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.


Written by: Ed Sherman
Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.