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Belts Without Suspenders


Understanding belt drives will aid in dependable—and safer—cruising.


This engine relies upon a serpentine belt to operate its multiple compenents. Just below the alternator, an idler pulley provides tension in order to keep the belt tight and in contact.

A few years ago I entered the lock in Port St. Lucie, Florida, bound for the Bahamas. As I was exiting the other side ofthe lock, smoke started coming through the engine room hatches. I immediately shut down the engine. Based on the odor, I had reason to believe there was no fire.

After opening the hatch to confirm the absence of flames, I grabbed a pair of diagonal cutters, made one cut, started the engine and got underway again. We’ll return to the cause of this failure and the quick solution in a bit.

Despite the highly evolved technology of the modern marine diesel engine, we still rely upon simple technology to provide critical functions. Alternators, seawater pumps, and internal circulation pumps are driven by a flexible belt that connects them to the flywheel. These components often share a single belt and if it fails you can expect the engine to overheat and the alternator to stop. The prudent cruiser will know how to identify signs of impending failure, maximize belt life, and install a replacement.

Belt Design

Belts fall into two main types: V-belts and serpentine belts. The modern V-belt was invented in 1917 by John Gates of the Gates Rubber Company (now known as Gates Corporation, which manufactures a full line of belts for all applications). A V-belt would more accurately be described as a trapezoid, since it lacks the pointy end of a V, and they come in two basic variations: standard and notched. The transverse notches on the underside of the belts increase flexibility, reducing strain caused by bending around the sheaves. Both styles run in the same sheave, or pulley, but the notched version runs cooler, lasts longer, and is preferred for running accessory components on a marine engine.

All belts face the same challenge of: transferring the power from one spinning source to another in the most efficient and reliable manner. V-belts accomplish this task by means of friction between the sides of the belt and the walls of the sheave. The narrowest portion of the trapezoid does not contact the sheave, only the sides do. An improperly sized or excessively worn belt will bottom out in the sheave, preventing adequate friction against the side walls.

Serpentine belts got the name from the way they snake around the various sheaves, but they are more accurately identified as multi or Micro-V®. They are flat and have small raised sections running parallel to the length of the belt. The number of sections varies depending upon the pulleys used to transfer power. These belts can handle higher loads than V-belts, suffering less wear, and one serpentine belt can replace multiple V-belts. Due to their advantages, vehicle manufacturers switched exclusively to serpentine belts in the early 1990s.

V-belts, though, offer advantages over serpentine belts in certain situations. When a belt breaks and you have no spares, rope, twine, canvas, or panty hose can be substituted, because the V shape of the sheave promotes the friction needed, while the multi-V style does not. For an application not requiring high loads, such as a seawater pump, the V-belt makes more sense.

The load demanded drives belt selection. A 12-Volt DC alternator with an 80-amp output can get by with a single, 3/8-inch V-belt. Increase the output to 210 amps and you will need dual 3/8-inch V-belts, or a single 6-groove serpentine. At 310 amps, you need an 8 groove. A serpentine belt is preferred for alternators due to the high loads. If you have dual V-belts, remember to always use identical belts from the same manufacturer when replacing them.

This notched V-belt drives the alternator, seawater pump, and coolant circulation pump


Your daily engine room check should include belt inspections. You are looking for proper tension, the belts’ physical condition, and belt dust. On modern engines, safety covers restrict access to the belts, which makes these inspections more challenging.

Tension can be checked by deflecting the belt at the midpoint of the longest span. A rough rule of thumb calls for 1/64-inch of deflection for every inch of span. More important, if you check this after each long run, you will get a feel for normal tension.

Visually inspect the belts for cracking, fraying, or glazing (shiny surfaces). Any of these conditions indicates significant wear and warrants replacement. Fortunately, belt composition has improved over the past decade, and most manufacturers have migrated from a synthetic rubber called chloroprene to another synthetic known as EPDM.

EPDM offers far greater resistance to cracking and temperature degradation, doubling the life expectancy. One word of caution: EPDM starts to lose material before it begins to crack. While it might not show visual signs, it will gradually loosen as it ages. Not all belts utilize EPDM, so it pays to ask.

Belt dust also points to a developing problem. If the area around the belts, perhaps on the engine or the nearby hull, shows a fine gray or black dust, chances are you have a problem. A degrading belt, a sheave alignment issue, or rust/abrasion in a sheave can throw this fine powder. Alternators run hot and utilize cooling fans to maintain acceptable operating temperatures.

Excessive dust can be pulled into the alternator, which can decrease cooling efficiency and cause overheating.

The final check takes place when underway. Whining or squealing when the engine runs can indicate a slipping or misaligned belt. If you hear a squeal, check the tension first. PassageMaker’s Technical Editor Nigel Calder, recommends this simple test: Spray water on the squealing belt. If the sound worsens, you have a tension issue. If the sound disappears, you have an alignment issue. Either way, you have a problem to correct. Keep in mind that not all belts are created equally. Price matters, and given the relatively low cost, the most expensive version of the correct size will also be the most durable.

Three belt types: notched V (outside), conventional V (middle), and serpentine (inside).

leaving the lock

Let’s look back at the smoke-filled engine compartment I described at the beginning of this article.

The smell of burning rubber gave me some hope. The bearing on the alternator had seized and the serpentine belt was spinning around a frozen sheave. On many engines this belt also runs the seawater pump, but on this particular engine that pump was gear-driven.

I quickly cut the belt loose and pulled it out of the way. By running the generator and battery charger I was able to provide the direct current needed, and the engine fired up and took me out of the lock.

It pays to understand the configuration of belts on your engine, to have spares, and to engage in a regular ritual of inspection. ■

Steve Zimmerman is the president and founder of Zimmerman Marine, which operates four boat yards in Maryland, Virginia, and North Carolina. Zimmerman has been repairing and building boats for almost four decades.