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