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Why is my alternator eating the belt?

2019 February 1

Dear Boat Doctor,

I recently upgraded the alternator on my Yanmar 4JH2DTE and things aren’t going smoothly. It is a Balmar 100A alternator with a Max Charge alternator. The alternator is working great, but I am tearing up belts. The belts don’t last long, and my engine compartment is covered in belt dust. Have you got some ideas for me? 

Corey Torres 

San Diego, California

Dear Corey,

Your problem is not uncommon, we all want a bigger alternator but then we struggle to drive it. I’ll give you some things to check out.

First off, make sure you have adequate belt tension, that black dust means the belt is slipping. You want about a ½-inch of deflection in the belt. But if the belt is too tight you can deflect the bracket and mess up the alignment, causing slippage and wear.

A 100A alternator on a single ½-inch belt is theoretically OK, but it is quite a bit of load. Your alternator will allow you to back off the output of the alternator with the belt manager feature. This will do two things, it will turn down the output (a little counter to your goal of a bigger alternator) and it can implement a “soft start” feature that will ramp up the alternator output at start up to not stress the belt so much.

The best answer to your problem is a better drive system. Cars did away with V-belts 30 years ago and sailboats should too. Actually most new marine engines now come with serpentine belts. Serpentine belts provide a much larger contact surface with the pulleys and run much smoother and quieter.

Balmar (www.balmar.net, 360-435-6100) and others sell serpentine belt upgrade kits. They aren’t cheap but they will make your problem go away. Your engine has just one belt that drives the water pump and alternator from the crankshaft, the kit consists of three new pulleys and a serpentine belt. The pulleys will fit directly on your engine with no modifications necessary. A nice side benefit is that they are beautifully made, nicely machined in aluminum with blue anodizing.

While you are at it, I would add a Balmar Belt Buddy. This is a tricky little device to positively mechanically tensioning your alternator, eliminating the need to lever the alternator over with a hammer handle while tightening the bolts.