Everything You Need To Know To Find The Best marine wet exhaust hose

Introduction

Back in the 1970’s I became heavily involved in the mechanics of boats and before I knew it they became my passion. I have been lucky to have gained over 25 years of experience and 10’s of thousands of engine operational hours (300+ diesel engine installs) and have learned (sometimes the hard way) what it takes to make a reliable engine / power train system, and all that that incorporates. The Marine Exhaust System is a major part of this.
My hopes are that the information in this article will shed some new light on the understanding (and misunderstanding) of one of the most important aspects of a successful boating experience: The Marine Exhaust System.
An important goal here is to remove some of the mystery surrounding much of the misguided and ill conceived “couch engineering” designs that have been imbedded in the boating industry for far too long. Some common sense is the first part that is needed to design a safe and reliable exhaust set up. And, the key word here is “GRAVITY.” Just putting some thought into the general placement of your exhaust system, and with the knowledge that I hope you glean from this article, should help solve many, many horror scenarios down the road. Also remember this (something you can take to the bank), doing it right the first time will leave your wallet much more in tact in the years that follow.
It has been a few years since I posted some thoughts, pictures and politics on Marine Exhaust Systems, so it is time to update. And, since exhaust system design, fabrication, and installation have become a large part of our business, the time is here to share some of what we have learned over the past two decades in this ever changing business.

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First and Foremost

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Well, this is where we have some “fudge factor” if you want to design it right and make it “fit” the boat. Going from a 5″ wet to a 6″ wet ( typically in 300-450+ HP engines ) is HUGE and opens many install capabilities. We like 6″ mufflers and piping from the typical engine room bulkhead aft to the exit near or at the stern. But we like to do our engine risers/custom elbows and wet mixers in 4″ pipe for the dry section and 5″ WET outlets where space is an issue. Using a properly designed dry riser w/ a 5″ mixing elbow, and then transitioning to 6″ at the engine room bulkhead, saves valuable space in smaller engine rooms, saves money for the customer, and is just about always easier and a better overall and “politically cleaner” install. The transition from 5″ to 6″ wet is always easy as 5″ to 6″ FRP tube is an easy make for a custom elbow of any degree at 45 or less. If a highly restrictive muffler is used, or the available space allows the use of 6″ close to the engine, then a 6″ mixer might be the best choice.

8″ WET SYSTEMS – Some “C” & QSC installs, QSM’s-400++ to 800-ish HP-Cummins has “recommended” both 6″ and 8″ and seems wishy-washy on this.

Well, I have yet to see a C, QSC, QSM, or any Cummins (or other make) engine UNDER 500 HP need an 8″ wet exhaust system to meet specs. But, I continue to see this as a RECOMMENDATION for the exhaust size in many cases. I guess if you have the room, the budget, and have an extraordinary amount of bends and/or an overly restrictive muffler, then maybe you should go for it. But from a vast amount of experience, we always stay with 6″ max if under 500 HP.

When we get up into the 500 – 800 HP-ish HP range, this is when we need to think about some, or all, of our WET side of the system being 8″. Since most of my work and the discussions on this site center around repowers of sportfishing boats like Bertrams, Vikings, Hatteras, etc, let’s confine the 8″ to this general style of boat in the 40-60 ft length range. And, with so many older Detroit powered boats/owners still out there that are now seriously looking at repowers, and most of these having 8-71’s , 6-92’s, 8-92’s, etc., this is now even more important to understand.

Usually, these boats have 8″ or even 10″ WET systems aft of the engine room. This is typically where the mufflers are located too. If all is good aft of the engine room – all piping, mufflers, outlets and hoses – (it is usually not), then this is a good size to start with for adapting to the newer 4-stroke engine. If some or most of this piping needs to be replaced because of age, it’s usually easier just to stay with the same size for the section aft of the engine room. If you’d like to size down for space or cost reasons, then a good look at the overall system and engine requirements will be needed.

Let’s look at a new QSM at 670 HP as to what will easily meet the spec. The factory supplies an 8″ fabricated wet elbow or mixer and IMO is of questionable use or quality. Yes, they do work in many applications when used correctly, but also seem to be subject to both internal and external leaks before their time because of the many welds and thin wall construction. Even worse, though, are the “installers” who re-weld or bolt these units in a dangerous “up” orientation in order to build a cheap “Mickey Mouse” type riser. I not only see this with the QSM’s but also in many other installs using the factory 5″ and 6″ “factory” wet elbows.

For those who cannot grasp what the issue is here, a simple explanation – This is a double walled 90 degree wet elbow that was never designed to “hold water” in this orientation but, rather, be mounted horizontally so as to exit downhill and self drain. When installed like this, the sea water that will eventually corrode thru the elbow will now go down the pipe into the turbo or worse. I hope you do not recognize this type of system on your boat!

Now, back to 8″ – This is what we have found with engines in the 600-700 HP range as to designing a wet exhaust system and working around the existing 8″ – 10″ piping or “book” recommendations. We typically use 4″ dry piping from the engine up to a well designed 6″ wet mixer. This usually includes 3 to 4 pieces of 4″ pipe size “weld” 90 degree dry elbows and then a double walled wet mixer of our design orientated on the downhill side of the system. The mixer is always pointed to a spot on the vessel so we DO NOT have to incorporate any WET bends / “90’s”, etc., BEFORE the transition to 8″ (or 10″) from the engine room aft.

With good design and thought, along with some first class fabrication, meeting exhaust restriction requirements can easily be done with a mix of both 4″ dry, 6″ and 8″ wet which will make for a much cleaner install while making the engine room more user friendly as to maintenance, etc.

To recap, exhaust size is something that can be adjusted to not only meet requirements for the engine, but also fit the vessel’s constraints. When sizing the system to meet acceptable restriction requirements in order to protect the engine from excessive back pressure, keep bends to a minimum, especially “wet” 90 degree bends. With some planning, most exhaust routing designs can eliminate a one or two 90 degree bends by building a custom riser/mixer and make for a less restrictive, less cumbersome and safer exhaust system.

Summary

In closing, please understand that with some good engineering and thought, a well designed exhaust system can make a big difference in the layout of the engine room and offer a superior design as to safety and functionality. The idea that rarely a month goes by without our shop seeing a disaster from water ruining an engine from a “factory” type exhaust system or from an OEM builder with his head between his legs still befuddles me. To this day I see drawings from architects who have no clue. The drawing below is from a boat being built that is just such – All that height in the engine room and the architect has the system riser backwards – Just ask yourself what would happen while cranking the engine when out of fuel. And just as bad is that he got paid to design it this way. Sad, very sad…….

Marine Exhaust systems are one of the more important construction features on a vessel. Done wrong they are not only a safety issue for the passengers and the vessel, they can also lead to premature engine failure and / or indirectly cause poor maintenance to be performed on a vessel because of lack of design thought in the engine room. So many times when the exhaust design is an “afterthought,” normal or routine maintenance is close to impossible.

If you give this some thought in the future, maybe this will help you in your next repower or new boat design – The energy that a typical 400 Hp diesel puts into the propeller is matched close to 100% by the energy that goes out the exhaust system. So if you think your power train needs to be well designed, don’t cut any corners on the exhaust design – It needs to be just as tough and built with the same degree of design to handle that kind of energy.

Hopefully the pictures and design ideas incorporated here will help remove some of the mystery in exhaust design that seems to elude much of the industry. I fully understand that a buyer of a new boat does not have much input as to exhaust design when purchasing a new or used vessel, but we hope the guidelines here help not only the buyer to understand what to look for in a boat, but also will prove to be beneficial to the builders in the design and planning phase of marine engine rooms.

It's a case of "out of sight, out of mind" when most people consider their exhaust and fuel hoses. Buried deep in your engine compartment, running under your decking, or winding from an invisible exit pipe to a pump, they're part of a system that keeps your boat running right and safely. For this reason alone, they warrant regular inspection and, when needed, rapid replacement.

Exhaust Hoses

Your exhaust system is constantly subjected to massive changes in temperature. This is particularly true of wet-exhaust systems, where water is used to cool the hot gases from your engine. We recommend rubber or silicone hoses for exhaust systems, because of their durability, flexibility, and strength. Rubber is more flexible, and is able to stretch and shrink repeatedly. However, it will eventually weaken and split. Silicone hoses are considerably stronger, and can last six times as long. They are much more expensive, but we think they're worth it. Fitting a pricey hose may seem pointless, but it will save you money and hassle in the long run.

Vibrations from the running engine can shake the clamps holding the various pieces of hose together. While exhaust hose is reasonably flexible, each separate connection represents a potential weak point in the system. We recommend that you use good quality stainless steel clamps. These are corrosion resistant and may actually last longer than the hoses they're holding together. We also suggest you use "hump" hoses at every connection possible. Simply a piece of hose with an additional "hump" of hose material, they provide more flexibility, absorbing much of the damaging vibration from your engine.

The best advice we can offer anyone fitting an exhaust hose system is to fit the best quality hose you can afford and inspect it regularly. Having your hose burst or split is not only inconvenient but dangerous too. Carbon monoxide poisoning or fire can result-a high price to pay for something relatively cheap to do right.

Fuel Hoses

The same guidelines we suggest for exhaust hoses also apply to your fuel lines. Obviously, anything carrying explosive material has to be strong and durable. Three types of hose are normally found on marine engines: flexible hose (A1, A2, B1, and B2), steel lines (on the engine), and copper tubing. What concerns us is the flexible tubing, since this is the part of the system most likely to fail. The metal hoses can be checked when servicing your engine, but the flexible hoses need constant monitoring. Due to vibration, it is a legal requirement that metal fuel lines be connected to the engine with flexible hose. You should check all joints for leaks at least twice a season. Use your fingers, and look out for stains on or around the fitting. For hoses without permanent end fittings, use stainless steel clamps.

Wipe your hoses with a dry rag and see if you smell gasoline. This is a sign that your hose has disintegrated and needs replacement. Hoses deteriorate more quickly if your engine uses gasoline blended with alcohol. Fuel blended with methanol makes fuel lines brittle, while an ethanol mixture makes them soft. This means that the best way to avoid alcohol damage is to use alcohol-free fuel. All flexible fuel lines must be USCG approved SAEJ1527 hose, which is resistant to alcohol.

So, which hose should you go for? Well, fuel lines use a standardized lettering system. This initially seems a little confusing, but is very simple. Of the two types available, A and B, A is more fire resistant, and has to be used at the fuel pump and carburetor where more than five ounces of fuel could spill if the hose is cut. Type 1 has less permeation loss than Type 2. This makes Type 1 the better hose for all applications. Therefore a hose marked A1 is a stronger, more leakproof, fire-resistant line. It is also more expensive but, as with the silicone exhaust hoses, we think the cost is worth it.

Keep Checking!

Our Marine Insurance Department frequently deals with boat fires. Leaking fuel hoses are a major cause. By checking your hoses regularly and you may spot a potential problem before it happens. After all, replacing a hose is a lot cheaper than replacing a boat!

Are you interested in learning more about marine wet exhaust hose? Contact us today to secure an expert consultation!