LASER ALIGNMENT AND DRIVELINE MAINTENANCE

Driveline vibration

A common complaint from boat owners is that ‘my boat has a vibration whilst under power’. So what causes vibration and what can be done to reduce the problem?

A marine driveshaft is expected to perform in quite different situations from an automotive driveshaft. The marine shaft is not only designed to transmit torque (rotational force) to the propeller but must also carry the thrust generated by the propeller to the gearbox/engine. This means that the shaft must be rigid (no flexible joints) and in most cases will be solid, i.e not hollow.

Further more, the shaft weight is required to be supported evenly between at least 2 or 3 bearings which are in good condition (with the correct clearance) and perfectly aligned to each other. The shaft itself must be straight (no bends) and running true to the gearbox output flange.

Diagnosing the cause of a driveline vibration is not straight forward and in many cases can be a combination of factors such as:

• Shaft not running true (bent)
• Excessive bearing clearance (worn bearings)
• Misalignment between strut bearing, stern tube and gearbox/engine
• Worn or incorrectly adjusted mounts
• Propeller damaged or imbalanced
• Worn or incorrectly installed drive-saver (shaft not running true)
• Engine misfire or exhaust harmonics transmitted to the deck above
• Excessive engine movement whilst under power

If any of these conditions are present, then a vibration will usually ensue which can be transmitted to the hull, deck and fittings of the vessel. Depending on the source and severity of the problem, the vibration may be evident at idle, at high power or during a particular RPM band. 

The importance of correct alignment

Improper alignment of marine drive line equipment is a common issue and can lead to excessive noise, vibration and premature bearing wear. If left untreated, severe damage may occur to the boat and/or propulsion components.

There are several types or procedure used for aligning drivelines, depending on access. Marine Power Systems have invested heavily in laser alignment equipment which allows shafts, engines, gearboxes and bearings to be aligned to a far greater degree of accuracy than traditional techniques. This leads to a quieter, smoother driveline that should give years of trouble free use.

Shaft straightening

A frequent cause of vibration is when the shaft has become bent and runs with excessive imbalance. Shafts can become bent by grounding, badly misaligned engines/gearboxes improper lifting of the boat or sometimes through internal stresses that manifest over the lifetime of the shaft. 

The good news is that most bent shafts can be repaired. Marine Power Systems use cold straightening techniques approved by most marine survey organisations to restore shaft straightness and remove imbalance from the driveline. A dramatic reduction of vibration and noise can be achieved, especially when this technique is combined with propeller dynamic balancing.