The belts that provide life support to your engine’s accessories, the alternator, auxiliary alternator, raw water pump, and circulator pump will typically provide reliable, trouble-free service, provided they are properly maintained and replaced on a regular schedule. Many boaters neglect this all too important system. 

A broken belt is always bad news because when it snaps, all drive power to whatever it turns is lost. That means the water pump quits circulating coolant through the engine and the alternator stops producing amps.

Belt preventive maintenance includes:

  • Checking for glazing, cracking, dust
  • Proper fitting and tension
  • Alignment
  • Replacement


The belts used to drive these accessories are of the “V” variety or ribbon-shaped flat serpentine belts. Often, one V-belt may do double duty, sending life from the crankshaft pulley to both the alternator and coolant pumps, while another belt will help drive the raw water pump.

Belt Life

The service life of a belt depends on the hours of use as well as load, tension, and heat. Every time a belt passes around a pulley, it bends and flexes, which produces heat that age-hardens the rubber over time. The wear process can be greatly accelerated if the belt is loose and slips because any added friction between belt and pulley makes the belt run even hotter. This can cause glazing on the faces of the belt and cause it to slip even more. So one of the most important factors that affect belt life is making sure it is properly tensioned when it is installed, and that the proper tension is maintained throughout its service life.

The good news is that replacing the belts periodically can go a long way towards minimizing the risk of a breakdown caused by belt failure. After all, it’s a lot easier to replace a belt at your convenience than having the belt fail unexpectedly, heaven knows where.

Servicing and Replacing Your Belts

Belts are easily serviced, adjusted and replaced. Begin by inspecting the engine’s belts. First, check that the belts are tight, tight enough, so they don’t slip, but no tighter. The old rule of thumb for belt deflection, allowing ½-inch of “give” between the furthest pulleys is not a very accurate guide for today’s engines. Follow the manufactures recommendations for belt tension. If a belt is slipping, it won’t turn its related accessory properly, and it will wear out and break prematurely. If it’s too tight, it will accelerate wear on bearings and seals. If you see any evidence of belt dust on the front of the engine or the alternator casing, then the belt is either slipping or misaligned. If the belt and/or pulley show signs of glazing, a particularly smooth, shiny appearance, it means the belt is slipping. A glazed belt must be replaced, but the glaze on a pulley can be “dressed” using 220 grit sandpaper.

Belt Tension

To tension a belt, use a breaker bar as a lever to pry the accessory away from the crankshaft pulley and then tighten the fixing bolt. Threaded “spreader” tools are available to make this task easier. Finally, ensure that each belt’s profile properly matches every pulley over which it turns. The belt should be even with or stand slightly proud, no more than a 1/16-inch, from the top of the pulley walls. On engines with a single serpentine belt, tension is usually self-adjusted automatically via a spring-loaded tensioner. No additional adjustment is necessary. If this is the case, check the tension pulley bearing. When the belt is removed, the pulley must spin freely with no rough spots detected under hand pressure.

Belt Tips

If your engine has been eating or twisting belts, misaligned pulleys may be your problem. Alignment can be checked with a straightedge. If a pulley is bent or not in the same plane as the rest, the problem should be corrected otherwise the “bad” pulley will continue to ruin belts.

Deteriorating belts can cause collateral damage to the alternator. A chronically slipping alternator belt can create large amounts of fine dust. The issue arises when this belt dust is drawn into the alternator with the cooling air. If the rubber dust-like blanket coating the bearings, windings, diodes, and heat sinks potentially causing failure.

Belts are comparatively inexpensive, so it’s cheap insurance to replace them every five years or every 2,000 hours, whichever comes first. Remember, a new belt may require adjustment and re-tensioning several times in the first 50-100 hours of use and should be checked for wear and tension during regularly scheduled maintenance.

~Deane Hislop

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