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Old 10-10-2004, 03:33   #1
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Exhaust System Maintenance

EXHAUST SYSTEM MAINTENANCE:

Often overlooked, exhaust systems are an important aspect of any boat's maintenance schedule. A few preliminary checks and regular care will not only save time but also prevent a costly repair bill. Unfortunately, many boat owners pay little attention to their craft's exhaust system - That is, until something goes wrong, and fumes and smoke billow into the boat.

Note: See important SAFETY note and links at the end of this article.

Over time, heat, acids, corrosive gases, vibration, rust and seawater all play an active part in deteriorating an engine's exhaust. However, if corrosion checks and maintenance are carefully carried out, exhaust systems can last up to 20 years.

Wet Exhausts

The role of an exhaust system is to remove spent combustion gases away from the engine. The by-products of this process are heat, vibration, fumes and, of course, noise.
As engine temperatures are usually high, water is injected into the exhaust pipe, to mix and cool the gases. This mix and the resulting condensed solids (such as soot) are run through a silencer and a pipe, which exits overboard. There are a number of practical aspects to be considered, which arise as a result of the engine's confinement in a boat's hull.
As the engine is often deep below the waterline, the exhaust pipe must run at an angle to prevent any cooling water running back directly to the hot engine. Such an occurrence can result in extensive engine damage. In the case of an engine placed below the waterline, water trap and drain devices are situated as close as possible to the engine. This prevents the water being sucked back.
Condensation water is also allowed to drain away.
In the case of an above waterline setup, vacuum valves are fitted in the exhaust line to avoid exhaust water being sucked back as the hot gases cool and contract along the length of the pipe. Also, it must be arranged so that water cannot siphon back into the engine if the boat is heeled over while underway.
Wet marine engine exhaust systems are generally designed to resist temperatures of not more than 120̊C. The exhaust gases, however, may reach more than 500̊C. To cool the gases, the exhaust cooling system depends on a free flow of cooling water from the engine. This flow can drastically decrease, due to a lot of reasons such as a plastic bag or seaweed being sucked into the seacock under the keel, or by a problem with the water pump. The exhaust temperature will rise immediately to around 450̊C, the exhaust will overheat and may be seriously damaged.

Dry Exhausts

Dry exhaust systems are less common than the wet system due to the fact components are more complicated and more costly.
In a dry system, the hot gases are led through a series of water-cooled exhaust jackets that surround the exhaust pipe. The water is sucked in from the river or sea by pump, and once the heat from the exhaust pipe is 'exchanged' to the circulating water, it is then pumped overboard again.
Sometimes the heated water is run through a heater or hot water system before being exited over the side.
With direct seawater cooling systems the temperature must be kept below 57̊C because the salt solidifies inside the engine/exhaust components. It is important that the engine is run at the correct operating temperature to prevent this. Regular flushing will help prevent this problem.

Exhaust Checklist

Vibration of solid mounted engines is a major culprit when it comes causing problems in exhaust systems — and thanks to other nuts, bolts and components that end up being shaken loose. If the exhaust gaskets break or leak as a result of this, the danger of carbon monoxide poisoning is high.

* Check all bolted exhaust fittings on a regular basis and remember, the weight of the pipes must never be borne by the manifold itself. This can lead to a disastrous situation where leaks and cracked manifolds result.

* The exhaust supports should be checked to ensure the pipe is supported at least every 4ft.

* Clamps should be doubled on flexible pipes and checked for loosening by vibration and corrosion. The clamps on exhaust muffler boxes are well known for freezing solid with rust. Keep them freed-up with heat-resistant graphite grease. And remember, a long run of exposed hot exhaust pipe must be insulated. The heat of a pipe is dangerous and allows excessive heat damage to woodwork, paint, and in extreme cases can be a fire hazard. In addition overheated air in the engine compartment allows the engine to run hotter than normal and this results in inefficiency. Ensure adequate ventilation is supplied along the length of the exhaust run.

* Rubber perishes quickly in an overheated environment. Check all hose clamps and double them on exhaust systems. The weakest link of the exhaust cooling system is the impeller of the engine-driven pump. Always keep a spare. (there’s an easy access kit that allows you to change the impeller quickly and easily)

* Kinks and sharp turns and elbows should be avoided at all costs to prevent back pressure in the engine.

* If exhaust pipes have to run through bulkheads ensure the correct insulated type of metal fitting is used. Hot pipes can warp, crack damage and burn interiors.

* Large craft often have exhausts which exit overhead at the cabin ends. It's easy to forget to inspect the pressure flaps for sticking or damage. It's surprising how much rainwater a three inch pipe will allow in. Once in, this water can corrode the bottom of the pipe.

Regular Check Ups

Thorough and regular checking and maintenance of your exhaust system will repay itself with saved cash dividends. On the other hand, a neglected exhaust system can be the source of smells, damage to interiors, components and a danger to wiring.
Worse still, it can be a noisy, vibrating and life-threatening hazard that could result in fire and the total loss of your craft!

Carbon Monoxide (CO), the "silent killer", is a by-product created by engines through the combustion of carbon-based materials such as gasoline and diesel fuel (also: propane, kerosene, charcoal, and wood). When CO gas collects in an enclosed space it can become fatal, even if a person only breathes it in for a very short period of time!

See also:
“USCG - Checklist”
http://www.uscgboating.org/safety/fe.../checklist.pdf

“USCG SAFETY ALERT - FUEL AND EXHAUST SYSTEM MAINTENANCE”
http://www.uscg.mil/hq/g-m/moa/docs/7-02.htm

“USCG - Carbon Monoxide Hazards on Recreational Boats”
http://www.uscgboating.org/safety/fe...saf_carbon.htm


HTH,
Gord
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Old 10-10-2004, 08:42   #2
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Dry exhaust

Interesting that the dry exhaust system describes a wet system, or sea water cooled system. My understanding is that a dry exhaust is a dry exhaust. I plan to use a dry exhaust when I put a diesel engine in my fishing boat. I will use a fan and insulation material. I do not want to have salt water anywhere near the engine or exhaust.
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Old 10-10-2004, 10:17   #3
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I am NOT an engine nor exhaust expert, and don’t intend to be taken as offering authoratative advice. What follows might be considered “conventional wisdom” - which may be wise (or not), complete (or not), and accurate (or not).

Exhaust gases are hot enough when they leave a typical diesel engine to set a vessel on fire and cause severe personal injury. A wet exhaust system lowers the exhaust temperature enough so that it can be routed thru FRP pipe or rubber hose instead of insulated steel, greatly reducing this hazard to vessel and crew. The author of “Voyaging Under Power” agrees when he states: “Unless every precaution is taken, including a generous contribution to exhaust housing space, it’s safer to stick with the wet system”.

On the plus side for dry exhaust, a well-built system will typically require less looking-after than a wet system; it has no raw water circuit to maintain.

But the benefits of the wet system are manifold: There is much less heat emitted from the exhaust line thereby reducing the demands placed on the ventilation fans and thermal insulation for cooling the vessel. The use of rubber exhaust hose absorbs the vibrational energy of soft mounted engines and generators and allows the mounting of the exhaust lines to the vessel’s structure without the requirement for sophisticated elastic mounting hangers and flexible exhaust couplings to control noise. When the exhaust gases are not cooled with water, the required steel pipe undergoes expansion as a result of the extreme heating and then contraction as it cools after engine shut-down. These repeated expansions and contractions can fatigue and over stress the exhaust line if not carefully designed and installed. This problem effectively goes away with the water cooled exhaust system. With water cooling, particulate and condensable/soluble gaseous emissions from the exhaust system are effectively scrubbed from the exhaust gases, reducing the possibility of air pollution. In other words, the exhaust is cleaner (less soot) and greener (easier on the environment.) Exhaust odor is also greatly reduced by this “scrubbing” process. The alternative is a non-scrubbed dry exhaust exiting out a smoke stack above deck. Since there does not exist this scrubbing process, such a system tends to dispose of its exhaust soot over the decks instead. A further disadvantage of a smoke stack is that it is a fixed vertical obstruction thereby increasing the vessel’s air draft. Voyagers might find this a problem if planning to cruise on inland waterways or canal systems of the world; the many bridges and sometimes fixed tunnels may require temporary re-piping of the exhaust, which may not be practical, or finding an alternate route of travel. On the wet system, the mast can be easily lowered for such a trip. Note also that with the smoke stack configuration, the hot exhaust must be routed thru the living space of the yacht. The steel exhaust pipe must be enclosed in a fire resistant and ventilated interior trunk. This trunk will occupy valuable living space and can be a source of noise and heat. The next advantage of water cooled exhaust is that the water injection has a sound dampening effect on exhaust noise thereby producing a quieter running vessel and permitting the use of a smaller muffler than would be required for equal sound attenuation on a dry system. This of course means more space and less obstruction in the machinery areas. A side benefit of wet exhaust is that the same water source that is used to cool the exhaust can be used for cooling and lubricating the shaft stuffing box. This type stuffing box lubrication also serves to flush debris from the cutlass bearing enhancing its longevity and smoothness. A most logical choice for this water source is the raw water discharge of an on-board heat exchanger. For cooling of an engine’s jacket water, the inboard heat exchanger has advantages over an external keel cooler. So, not only is the heat exchanger advantageous for engine jacket water cooling, its raw water discharge provides a ready source for cooling the exhaust and for lubrication of the stuffing box; the systems all work together in harmony. To explain the heat exchanger preference, some of the problems avoided by not using keel cooling can be stated: Heat exchanger cooling is factory designed and installed for optimal performance for each engine model; it is part of the engine package and the installer need only make the appropriate raw water hook ups to the exchanger; there is no tampering with the engine’s jacket water circuit. The jacket water is the fresh water (typically chemically treated) which passes thru the engine galleries removing heat from the engine. This is a continually re-circulated closed circuit. When keel cooling is employed, this circuit must be tapped into and re-routed to the remote keel cooler. In doing so, the design and installation must be carefully executed to avoid high spots in the keel cooler branch where air pockets can be trapped. Air trapped in this way would not rise up to the engine’s expansion tank to be purged from the system. Such an air pocket would reduce the heat removal capacity of the keel cooler and be difficult to detect. Likewise, if marine growth is not kept from developing on the exterior of the cooler, its heat removal capacity will also be reduced. Marine growth must be controlled by periodic cleaning as bottom paint will also diminish a cooler’s heat removal capacity. A rise in jacket water temperature resulting from these faults will contribute to increased engine wear or failure of components. These problems are avoided if the factory designed heat exchanger package is used. The next disadvantage of keel coolers has to do with their reliance upon the flow of water past the cooler for adequate heat removal. When a vessel is moored or anchored in still-water conditions, there may be insufficient flow -or no flow- present for keel cooling of a generator’s jacket water. The solution is to have a cooler with a heat transfer area large enough to handle this condition; but, such a cooler may be excessively large for conditions where flow is present. However, with the heat exchanger system, the engine’s raw water pump provides a continuous flow of water over the heat exchanger tubes, so, even at times when no flow is moving past the hull, the on board exchanger is still working efficiently. Since the raw water pump provides such an important function, it should be well cared for and monitored- as should be the rest of the cooling circuit. The pump’s operation is usually easily checked by inspecting the transom exhaust for the tell tail stream of water. The raw water strainer should also be checked as part of the ship’s routine engine room watch. If cooling flow were ever lost, however, the engine’s high water temperature alarm would eventually sound. For added safety, a flow sensor could be installed in the exhaust riser water injection line. If raw water cooling flow ever fell below a pre determined level, the sensor would immediately activate an alarm providing valuable early warning. By the way, one potentially important side benefit of this raw water pump is its capability for use as an emergency bilge pump. Several common piping arrangements have been developed which allow for such emergency rerouting of the pump’s suction to the bilge. This ability is not present with dry exhaust and keel cooling. Keel coolers are also vulnerable to damage from passing debris or grounding and they add drag to the hull. A hull indentation may help to better protect the cooler and reduce drag but recessing tends to take the cooler from clean flow into stagnate water thereby diminishing the cooling effectiveness. Hull indentations may also produce stress concentrations in the hull structure and make fiberglass layup more difficult. Furthermore, even with a recess, the cooler remains vulnerable to damage. This vulnerability is virtually nonexistent for the heat exchanger system because of its location entirely within the ship and, of course, being within the ship, there is no parasitic hull drag. The keel cooling is good however for vessels operating primarily in dirty water where silt and other particles in the water continuously clog the raw water strainers. In summary, wet exhaust with heat exchanger cooling is preferred over dry exhaust with keel cooling because the system is safer, cooler, smoother, simpler, cleaner, greener, less odorous, allows lower air draft, quieter, provides shaft lubrication, factory designed and installed, not susceptible to air entrapment, not susceptible to marine growth fowling, less vulnerable to damage and provides continuous cooling flow.

Some further reading:
Noise Reduction on Power-Driven Vessels - Appendix D (Transport Canada)
http://www.tc.gc.ca/marinesafety/tp/.../appendixD.htm

DRY ENGINE EXHAUST HAZARDS (USCG Safety Alert)
http://www.uscg.mil/hq/g-m/moa/docs/sa0697.htm

US Code of Federal Regulations - Title 46 Sec. 182.425 Engine exhaust cooling.
http://a257.g.akamaitech.net/7/257/2...cfr182.425.htm
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Old 10-10-2004, 13:01   #4
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Just to condense Gord's excellent article, the main advantage of a Wet system is reduction in NOISE, Vibration and heat. Especially NOISE, as this is reduced right from the injection point of the water and helps reduce the emmision noise through out the path of the system, which of course is through the boat.
And just to add, a well designed Wet system is just as safe as a dry, as far as water getting back to the engine.
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Old 10-10-2004, 16:25   #5
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Dry exhaust

I will check the links you have posted. While not disagreeing with the points you have posted I would like to add some thoughts. There are marine mechanics that will tell you a dry exhaust is more trouble free than a wet system. There are many boats providing good service with a dry exhaust system including fishing boats and sailboats. Muffling is fairly easy, a skeg can be used for cooling with a sail boat. A dirty bottom is a dirty bottom. A bucket is a more effective bilge pump than the engine which uses very little water. An aluminum hulled boat can have the cooling tanks inside. Quite a few automobiles have a dry exhaust system and the engines are operated at higher temperatures. Heat exchanges are not trouble free. Salt water causes damage, the lack of it helps eliminate some problems.
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Old 13-10-2004, 13:55   #6
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Hi BC, I think it is going to be "horses for courses". It really depends on the design/layout of the vessel as to which situation is going to be best. For instance, many of the commercial boy's that have dry exhausts, have it piped vertically and the exit is above the cabin/pilot house. Noise is not so much an issue on a work boat. There is a lot more room and a housing of some form is often built around the pipe to insulate it from heat and noise transfer. Some boats, that system is not so easy. On a yacht, or motor sailer, who wants an exhaust outlet blowing over your nice white sails.
For a commercial fishing boat, who wants to have there head over the exhaust while they are leaning over the back fishing.
But there is good points and bad points for both systems. I remember a few years back, actually you may have been here then. The Police boat Deodore was built. It had a very flash SST wet exhaust system fitted. They wanted it to last for as long as the boat and the engineers were very proud of the workmanship. Well the thing kept cracking and falling in bits. They couldn't work out why for awhile, till they discovered that SST work hardens. The pulse from the exhuast and water was cuasing enough expansion/contraction in the pipe to work harden it. And it would simply crack through. ooops
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Old 13-10-2004, 17:59   #7
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Exhaust

Good points. I am going to put a 2.5 litre Nissan diesel in my 18 foot aluminum fishing boat. Will likely use a Volvo stern drive leg as that would be the least work and cost. I could use the leg to pump water through my engine but I do not want to ruin it with salt water. So I will make cooling tanks for the inside of the hull. The exhaust is easy for me to insulate and run dry with a fan if needed and it is easy to keep in out of the way and clear of the water. I want to read a book on the subject by Nigel Warren before I do too much work. If I keep the salt water out the engine should run a long time, at least that is the plan.
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Old 14-10-2004, 00:31   #8
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The LD28 (2.8Lt)is a very popular engine used in NZ. I guess it is cheap and easy to get hold of a replacement. Bit of a brick though. Everyone I know who has used one have always had to Turbo it to get it to perform.
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Old 02-12-2004, 13:11   #9
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Ceramic Coating

Has anyone had any experience with Ceramic coating the elbow in the exhaust system. I have used it extensively in my turbocharged auto exhaust systems, heads and valves. It has improved performance in those applicationm and the service life has been wonderful. I tore one of the engines appart after 4 years and there was no signifcant degradation in any of the parts that had received the coating. The coating should decrease the wear associated with the corrosive nature of the exhaust/water mixture.

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Old 02-12-2004, 21:16   #10
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Thumbs up Thanks for that Idea!

We are constantly replacing 6" ID elbows on some of our Vactor rigs (not boats) and I'll bet that'll help with the wear factor. Now just need to see if it's cost effective.
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