Engine Mounting

Main propulsion engines may be rigidly or flexibly mounted.

Flexible mounting is now universal in pleasure boat applications a increasingly so in commercial applications where noise and vibration suppression is important.

Flexibly mounted machines may be fitted with either rubber or coil spring type flexible mounts. Care must be taken in the selection of engine mounts. Factors that must be considered are the engine “mass”, torque reaction, propeller trust and the deceleration caused by collisions. The mounting should survive a 5g force in event of collision plus the propeller thrust without damage.

Because the engine is free to move in all directions the connection to the propeller shaft must be via at least one flexible coupling. All Watson vessels are now fitted with Centa™ AM couplings. These couplings incorporate rubber elements to accommodate torsion, thrust, angular and radial forces and greatly reduce bending forces in the propeller shaft and gearbox components. They also have a “failsafe” feature and are torsionally soft and backlash free.

There are other types available, notably the double constant velocity cardon type with integral trust bearing. These are a technical marvel, but suffer from a total lack of torsional flexibility or failsafe feature.

Often, and generally as a cost cutting measure, an engine may be flexibly mounted, but fitted with a solid steel shaft coupling. This arrangement offers no radial or angular flexibility whatever. With the engine being free to move, extreme forces are applied to the gearbox and propeller shaft components with the result that at some unpredictable time, failure of either or both is enviable.

An alternative is the plastic insert type coupling which fir between the gearbox flange and the propeller shaft coupling flange. These have very poor flexible properties and at best are an excuse for a coupling.

Rigidly mounted engines have the great advantage of simplicity which is why this option, combined with an oil filled stern tube, is preferred for most commercial craft.

Last Updated (Wednesday, 15 September 2010 09:29)