Heat is the greatest enemy of all things electrical, and your alternator is no exception. Alternators come equipped with cooling fans, either internal or external, but with a tightly enclosed engine compartment, high ambient temperature means an even hotter alternator. One way to keep your alternator cool is to build a simple fiberglass shroud that, when fed by a fan and ducting, will supply cool air to the alternator.
The classic equation of heat transfer: Q=UAdT simply indicates that the amount of heat transferred per unit area increases directly with the difference in temperature. Cooler air from outside of the engine compartment will lower the operating temperature of your alternator and prolong its life. We now have almost 5,000 engine hours on our alternator and more than 14 years of trouble-free operation, without doubt due to this cooling system. Here is a fairly easy and low-cost project that you can complete in a weekend for less than $35 in materials.
If you have a spare alternator, use it as a plug to mold a shroud out of fiberglass cloth and resin. No spare? Then remove the alternator from your engine and use it, but first carefully label all of the wires and terminals. (Now is a good time to inspect the drive belt and bearings and give the outside of the alternator a good cleaning. Carefully wrap the alternator with plastic kitchen film so you won’t have to worry about fiberglass resin getting in or on it. Use stiff paper such as card stock or poster board to fashion a mold for the shroud over the back half of the alternator, leaving sufficient room for the wires to exit easily. Use masking tape to hold the pieces of paper in place. A connection for the cool air duct will be the hardest part to make, as it will have to fit the cooling air duct (three or four inches or 75 to 100-mm diameter) and not interfere with the various engine mounts, water pump, hoses, etc.
Once the paper mold is complete, mount it and the alternator on the engine to make sure that everything will fit properly. Make any adjustments to your design now, as once the fiberglass is hard it will be too late.
With the paper mold still on the alternator, cover the paper with plastic kitchen film so the fiberglass won’t stick to it.
Carefully apply strips of lightweight fiberglass cloth saturated with polyester resin over the paper mold. Use the auto repair type resin, as it’s a lot cheaper than boat-grade epoxy resin. Use your own judgment on how many layers of fiberglass cloth to use, but use a minimum of two layers, more on the connection to the air duct. It’s a very good practice to use “Gloves in a Bottle” or similar barrier cream and vinyl gloves when working with fiberglass. Plain white vinegar will remove any uncured resin from your hands and tools.
After the resin has cured, lift the shroud off of the alternator and peel out the paper and plastic film. Use heavy scissors or shears and coarse sandpaper to trim and smooth the rough edges.
Paint it to match the alternator, if you wish. Install the alternator and reconnect all of the wiring. Fit the shroud to the alternator and hold it in place with strips of duct tape. Install a 12-volt ball bearing muffin fan in an opening in the engine compartment as close to the hull as possible as the air is coolest there. Make sure that the fan is installed so that it blows air into the duct! Connect the fan to the alternator shroud with the cooling air duct (clothes drier vent hose works fine). The fan can be powered directly from the alternator, but be sure to install an in-line fuse in the positive power lead on the fan. We have a practice of letting the fan run for 15 minutes after we shut down the engine to remove heat from the engine and alternator. A simple kitchen timer reminds us to shut off the fan.
Harry Hungate and his wife, Jane Lothrop, live aboard their Corbin 39 cutter Cormorant, and are cruising in the western Mediterranean in 2011.