Exhaust Back Pressure
As you can imagine, any exhaust system other than a straight, short pipe will set up some resistance to the exhaust gas escaping from the engine. Pushing its own exhaust gases and the cooling water through a silenced exhaust system obviously saps some energy that would otherwise make its way to the propeller. This resistance can be measured as a pressure within the system, and many engine manufacturers set an upper limit for each of their models.
Guarantees and Performance
Engine manufacturers are not worried quite so much about poor performance as the overheating effect of gases unable to escape cleanly from the engine. Compliance with back pressure limitations is often a condition of the engine's warranty. On new series production boats it is common for the complete engine installation to be certified as satisfactory by the engine manufacturer -and this will include measurement of back pressure. But when it comes down to one-off production or re-engining, it is vital to check the small print in your warranty document. If the engine fails, it can be up to you to prove that the back pressure was within limit. Contact your engine or exhaust system supplier if you need further advice.
The most common factors influencing back pressure are:
• The amount of water injected
• The length of the system
• The diameter of the piping
• How many bends there are
• The design and type of silencers used
• The position of the exhaust hull outlet in relation to the waterline
Taking these in order;
The water needed to cool the exhaust system takes up room that could otherwise be occupied by gas and so increases back pressure. Obviously one can't do without it, but one can ensure there's no more than necessary by water separation immediately after the silencer. (See water separation for more information).
The bigger the diameter of the hose, the less the back pressure. Increasing the diameter over and above the diameter of the engine exhaust outlet can be done with a special adapter, a riser with large diameter outlet, or even by fitting a silencer close to the engine that has a bigger outlet than inlet. That keeps the small diameter hose run to the minimum.
Bends add back pressure so you should aim for the straightest run possible.
The type of silencer used in a particular exhaust system may be dictated by the height of the engine and the hose run. So it's clear that back pressure is yet another factor that has to be kept in mind during exhaust system design and balanced against other conflicting demands, including adequate cooling and silencing. Unfortunately, back pressure in any particular combination of exhaust system and engine is almost impossible to predict. The sensible approach is to consult your exhaust supplier for advice on the correct hose diameter and components needed. But generally, the larger the silencer, the lower back pressure will be.