Boat alternators

Maintenance of boat charging systems

A modern charging system with its solid state electronics requires very little maintenance. In some ways this is a bad thing because it tempts you to leave it alone until something goes wrong. Unfortunately, this usually occurs at a most inconvenient time, so a little maintenance, even if it just means checking the system, can be justified.

The most vulnerable part of the charging system is the driving belt for boat alternators. Whilst there is a trend amongst engine manufacturers to use shaft or gear drives for many engine accessories, the alternator remains almost universally belt driven. Perhaps this is because alternators are not essential to make the engine run, but the vee belt drive of the alternator is subject to breaking and does require adjustment.

This is a weak point in what is an otherwise reliable system. The belt drive to alternators requires regular adjustment to give it the correct tension to drive the alternator efficiently. At full charge alternators take a surprising amount of power to drive them, so the belt will be working quite hard.

The correct tension, which will be shown in the engine manufacturer's handbook, helps to give the belt a long life and prevents it from slipping on the drive pulley.

To adjust the tension you have to slacken off the adjusting bolts. Whilst you are doing this take the opportunity to either take off the drive belt or at least slacken it right off and look at its condition.

Deterioration usually starts on the inside of the belt and you may see cracks in the rubber which can extend part way through the belt. These obviously weaken it and if you find these cracks it is time to renew the belt, on the basis that it is better to do this at a convenient time in harbour than wait until it breaks at sea.

In many cases, renewing the drive belt is simply a matter of taking the old one off, slipping on a new one and then adjusting the tension correctly. However, on some marine engines it is not as simple as that because a water hose or other connection runs between the drive pulleys and this has to be disconnected first. If this is the case, it pays to fit two belts at the same time, taping the second belt out of the way where it is ready for fitting quickly if the drive belt breaks at sea.

Alternator drive belts are comparatively cheap items so it is a good idea to replace them on an annual basis. This will give you peace of mind but you will still need to check the tension every couple of months or so. At the same time check the connections and the wiring on the charging circuits, the former to make sure that they are all tight and clean a and that there     is no sign of corrosion, the latter to ensure no sign of chafing.

Corrosion can be a problem with alternators because these tend to be based on automotive units, and the aluminium body of the alternator can react with nearby brass or steel fittings in a salt-laden atmosphere. The situation can be made worse if there are leaks from the engine cooling system; apart from curing the leak, giving alternators a spray with silicone grease will help keep corrosion at bay.

The same spray can also be used on the wiring connections to reduce the chances of corrosion. The alternators connections should have rubber boots over the exposed metal parts to prevent accidental short circuits when using tools in their vicinity.