Quiet and Safe Exhausts
The water used to cool a wet exhaust is the raw (sea or river) water used by the engine cooling system once it has done its job of cooling the engine itself. You might think this would be too hot, but it's a lot colder than the exhaust gas, and perfectly effective -assuming the correct materials are used in the exhaust system. Injecting the used raw water down the exhaust pipe also kills three birds with one stone: cooling and quietening the exhaust, and eliminating the need for a separate overboard discharge. Fortunately, the amount of water injected into the exhaust is proportional to engine rpm, as normally the raw water pump is driven directly by the engine. I say fortunately because the faster the engine runs, the more heat it produces, and the more cooling the exhaust needs. The connector through which cooling water is introduced into the exhaust pipe is called the water injection elbow -for the simple reason that when wet exhausts first appeared it was usually nothing more than a 90deg. plumbing elbow welded into the engine's exhaust outlet! The top image shows a very basic wet exhaust system.

So far, so good, but haven't we just created a monster -an exhaust system made up of GRP tube and/or rubber hose, perhaps with a number of joints in it, full of noxious exhaust gas and gallons of water -all running inside the boat? Surely a leak could very well poison everyone on board, assuming they weren't drowned when the boat sank under the weight of the escaping water! The answer to both questions is yes, which is why proper design, use of the correct materials and meticulous assembly are all absolutely essential for safety. (Makes the good old funnel look attractive, doesn't it?)

As if that weren't enough, having the exhaust outlet close to -or sometimes level with -the waterline can lead to more problems. When the engine is running, the outward flow of water and gas prevents seawater getting in. But with the engine stopped, waves can push quite large amounts of water up the exhaust pipe towards the engine. For this reason, vessels where the exhaust pipe exits through a flat transom are often fitted with a flap over the outlet as a first line of defence.

If sea water were to find its way to the engine end of the exhaust system, there would be nothing to stop it entering the exhaust manifold and from there, getting into the cylinders themselves through any open exhaust valves. With cylinders full of water, you can imagine what would happen when you innocently turned the key for the next start. The technical term 'hydraulic lock' may sound fairly innocuous, but can mean bent or broken connecting rods, collapsed pistons and wrecked bearings!

The good news is that all these potential problems can be readily eliminated. The principles of sound exhaust design are well established -even if not universally applied -and modern materials are well up to the job of coping with the various stresses.

Exhaust system design depends very largely on whether the engine is located close to, above, or below the waterline. Let's have a look at some popular options.

cont....

The Transom Flap is the first line of defence against water entering into the exhaust system.

Exhaust Basics continued.....