Marine Forensic Technicians
Marine Forensic Technicians, Stevensville, Maryland - James R. (Randy) Renn, SAMS® AMS® Marine Surveyors
Stevensville, Maryland

James R. (Randy) Renn, SAMS® AMS®
Principal Marine Surveyor
Damage & Insurance Loss Specialist

Worldwide Service




Boat Mufflers-An Exhaustive Journey
Greetings fellow boaters, we are here in this article to polish up on the ever titillating subject of Marine Mufflers. WOW what a subject! Hey now how often do you hear the query, "have you seen your muffler recently?" Over the many years of pleasure and commercial boating I have never replaced a modern muffler, lots of manifolds, metal boxes, barrels, risers, stand pipes and hoses but never a modern muffler. So why do we address a subject with such a low failure rate, or high durability rating? Glad you asked. The reason is quite simple. We are an aging fleet and while it is true that most of our boating population is made up of Outboard and Inboard Outboard vessel which do not have mufflers as such they do have some form of exhaust and many principals overlap.

We understand that our primary concern must be safety. No exhaust leaks whatever can be tolerated. That means no seeping riser gaskets (Inboard Outboards are famous for this) at any point. If it is leaking out it likely is seeping in so change gaskets at first signs of rust streaks. Fiberglass mufflers can be weeping tracks of salts from seams and drain plugs. Same principal leaking cannot be accepted. Metal units rust, fiberglass ages, exhaust hoses cycle millions of times. The exhaust system, whether it is a simple barrel muffler or a complex with required dry sections, is a dynamic thing. Systems get hot, expand, and are repeatedly cooled by seawater in an extremely hostile environment.

So where to begin? We let us discuss the major concern in system design. While noise control is a consideration it is not the most difficult to plan. The large issue is in the form of "Back Pressure". This area is where many boats/owners get into difficulties. Back pressure is the measurable entity in the form of Pounds Per Square inch or Inches of Mercury that your engine will create in an effort to evacuate exhaust and cooling water from your vessel. How much can this be you ask? Well let us calculate some little figures. Lets us say you have a 150 horsepower diesel that rests two feet below the waterline. From the back of the engine to the transom is perhaps 14 feet and with all the twists and turns the total exhaust run is 25 feet, add a muffler and 3 feet of rise in the system. Given that at some times-several times a minute- the system run is a least partially filled with water how much pressure will the system need to pump the water out? This figure is recommended by the engine manufacturer and is can be as low as 3 PSI-not much. Which means system runs must be smooth and large at times. We care very much about this figure because too much back pressure affects engine cooling, turbo life, cruise speed, smoking and engine life in general.

How do I measure my system and why would it change? Back pressure readings can be taken by a reasonable wrench with simple analog gauges at basically any point in the system close to the engine in the system that can be tapped and a fitting added-some mufflers have usable tap point. Many diesels come with pressure taps-not to be mistaken for Pyrometer taps although they may be the same location. Why would the pressure change from when the vessel was built? Several reasons, mufflers and hydro lifts can become clogged with residue, rust, hoses can collapse internally-large cause, vermin can built nests, metal muffler can scale and fiberglass mufflers can lose internal baffles. Mufflers maintenance items but frequently overlooked when considering performance issues. Your engine is not as strong as it once was so additional resistance can multiply in systems.

Most of what we see in exhaust layouts are in the form of exhaust percolation arrangements. This is where exhaust and water are fed into a barrel type unit, and pressure builds to a point in pressure higher than the weight of the water in the barrel. The water and exhaust are then "percolated" out of the vessel. While this is a tried and true method there are a couple of drawbacks. First the system is constantly loading and unloading the exhaust line (turbos do not care for this-actually nothing does). Secondly these systems can be noisy in particular when in a quite harbor with a generator whooshing and burping at night.

Enter the newer kids on the dock. Bypass and Halyard systems are fairly new on the market in large numbers. By pass systems divert off some water flow and exhaust out of the side of the boat or to an underwater outlet to help with the loss of water dampening for exhaust noise. Halyard systems and Halyard type systems separate water and exhaust after the exhaust gas is cooled and sends the cooling water overboard below the water line. This system eliminates exhaust cooling water splash and pulse reducing system noise output considerably. In a recent mid-sized power boat at anchor we turned down the stereo to see if we could hear the generator-amazing. Another newer system is the vertical dry stack, similar to commercial boats but the system starts out "wet" separates the gases and sends them out above the vessel top deck.

A fine source concerning exhaust systems materials and installations will be ABYC (American Boat and Yacht Council section P-1. This portion of the ABYC Manual speaks to outlets terminus, back pressure, what maximum system temperature should be in relationship to structure and a myriad of system parts description. Speaking of temperatures a great tool for this is an infrared thermometer. They run around $30 from your local Auto Parts Air Conditioner tool catalog and useful all about the boat. A further point is that the machinery space must be properly ventilated to reduce maximum temperatures that affect engine performance. Modern boats can be so air tight that closing an engine hatch can slow them down, help them overheat and be very tough indeed of hose and electrical component life. Crushed vent hoses, poor runs weak blowers can all contribute to performance loss. Add to this the heat that a newer high speed diesel can generate if only from its inner cooler and reject into machinery spaces and things can go south.

Mufflers, tubes and connections come in a wide variety of configurations and these days mostly in configurations of standard shapes for fiberglass and roto-molded types. The combination of available sizes and shape patterns allow the skilled weekend warrior to assemble a custom system with little trouble and not great expense. Have a care about system support and back pressures. Stainless systems or combinations that use flexible steel tubes should be left to professional builders and tend to be painfully expensive and far too difficult for a DIY person. Always use materials that conform to your situation and SAE Standard J2006. Flexible plastic/rubber hoses vary considerably in price and temperature ranges; suggest Trident Marine as an information source.

Hose clamp/tee bolt failure from over tightening, external corrosion, dissimilar material attack or physical damage can result is several conditions that do not seem obvious on the surface. Over tightening clamps will deform hoses and greatly shorten life span-see photo of a leaking system from over tightening. If you can see surface rust or wire coming through the hose or the hose is surface cracked it is time for replacement. Leaking hoses can very rapid cause vessel flooding so even the smallest leak should not be ignored. Any metal union or fitting found to be seeping rust must be replaced. Concern over the seep would be one item but complete union failure due to internal corrosion can really spoil a season if not just a day.

Home or shop made systems of iron pipe are generally unsuccessful in the long run and usually problematic. If you have an older iron "Stand Pipe" system consideration should be given to system redesign due to catastrophic failures almost always at the wrong time. Any part of the systems that is bolted, clamped or welded to the engine must be supported-really supported as to not load hard bolted components with systems weight. A mid-sized diesel exhaust section can easily weigh 90lbs per part and larger units very much more. Stainless adjustable braces are suggested.

No matter what your system is a seawater flow alarm of some nature should be added. Either in the seawater pick up line or near where the seawater joins the exhaust a system should be installed for early warning of seawater loss. Seawater loss can result in the system being damaged or catching fire in very short order. You can damage a seawater side exhaust system before the freshwater cooled engine it is attached to will sound its alarms.

For some thrilling reading, wonderful diagrams and a lot of information try among others, ABYC P-1, The Vetus Den Ouden Catalog-1-410-712-0740, Halyard Marine-www.halyard.eu.com and www.tridentmarine.com.

All the best boating and stay off the lee shore.

Engine mythology: You can tell if the sea water impeller is going bad by the type of spray the exhaust puts out. If you believe this please contact my office as I have a boat for sale that needs just a "little" work.

James R. Renn, AMS-E


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