Banishing bad vibes

A perennial problem faced by boat owners is isolating vibration generated by the engine. It’s certainly irritating – but it can also become an expensive headache.

Unchecked vibration can also cause fatigue and eventual failure of the boat’s structural elements, so it’s worth eliminating or at least isolating all sources of vibration.

In general terms there are two approaches to reducing vibration: use flexible mounts and couplings to isolate the vibrating components from the rest of the boat, or eliminate the vibration by careful balancing, tuning and support of the moving parts.

The isolation approach sees the engine and gearbox ‘floating’ on flexible elements. These allow limited movement and only transmit essential power to the drive shaft and hull. They comprise two main items: engine mounts and flexible couplings.

ENGINE MOUNTS

The engine mounts are the primary components for isolating the engine from the hull, and are also used to correct a minor misalignment between engine and driveline.

Engine mounts contain a flexible material bonded to a base plate, which in turn is attached to the hull’s engine supports. A stud or bolt is also bonded into the flexible material, and this fits into the engine’s mounting points. They absorb engine vibrations.

But this simple design belies a lot of technology. Because the engine mounts are in the bilge, they typically get covered in contaminants, usually engine coolant, oil or fuel. Which is why rubber, which degrades after extended contact with petrochemicals, has largely been replaced by modern polymer materials.

These are also more heat-resistant and can absorb greater forces than the equivalent rubber-based unit. Polymers also retain their flexibility over long periods of time, unlike rubber which tends to harden and eventually becomes brittle.

The basic design has also changed considerably, especially in engine mounts designed for marine applications. Most manufacturers’ standard mounts are based on industrial applications, where the engine is held in a stable, horizontal position.

But a yacht motor-sailing and heeled over at 30 degrees adds considerable additional lateral force to the conventional thrust

and axial movement the engine exerts.

Mounts also need to be fail-safe. Should the flexible material shear in extreme circumstances, say during a collision or capsize, the mounts must be able to completely contain the engine to prevent its considerable mass from breaking free.

Some marine-designs, such as the Isoflex range from Henley Engineering, provide all these advantages. In many cases they’re also less expensive than the OEM mounts.

Engine mounts are often neglected until they fail, which isn’t a good maintenance approach. They are, of course, difficult to access, as the engine has to be lifted to remove the mounts. The limited space in most engine compartments means the engine may need to be removed completely to achieve this.

But they should be inspected as part of the annual maintenance routine, especially if they’re the older rubber-based type. If they are, consider replacing them the next time any major work is planned for the engine.

FLEXIBLE SHAFT COUPLING

The thrust from a boat’s propeller has to be transmitted to the hull to push the boat forward, without adding to the stress exerted on the engine and its mounts.

In traditional engine installations, the engine is precisely aligned to the propeller