How To Use
The Right Fastener

By Don Casey
Published: August/September 2013

First, assess the load this fastener will need to handle. Then choose among very different fasteners for different challenges. Don't worry, Don's here to help you figure it out.

In general, the weak link in the attachment of a fastener is the internal threads that the fastener's external threads engage. Threads cut by wood screws into wood, or by sheet-metal screws into fiberglass, lack the strength, consistency, and durability that machined threads inside a metal nut or other thick metal component can provide. Bolts or machine screws must be used to secure any component or piece of hardware essential to boat function and/or safety that might experience a load that will or could pull or pry it from the boat. In addition to nuts on the backside of the fitting, through-bolt mounting should include a strong backing plate made out of aluminum or stainless steel or, at the very least, heavy fender washers to spread the load.

Besides eyes, cleats, and pedestals, the items that need to be secured with bolts that pass through the deck or hull include handrails, windlasses, and safety rail or lifeline stanchion bases; davits, cranes, and pole mounts; wakeboard towers and ski pylons; and for sailors, winches, travelers, and headsail tracks. Hardware of lesser import or subject to little stress can be fastened with screws. This would include things like deck fills, ventilators, rod holders, flagpole sockets, and running lights.

Wood screws are for wood only, never fiberglass. If you think an underlying wood core alters this, you are doubly wrong. You should never screw any type of fastener into (or through) core material. In all cases, core around the fastener must be removed and replaced with epoxy putty sufficient to isolate the core from the fastener hole. Sealants won't prevent water from reaching and ultimately destroying core. So whether installing screws into solid laminate or through a fiberglass skin into a core plug of cured epoxy, use stainless-steel sheet-metal screws. But sheet-metal screws should never be used to fasten hardware to extruded aluminum spars (masts, booms, spinnaker poles, and so on) because sheet-metal screws are designed to be held in place by a single thread and are not meant to cut threads — they distort the walls of the hole into a helix. Your choice here will be between machine screws and blind rivets.

Blind rivets are the easiest, although you'll need a riveting tool. Blind rivets for marine use can be made out of aluminum, Monel (a nickel/copper alloy), and stainless steel. Aluminum rivets won't cause corrosion due to dissimilar metals, but they are the lowest strength. 316 stainless steel and Monel rivets are more than twice as strong as aluminum and do well in the marine environment, but both require a pre-installation coating of Tef-Gel, a PTFE paste, to reduce the risk of corroding the aluminum. Some aluminum and Monel rivets come with steel mandrels (the post inside the rivet that helps to create the head on the blind side of the hole), and those invariably rust in the marine environment. Aluminum rivets should have aluminum mandrels; Monel rivets should have Monel mandrels.

Mast-hardware brackets often extend around to the sides of the spars so that the forces on the fitting will be shear instead of tensile. Stainless-steel blind rivets are often specified for this type of bracket. Where the loads will be tensile, a stainless-steel machine screw can deliver roughly half again the holding strength of a stainless blind rivet. This assumes a pilot hole drilled to the proper size, internal threads cut carefully with a sharp tap, and a wall thickness sufficient for at least three threads — which is 0.150 for a 1/4-20 screw (20 threads per inch). Note that tensile loads on screws place shear loads on the threads, so in soft material like aluminum, you'll generally get greater holding power with thicker, i.e., coarser, threads. For example, select 1/4-20 rather than 1/4-28 screws for mounting hardware to a mast.

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