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 12in (300mm) into a waterlift silencer below the waterline. The outlet hose must then rise at least 18in (450mm) 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 can not 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.

The addition of a water separator to replace the gooseneck will dramatically reduce exhaust back pressure. Noise will also be reduced by up to 80 % at the exhaust outlet, making the engine exhaust almost silent when running on a mooring to charge batteries or to provide hot water.

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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