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