Features May 1, 1999 Issue

Offshore Log: Alternator Breakdown

When you don’t have a back-up means of charging batteries, you carry a second alternator and learn about belts and tension.

Because we have engine-driven refrigeration, our Perkins 4-108 must be run about 1.5 hours per day to maintain the contents of the freezer in a rock-solid state. This engine schedule also does a good job of keeping up with our basic electrical demands.

We replaced the original 70-amp Motorola alternator with a high-output Balmar 901-100 isolated ground alternator, more than doubling low-rpm electrical output. Matched with a Link 2000-R monitoring and regulating system, we had almost exactly 1,000 hours of trouble-free generating until the alternator gave up the ghost.

Just before heading south two years ago, we purchased a second, identical alternator as a backup. In addition, at Balmar’s recommendation, we carry a complete rebuild kit for each alternator, containing diodes, brushes, and bearings. It is fairly easy to find an electrical shop that rebuilds alternators anywhere you go, since virtually every car and truck in the world has one, and failures are not exactly rare. Having the necessary parts in hand for your particular alternator, however, can make the difference between getting your alternator rebuilt in a couple of hours, or waiting days or weeks for parts to be shipped to some remote island.

A post-mortem on our alternator, performed by Dockyard Electric in Chaguaramas, Trinidad, was not definitive in revealing the exact cause of death, but taught us a lot about installing and operating high-output alternators.

In our case, the alternator showed evidence of extreme overheating. Small-case, high output alternators in the typical sailboat’s tightly packed engine compartment get very, very hot. There must be a constant, high-volume flow of clean cooling air through the alternator if it is to last a long time. This is achieved in several ways. First, you must have the right type of V-belt, maintained at the proper tension. All V-belts are not created equal. We use NAPA A-series industrial belts, available by special order through NAPA stores anywhere in the US, as well as in some areas overseas. A quirk of these belts is that the belt number is not identical to the belt length: an A 41 belt is 43" long!

The belt must be properly aligned with the engine and alternator pulleys. Do not assume that they are, even if you have a “factory-installed” alternator.

Proper belt tension is an equally important issue. No precise figure for belt tension can be given by the alternator manufacturer, since it depends on the pulley arrangement on the individual engine, as well as the type of accessories driven by the belt. In general, the belts on the engines we began checking after our alternator failure have been too loose, rather than too tight. A loose belt will both reduce output and contribute to overheating. If there is a significant amount of black belt dust on your engine or alternator, either the belt is too loose, the pulleys are misaligned, or you have the wrong type of belt.

We recently purchased a NAPA Krikit V-belt tension gauge (part #7401-0101) to help us keep belts properly tensioned. This $8 , pocket-size gizmo should be in every boat owner’s tool chest. It is easy to use, and the instruction sheet gives belt tension guidelines for a variety of pulley and accessory combinations.

The engine compartment must be kept clean. A lot of air gets sucked through an alternator. If your engine runs dirty, that dirt will find its way into the alternator, coating the windings and other components. Wipe the engine down regularly, and clean the alternator case and fan as well.

Make sure the alternator gets plenty of airflow. This may mean increasing engine compartment venting. In our case, a preoccupation with engine noise resulted in a beautifully insulated engine compartment that coincidentally was very efficient at keeping the heat inside. We are now re-working the air supply.

We had made minor modifications to the first alternator’s mounting feet to accommodate an engine bracket that did not quite fit the alternator, and had to do the same thing on the replacement.

It is very important that the alternator’s cooling fan run in the proper direction for the configuration of the fan blades. During the rebuild, Dockyard Electrics swapped out our original fan for a bi-directional model, which resulted in a much cooler-running alternator on the test bench.

Now we’re looking at buying a solar panel for back-up charging and make us independent of the engine altogether.

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