This section follows the Cooling System because of the close relationship between both systems on wet exhaust boats.
The overriding consideration in designing an exhaust system for a boat is that water must not be allowed to find its way back into the exhaust manifold and thus into a cylinder. If this happens on a diesel you will be paying for an extensive overhaul or a new engine as soon as the engine spins.
Ideally the system will also be more than capable of passing the exhaust gasses with minimal back pressure, but backpressure will only make smoke and reduce power – not instantly wreck engines.
Inland boats with the exhaust manifold well above the waterline can use almost any form of exhaust system, but sea-boats and any boat that has or may have the manifold below the waterline at any time must have a system that minimises the danger of water entering the exhaust manifold.
These systems are much like a car or truck exhaust and those parts close to the manifold may well glow yellow hot under high power, so they may not be the best of systems to use on GRP or wooden hulls. They will require a silencer of adequate size to minimise backpressure.
A length of flexible metal pipework must be fitted between the main system and flexibly mounted engines. This is normally hidden beneath lagging.
All parts of these systems should be adequately shielded or lagged to prevent injury and fires.
Metal inland boats are likely to have an exhaust exit through the hull, but wooden, GRP and sea boats would probably be safer with an exhaust funnel to keep the system well away from flammable materials and as far away from water as possible.
A "normal" silencer that almost totally silences the exhaust will create too much back pressure, but if several of these units are connected in parallel (giving a number of different paths) the result is a very quiet exhaust and acceptable back pressure. The whole lot are packed into a large case with a single inlet and outlet.
The main problems on a smaller boat are finding the space to mount the unit and the cost.
Dry exhaust problems
Over time dry exhaust silencer tend to block with carbon. This causes excess exhaust smoke and in extreme cases bad starting. The cure is a new silencer.
As the whole system should be lagged pipe failures can be hard to detect, but if an area of lagging or even the whole engine compartment is getting covered in black dust it is a fair indication of an exhaust leak.
In these systems water is introduced into the exhaust at a point close to the exhaust manifold. This will ideally be in to a downward pointing elbow and in such a way that the water will naturally drop "down" the exhaust hose and slightly below exhaust manifold level. In this way it will be more difficult for water to work its way back into the exhaust manifold and then the cylinder(s).
The exhaust water is usually taken from the "exit" of the raw water part of the cooling system.
The water ensures that beyond the mixing elbow the exhaust gas temperature will be at or below 100 C. This allows the use of special "rubber" exhaust hose that damps the tendency of metal exhausts to resonate internally and amplify the exhaust noise. When the water hits the hot exhaust gasses it causes the gases to cool and contract, this also serves to minimise exhaust noise.
The "self silencing" inherent in wet exhausts means that the silencer (if fitted) can be far simpler, often not much more than an expansion box and weir plate. The silencers are often made from various plastics or rubbers.
Exhaust dangers from raw water failure
It is all but inevitable that the raw water system will fail many times in the life of a boat. This will allow exceptionally hot exhaust gasses to pass down the system. This chars the inside of the exhaust hose and may eventually burn its way through the silencer.
Sometimes the inner layers of the hose de-laminate and "drop" into the exhaust flow. At low speeds this may not be noticed, but at higher speeds the back pressure in the exhaust and therefore the raw water pump rises causing the pump vanes to bend back. This stops the raw water pump working so a hard to diagnose overheat occurs.
The danger of the silencer burning through means that it is vital to inspect the bilge several times during after a raw water failure. A rising bilge water level is a good indicator of a hole in the silencer.
Preventing the engine flooding.
All sea boats and those with the exhaust manifold close to or below the waterline must take extra precautions to prevent water from the exhaust flooding the engine.
Water flowing back up the exhaust
Just before the exhaust exits the boat the exhaust pipe should be taken as high as possible in a "swan neck". The top of the swan neck must be well above the waterline. This prevents waves etc. flooding back through the exhaust, but it is possible that a siphon may be formed. To prevent this a siphon break should be fitted at the top of the swan neck. A small pipe Td into the top of the swan neck and exiting high up on the hull will also prevent siphoning as long as the exit remains in the air.
Sometimes a water separator is used close to the outlet that separates the water and exhaust gases. The water exiting below the waterline so you can not hear sloshing water and the gasses exit above the waterline as normal.
Sailing boats and any boat on an exposed mooring may benefit from having a seacock fitted into the exhaust system close to the hull outlet. The lower the engine the more important this becomes. The cock will act as backup in case waves etc. manage to enter the exhaust and overcome the swan neck when the engine is stationary. Just remember to turn it back on BEFORE trying to start the engine.
Water filling the exhaust from the raw water system
Although the raw water pump should not pass water when it is stationary it will almost certainly do so as it or the impeller wears. This will allow raw water to fill the exhaust pipe and back-feed into the manifold and engine. To guard against this at some point the supply from the raw water pump should be taken through a high swan neck that is well above the waterline. The top of this swan neck should be fitted with a siphon break cock at the highest point.
Ideally the sea cock would be turned off whenever the engine is stationary but an open siphon break cock at this point is a back up - especially with a sea cock using gate valve that can be difficult to fully close. The majority of inland boats would not bother with this because their pumps are often above the waterline.
A wet exhaust system showing swan necks and vacuum breakers
Wet exhaust problems
Engine failing to start
If a fault causes the engine to spin over slowly and fail to start quickly continued attempts to start the engine might result in the exhaust filling with water and leaking back into the engine. The engine should blow enough air into the exhaust to purge it, but as a precaution you might consider turning the seacock off. If you do this and the engine starts it is important to turn the cock back on quickly to prevent the raw water pump impeller running dry and damaging itself.
Apart from the delamination of the exhaust hose problem there is another related to the mixing elbow.
When the water hits the exhaust gasses and "flashes" off into steam any minerals it contains are deposited on the mixing elbow walls. Over time these will start to block the exhaust and increase the backpressure. This will prevent the raw water pump delivering its full output and lead to overheating.
Any exhaust blockage may cause the engine to smoke – usually black – and even induce poor starting