Tension on the Beltway

Posted: June 1, 2011

We’re talking serpentine belts, not D.C.

By: Capt. Wilson J. Sheppard

Inboard and sterndrive engines not only power the propulsion system aboard a boat, they also indirectly provide power to other systems. 

To utilize the rotational power produced by an engine, a pulley is connected to its crankshaft. The crankshaft pulley is located outside the engine block and is connected to pulleys on accessory components with either a serpentine belt or a V-belt.

The most common component powered by the belt system is the alternator. As the pulleys rotate, the alternator produces electricity to charge the battery bank and power the electrical system. On larger engines, the belt system may also connect to pulleys on other components such as a water pump. In any setup, sufficient tension is required for the belt to keep all of the pulleys rotating in sync while the engine is running. After several operating hours, the belt may begin to stretch. Without adjustment, it will begin to slip and buckle as it travels around the pulleys. If the belt continues to lose tension, falls off the pulleys or breaks, the engine or its components may be damaged.

As a belt ages, adjusting the tension becomes less effective. There are a few indications that you need to replace a belt. One is dust. Black dust collects on pulleys, hoses and engine parts near the belt’s path. Another is the belt’s seating. It’s time to replace when the belt travels inside the pulleys rather than along the outer edge. The last is wear. If the belt appears worn, cracked or frayed, replace it.

The belt-and-pulley system is a series of moving parts that presents serious danger. Your engine needs to be shut off before you attempt to adjust or replace a serpentine belt. In fact, you should not have the keys in the ignition. You may even want to consider disconnecting the battery to avoid accidental operation of the engine.

Taming the Serpent

To adjust or replace a serpentine belt, you must adjust the position of the alternator. There are two bolts that secure the alternator to the engine. One is a  stationary bolt  that serves as a pivot point for the alternator. The other is an  adjustment bolt that secures the alternator to a rail. With both bolts loosened, the alternator will slide along the rail and adjust the tension of the belt. Sliding the alternator away from the engine will move the alternator pulley away from the crankshaft pulley and increase tension on the belt. Moving the alternator toward the crankshaft pulley will decrease the tension. As you are adjusting the position of the alternator, be sure not to damage the thin metal of the alternator’s housing, break or crimp the wires, or kink the hoses.

Out with the Old

To find a replacement belt, check your engine manual or use the specs printed on the belt. When installing the new belt, you may need to loosen the adjustment bolt and completely remove the stationary bolt. Position the new belt around the pulleys exactly as the worn belt, then reinstall the stationary bolt and adjust the position of the alternator. You should use a tension gauge to check the tension, but you can estimate it by pressing on the belt. You should only be able to press it about ¼ to ½ inch. Avoid overtightening the belt, because you can damage bearings in the pulleys. Once sufficient tension has been applied, tighten the adjustment bolt and then the stationary bolt.  The belt should be positioned flush with the other edge of the pulley.

Start ’er Up

Before starting the engine to test the operation of the belt, be sure there are no objects that can get caught in the belt and/or pulleys. This includes tools, clothes, fingers, hair or anything else you don’t want to be separated from.

Once you start the engine, pay close attention to your engine’s tachometer. It may display a low or erratic reading upon startup if there is not enough tension on the belt. While the engine is running, listen for any high-pitched noises from the belt or pulleys. If you can get someone to help you, have him slightly increase the engine’s idle to see if you notice the belt slipping or buckling. Remember that you should use protective eyewear when looking at a running engine. If everything looks good, remember to check the tension of the belt ­periodically after a few hours of operation.

Regularly checking the condition and operation of the belt system will go a long way in making sure that the connected accessories function properly. Maintaining sufficient tension on the belt will remove some tension from your boating experience. Ironically, a little tension can provide a lot of peace of mind. 

captcha f82ebb47ac0241b5884983116fb38bee