What Is an Engine Mount (Mushroom Mount)?
Rubber-mounted feet that connect the engine to the hull - both a vibration-absorbing cushion and the millimetre reference for shaft alignment.
An engine mount, or mushroom mount, is a vibration-absorbing foot with a steel body and a rubber core that secures the engine to the boat structure. The adjustment nuts on top allow the engine height and tilt to be set to the millimetre; shaft alignment is carried out on these mounts.
Rubber is a material that ages; oil, heat and the years soften the mount or cause it to sink. The table below shows the symptoms.
Mount symptoms
Symptom | Cause |
|---|---|
Increased vibration, loose parts | A sagged or softened mount |
Rubber contaminated with oil | Being fed by leaks; the leak must be put right first |
Height difference between the four mounts | Alignment has gone out, with one mount compressed more than the others |
A sagging mount creates two bills: increased vibration (and everything that loosens as a result) and disturbed shaft alignment (leading to wear on the shaft seal, bearings and gearbox). The usual answer to ‘the engine is shaking much more than before’ is the mounts.
Replacing mounts is an engine-lift job; the best time is when another major engine job is already due, so it can be done while the engine is already lifted. After replacement, shaft alignment must be checked without compromise, and the verification measurement repeated one month later.