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, Sod's law ensures that 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 -which we will come to a bit later.

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 degrees, 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. Later we'll take a close look at the various types of silencer and their pros and cons. 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 (12in) 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.
cont......

Exhaust Configurations

System Variations

High mounted engine with In-line silencer

High mounted engine with lift silencer

Medium mounted engine with high riser and lift silencer

Low mounted engine with lift silencer

Low mounted engine with lift silencer and water separator

Positioning of In-line silencer