PassageMaker - September 2009

WHAT’S WRONG WITH THIS PICTURE?

FASTENER LENGTH

Nuts, bolts, and other fasteners are an essential part of every cruising boat, no matter how small or large the craft. Even an engineless dinghy relies on fasteners to hold oarlocks, thwarts, and towing eyes. Bigger vessels, of course, depend on a vast assortment of fasteners of every imaginable shape, style, and size. In fact, the “fasteners parts list” for the average 40-footer can easily number in the hundreds.

The importance of properly selecting and installing all fasteners cannot be overemphasized, nor can the insidiousness of their potential failure. Choosing the wrong fastener for the job—using a brass machine screw rather than a bronze one to secure a hull strainer, for instance—could mean the loss of this gear, an unfortunate but hardly ruinous event. In contrast,

replacing a high-strength mild steel coupling bolt with one that’s the wrong size, of a lesser grade, or not made from the correct alloy could lead to a catastrophic failure.

It’s also important to point out that most machine-screw-type fasteners, common nuts and bolts, and bolts that are mated with female threads located in large objects (such as engine blocks) enable mating of different parts via clamping action that is only achieved when they are properly tightened or torqued. Thus, all

fasteners—particularly those used in critical applications such as engines, steering mechanisms, and running gear—should be installed using a torque wrench and recognized torque specifications.

The coupling fastener shown in this image has loosened as a result of inadequate torque,

insufficient length, or
both. It’s clear that the
bolt is too short; its threads have
not engaged the nut’s nylon locking
insert. When using nuts and
bolts, standard practice calls for
a minimum of two bolt threads to
stand proud of the nut, regardless
of whether a locking-style nut is
used. This helps to ensure proper
engagement and load transfer
between the nut and bolt threads.