A version of this article originally appeared in the  June 2017 issue of the Good Old Boat Newsletter. A follow up article is also available in the August issue with another take on the same idea submitted by GOB Reader Jim Shell. 

Edit (12/1/2018): Rod Collins of Compass Marine recently published an exhaustive article about alternator mounting and belt tension. Far more exhaustive than this article, it is worth reading. Here's a direct link to the article: Marine Alternator Installation – Tips & Tricks RC gives tirelessly of his time and knowledge to help DIY boaters, he deserves our support. Be sure to visit his store!

As the story goes a kingdom was lost for the want of a nail. A sailor’s lament might be a vessel was lost for the want of a v-belt. Many systems on a modern sailboat are dependent on a functioning inboard engine, electricity, refrigeration, hydraulic pumps, and cooling the engine. Yet v-belt maintenance is often avoided. 

V-belts die of old age, poor alignment, and improper tension and seldom at a convenient time. (See Good Old Boat April 2017 Newsletter) Proper tension allows accessories to function efficiently with minimal belt wear. A tight belt increases wear on pump bearings and seals, while loose belts slip, shortening belt life and spewing belt dust damaging critical equipment. Sucked into an alternator the dust covers the windings overheating the alternator.

A simple task, achieving proper belt tension in the confines of an engine room is difficult. Holding the alternator in place while tightening the adjusting bolts is a task only a contortionist could love. If there was only room for another set of hands.

One option is a commercially available belt jack (see Good Old Boat Jan/Feb 2016). However, for about a dollar a frugal sailor can make one from a length of 3/8” threaded rod, a couple of nuts and washers, and scrap wood.

Cut two wood blocks so that one edge curved to fit the pulleys leaving the opposite edge flat. Drill a 3/8” hole perpendicular to the grain for the threaded rod. Cut the threaded rod slightly shorter than the distance between the two pulleys. Assemble the jack between the pulleys and tighten the nuts until proper tension achieved. With the alternator secured, the adjusting bolts are easily tightened.

With the savings from the DIY jack, I was able to purchase a Krikit belt-tensioning gauge. Sold by v-belt manufacturer Gates, this gauge helps to ensure proper belt tension and is more accurate than the “well calibrated finger” often used to measure tension.

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