When it comes to preventing seawater getting into the engine, the first step is to angle the exhaust system -or certain sections of it -downwards from the engine to the outlet. However, if a good slug of water gets pushed up the exhaust pipe by a wave, it will be helped along towards the engine by the boat pitching bow down just at the wrong moment. So if we are relying on down-slope alone, we need to be confident that the system will never approach the horizontal even in extreme conditions. For this reason, ocean-going yachts should always employ some additional protection.
High Mounted Engines
Fast planing boats with hard chine hulls often have the tops of their engines well above the waterline. In this case, a simple gradient of 1 in 8 may offer sufficient protection as shown in the illustration at the top. An inline muffler, like the one shown, offers a cost-effective silencing solution. Whenever possible, the silencer should be mounted horizontally, or slanting downwards at not more than 7°, bearing in mind that the transom outlet should be clear of the waterline by at least half of its own diameter, and that a transom flap offers further protection.
For even more effective silencing, a dual chamber lift silencer can be fitted as above.. As with the inline muffler system, the gradient on both lengths of tube must comply with the 1 in 8 rule and the silencer will normally need to be at least 300mm (12") below the water injection point.
Note: This is to prevent water in the lower part of the silencer from back flooding the exhaust manifold. In practice, fitting a dual chamber silencer results in an increase in gradient in both sections due to its design.
Medium Level Engines
If the engine is mounted on or about waterline level, it may be necessary to fit what's known as a riser to the exhaust manifold or turbocharger outlet to artificially lift the engine end of the exhaust to provide a proper gradient. Risers incorporate a spray ring to ensure better mixing of water and gas which helps all-round cooling of the exhaust pipe circumference.
From the riser outlet aft, the system is essentially the same as for the high mounted case and the same considerations regarding silencer inclination and transom outlet apply.
Low Mounted Engines
A low engine position poses special problems. Firstly, the water inlet hose must extend above the waterline and be fitted with a siphon break. Otherwise, if the hose were full of water when the engine was switched off, it could continue to siphon raw water through the engine cooling system. This is dangerous because it could lead to the exhaust system filling with water to the extent that the engine cylinders could eventually become flooded.
The exhaust must then drop vertically by at least 300mm (12") into a water lift silencer below the waterline. The outlet hose must then rise at least 450mm (18") above the waterline (bear in mind that this is the heeled waterline on sailing yachts) to a U bend or gooseneck. This makes sure the final run to the hull exhaust outlet is steeply inclined to prevent run-back. These dimensions can vary from one boat to another.
This layout offers triple protection for the low mounted engine. Firstly, any water entering the outlet of the exhaust pipe will find it very difficult to ascend the steeply rising section of the gooseneck. Secondly, the hose from the silencer to the transom outlet should never be completely full of water, because when the engine is running, there is a considerably amount of gas present as well and a natural siphon break is created between these two components. And thirdly, water lying in the bottom of the silencer cannot get up the vertical inlet pipe. As a bonus, any water remaining in the first section of pipe run when the engine is switched off will naturally drain down into the silencer rather than run uphill into the exhaust manifold.
Sailing yachts present special problems as another factor enters the equation, that of heel. Yacht exhausts are often offset from the centreline, and while the outlet may be well above the water normally, it can be well below it when the yacht is heeled over.
Overcoming the heeling problem is a question of making sure that water can't run down towards the engine in normal circumstances. The designer must ensure that the top of the swan-neck is always well above the waterline, even when the boat is heeled to its maximum. Obviously, nothing other than a closed gate valve or seacock at the hull skin fitting can protect an engine if the exhaust system goes completely underwater -as in a knockdown or capsize -and many yachtsmen choose to close the gate valve as an additional precaution once the engine is switched off. Of course, one must remember to open it again.