Question: I have been thinking of adding a second high-output alternator to my boat to service the batteries that supply my house loads. This would be in addition to the alternator that came with my engine that I would keep for charging my engine cranking battery. My friends are telling me that with high-output units I might have to add a double-belt pulley set to my engine. The original equipment alternator only uses a single belt arrangement. Is there a cut-off point in terms of alternator output where this becomes a problem?

Shown here is a double-belt pulley system on a high-output Mastervolt alternator.

Shown here is a double-belt pulley system on a high-output Mastervolt alternator. At the top of the photo, the standard alternator runs on its own single belt.

Answer: This a situation where a lot of mistakes get made. The general rule of thumb is that an alternator with a rated output of 100 amps or more is probably going to need a double-belt pulley arrangement to keep the alternator spinning when under load without any belt slippage. The exception to this is on a really new set-up where the engine manufacturer is using a flat serpentine “cog” belt set-up. Those units can handle a lot more torque than a traditional V-belt set-up.

The arrangement you see in the photo here is similar to what you are describing. The Mastervolt high-output alternator is using a double-belt arrangement, while at the top of the photo the original equipment alternator, only having a 65-amp rated output, is still being driven by a single-belt pulley. The trick with all of this is to check with your engine manufacturer and the alternator vendor to make sure that double-pulley kits are available for your engine. Also, understand that the alternator bracketing might have to be custom made by your local metalworking shop.

Another often-overlooked consideration here is the fact that alternators use up a bit of your engine’s horsepower when they are driving an alternator. Typical horsepower numbers used are in the 2-7 hp range depending upon the rating in amperes of the alternator. If your engine is a really low-horsepower sailboat auxiliary engine, this may be a concern, since it is possible to overload the engine by driving alternators, not leaving much to actually run the boat.