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ENGINE MOUNTING

Engine mounting cannot be considered in isolation from shaft/coupling/stern gland type.

If a shaft assembly has no flexibility built in it MUST have a solid mounted engine, otherwise the engine will shake about, flexing the shaft, causing stern gland/bearing wear and noise/vibrations to be transmitted to the hull.

There might be a case for bolting very slow revving engines directly to the bearers as long as they produce no vibration frequencies that cause plates, windows, and other boat parts to vibrate.

Unless there is some sort of thrust bearing on the propeller shaft the boat moves by the propeller pushing on the shaft, the shaft pushes the gearbox & engine, the engine pushes the engine mounts, the mounts push the engine beds and the beds finally push the hull. This means that ordinary automotive or machinery mounts are not suitable. They are not designed to be pushed, only compressed or twisted.

Normal revving engines should be flexibly mounted, with a suitably flexible shaft coupling. The solid mounting of these engines will almost certainly cause vibrations elsewhere in the boat.

 ENGINE BEDS

It is vital that engine beds are effectively part of the hull. There is no problem on steel boats (they are welded to the hull), GRP boats are usually OK as long as the beds are moulded into the hull and filled with wood, rather than paper rope.

Wooden boats have a problem in the fact that their beds often work loose and amplify engine vibrations. The easiest way for an "amateur" to improve this is to use steel angle and big bolts to tie the beds together and then strong plywood fillets to bolt the beds to a number of frames.

 

ENGINE MOUNTS

The engine mounts must also be tight, both tight to the bed and tight to the engine.

Flexible mounts must be checked for perishing and dropping (shaft alignment) and to make sure no part of the engine contacts the bed. (Newage BMC 1.5s rear mounts have a habit of dropping so a retaining bolt on the engine drops onto the foot, causing vibrations throughout the boat).

Whatever type of engine mounting is used your lecturer would always recommend using flexible hoses to connect the fuel system to the engine. (see fuel section)

Adjustable mounts

Mounts that allow the height of the engine to be adjusted need watching like a hawk. These and twin height adjusting wedges have a habit of working loose. The engine will then drop causing misalignment of the propshaft. This again results in vibration, noise, and most importantly wear and leaks in the stern gland/bearing area.

 

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