hour commercial application.

3) Under the deck is where most care must be taken as to spacing and supporting or "hanging" the system. You must allow enough space to install proper insulating blankets or covers and all support brackets must allow a minimum of heat transfer to the adjoining surfaces.



Cummins N-14 in the 75 Ft commercial seiner "Anthony G" with 5" dry outlet to 6" bellows and muffler.

4) Once the exhaust exits the deck and is now in the typical "muffler" compartment above the deck, the use of natural "drafting" from the heat is critical to safety. Ever wondered why the stacks on ocean liners are so large on the outside? That's because they use the heat that goes out the exhaust to "draft" the internal piping and engine room - just like a chimney, the heat draws excess air thru it.

5) Dry mufflers are large, heavy, can be quite cumbersome to deal with, and get extremely hot. The bracketing to hold and support a muffler on a vessel must not only be strong, it also must not transfer heat to the surrounding support surfaces. We always leave plenty of room in this area and allow the use of draft air to cool the support brackets. If the muffler compartment is large enough and well drafted, then using an insulating blanket around it is not necessary as the draft air becomes the insulator. But since each design is different, this is now an option based upon the uniqueness of the overall installation.


Cummins QSC w/ entire dry run and muffler support "on-engine".	Simple John Deere 99 Kw generator and attachment to existing "under the deck" dry muffler - Installed on the 90 Ft, 300 Ton commercial long liner "Ventura II".

Hopefully if you are going dry, some of the designs and ideas we employ will help you build a safe and well constructed system.

Some Thoughts on Underwater Exhaust Systems

Underwater exhaust systems can offer many advantages (and some disadvantages) to the overall design on a boat. In some cases, going "underwater" may be the only choice in many applications because exhaust routing from the engine room to the stern is close to impossible, or good planning was not done during the initial design. Having underwater exhaust may seem like the ultimate way to go, simple, "it must be super quite", and "maybe I don't even need a muffler". Well, this is not the necessarily the case at all as going underwater adds another dimension to the exhaust system many builders soon find out at first seatrials. "RUMBLE"

Typically, underwater exhaust is what is normally referred to as "exhaust exiting" through the bottom of the boat, but in some cases, underwater exhaust could also be exhaust exiting at the transom or the side of the boat that is below the waterline. Since exhaust exiting at the transom (or on the side of the boat below the waterline) may or may not be "under water" when the boat is traveling forward I am confining this page to exhaust exiting thru the bottom of the boat, which is always under water.

As many boat owners may have already figured out from personal experience, underwater exhaust may not be the perfect solution to noise control, although many with no experience in underwater exhaust always seem to think, "What could be a simpler way of muffling an engine's exhaust system." It seems that most underwater exhaust systems that I have seen in the last ten years have had issues with rumbling at low to medium speeds, causing a different type of "discomfort" in the confines of the vessel. Although the outside noise may be low, the exhaust energy that is dispersed beneath the vessel's hull can have a tendency to create a low frequency and very uncomfortable "rumble" or vibration inside the vessel at some speeds.

What we have learned about underwater exhaust in the past 20 years is that mufflers still need to be an integral part of the exhaust system so that the exhaust energy is reduced / absorbed before exiting the vessel.

Some advantages of underwater exhaust are that it allows the exhaust to exit in the engine room floor and not have to travel at its full size dimensions all the way to the transom. Over the years we have found many boats have no provisions for increasing the size of the exhaust piping aft of the engine room or is close to impossible to do, leaving us no choice but to exit the side of the vessel, OR, underwater. Sometimes we even use a combination of underwater and a stern exit / bypass. The exhaust system on any vessel needs to be carefully examined to see if an underwater solution is feasible, and / or is, the best alternative.

One of the main goals with any underwater exhaust system that can be used to the advantage of the vessel owner is the forward movement of the boat, in that it can and will create a low pressure area with the forward movement of the vessel keeping back pressure to a minimum when designed correctly. In many cases, this is a distinct advantage and allows a smaller size to be used which would go against most ideas as to how exhaust systems actually work.

Below are some pictures and a simple "outlet drawing" drawing showing the basic layout of a typical underwater exhaust system with a horizontal inline muffler that has been modified to operate vertically, and a smaller bypass muffler. Again, this is only a typical representation, as many times, a lift muffler is used as the main muffler before the underwater exit, as it can be the best choice for some applications.

Cummins QSM 670 UNDERWATER exhaust system. The system uses a 4" dry riser w/ 6" mixer, to a highly modified muffler, and under water outlet. Notice the 4" wet bypass to the transom that incorporates a small inline lift muffler - This particular system showed under 20" H2O exhaust pressure at WOT and was very quiet at all speeds.

6" input

4" bypass to side outlet

Underwater 8" outlet with 24" reverse scoop

Basic layout for this particular vessels exhaust system. When the builder designed this vessel, he left zero provisions for a conventional stern exhaust, so an "underwater" solution had to be found.


Starboard Exhaust for a 670 QSM - 4" Dry Riser to 6" wet mixer.	Port Exhaust for a 670 QSM - 4" Dry Riser to 6" wet mixer.


Close-up of the Starboard riser/wet mixer to muffler connection. - Because of the short hose, a "double hump" was used for added flexibility and safety".	Custom in-line heavily modified Centex muffler glassed into the vessel bottom.


In-line lift muffler makes for 4" low pressure by-pass to a side exit just before the transom.	Base mock-up of port riser w/ measurement for a custom wrap.

And last is the most obvious criteria for any underwater exhaust design - With any underwater exhaust system, the exit thru the bottom of the vessel needs to be designed in such a way the it is no more vulnerable to cause an issue with as to hull damage, physical penetration, water intrusion, failure of components , or ? as the main hull itself. So leave this part of your vessel design or repower to a company or builder well versed in this type of work and has a proven long term record in building underwater exhaust systems.

Tony Athens | October 1st, 2006