Early wooden boats were often held together with lashings of roots, vines, cordage or animal parts. As boats became larger, many were pegged together with wooden rods called trennels (basically tree nails). A hole was drilled in the plank and frame, then the trennel was pounded in, wedged in place, and sawed off flush.

As time marched on, metal nails were used, sometimes backed up with roves on the inside as rivets. Manufacturing became more sophisticated, and screws and bolts could be created in mass quantities suitable for even stronger assemblies.

Regardless of the method, the strength of a fastener and its installation have always been the difference between a heavy piece of equipment remaining in place or coming adrift—or a crew staying on the boat or going overboard.

Measurement Woes

Since boats and their equipment are globally produced products, there are as many ways to hold things together as there are languages. It seems like everybody has their own design and measurement system. Some of these measurements are indexed with the length of a king’s toes to his heel. A fathom is 6 feet, about the length of most men’s arm spans. A tape measure was built into the man for measuring out the anchor line. A foot needs no explanation.

The problem with measuring this way is that we are all different lengths. Even once the units were standardized, the fractions used to divide up smaller proportions make for mathematical nightmares.
Specifically referencing U.S. fastener sizes, we commonly call them SAE, which stands for the Society of Automotive Engineers.

Some time ago, the SAE gave up the job to ASME and ANSI, the American Society of Mechanical Engineers and the American National Standards Institute.

And I’m still waiting for the metric system to take over the United States, as my teachers said it would back in the 1970s.

So Many Choices

The environment that boats inhabit requires special considerations for fasteners. You have to protect against corrosion. And, of course, you need the correct tool to match each fastener if you are ever going to install or take anything apart.

A screw typically has fewer threads per length than a bolt, and does not penetrate the bottom of the surface. The hole should be sized to allow the shank to fit and the thread to cut the material. The material needs to be thick enough for enough threads to be formed in the material, so the screw won’t pull out.
The material also needs to be strong enough to resist whatever the pull is.

We’ve all hung pictures or towel racks at home and soon discovered that the screw easily pulled out of the drywall. A similar thing can happen in cored fiberglass if the coring is not backfilled with thickened epoxy, or if the skin is too thin.

Countersinking the outer surface of the hole will also help prevent any cracking in the gelcoat, and will make a fine O-ring of sealant when the object is bedded.

A bolt typically penetrates the bottom of the surface and utilizes a nut. Smaller bolts are sometimes called machine screws, just to complicate things.

Depending on the load, washers and backing plates will prevent the nuts from pulling through the surface. Plain nuts have a tendency to loosen when there is vibration, which is why nyloc nuts, thread-locking compounds or Nord-Lock washers should be used in high-stress or vibration applications.

Heads and Drivers

Early fastener heads were typically slotted. Slotted screws still have their place because they can take a proportionally larger amount of torque when stuck without stripping. However, they have an irritating shortcoming: They won’t stay put on the end of a driver during installation. If you are assembling many screws or reaching into a one-armed, blind location, this can really slow you down.

Englishman John Frearson invented a cruciform (cross) head recess in 1873. It was later manufactured by Reed & Prince Manufacturing Co. in Worcester, Mass. In 1933, J.P. Thompson in Oregon designed a slightly different cross shape that Henry Phillips began promoting. Phillips’ efforts quickly cornered the manufacturing market, and even today, half the screws produced in the United States are Phillips. Even still, the Frearson design is less likely to be stripped if the driver slips under torque, an irritating situation called “camming out.”

Another cruciform design is the Japanese Industrial Standard. JIS can be distinguished by a small dot on the head next to the cross. Adding to the confusion, there is another known as the Pozi head, which incorporates a small square in the center of the cross.

The big thing to know is that there are all these similar shapes. Your screwdriver or bit needs to match the exact shape, or you’re likely to strip the head.

There are also Robertson (square), Allen (hex shape), pentalobe (we see you, Apple), Torx (six-pointed star), XZN (triple square) and spline (multipoint). By the time this article is published, there are likely to be a few more. Some of these heads are designed with a small stud in the center, requiring a corresponding hole in the center of the bit as a theft deterrent. And lest we forget, there are also hex-headed bolts and carriage bolts with a square shape under the head that locks into the substrate to keep them from turning. They require a hammer to knock them out.

And, of course, each of these can be produced in multiple sizes measured via metric or SAE.

Some boats seem to have every variation of fastener in metric and SAE, as well as slotted, Phillips, square and Pozi screw heads. This can be a bear, as it seems you almost never have the right tool. I’ve known owners who choose one style and replace all the fasteners just to get as many the same as possible. The struggle is real.

Threads and Nuts

Like the head size, threads are dependent on their measuring system.

There can be British Standard Whitworth (BSW), Unified Thread Standard (UTS) and metric. The best advice here is to purchase a thread pitch gauge or fastener size gauge. Many are available, and even the most inexpensive plastic size gauges will allow you to screw in your fastener or nut to check for fit. This can save lots of time and trouble by allowing you to identify the correct diameter and thread on the first try.

Nuts are made as standard, nyloc and flanged, possibly with locking mechanisms formed in. Each can be made in its own measurement system suitable for wrenches or sockets. Fortunately, most are hex shaped.

Acorn nuts can help finish a pretty installation or protect the sharp edges of a bolt that protrudes past the nut.

Materials

Inside the cabin, mild steel fasteners, especially if they are anodized or coated, are cheap, strong and durable. Brass, an alloy of copper with a large percentage of zinc, also works well inside.

Metals better able to resist corrosion are in order in the bilge, damp engine rooms or underwater. Bronze, another copper alloy, utilizes a much smaller percentage of zinc in its alloy. It still is a favored solution for underwater fasteners.

Plain magnetic steel fasteners have a major drawback when used outside on a boat or in a wet bilge, especially if there is salt involved. Unless the steel is fastidiously kept painted, it will rust away, sometimes quite quickly.

Stainless steel, a generic name for different alloys, adds different compositions of chromium and molybdenum to the steel to create a more corrosion-resistant alloy. The most widely used stainless fastener is 304, sometimes called 18/8. This is acceptable for many applications but can still rust if continually exposed to seawater. The next-higher grade, 316, has even more corrosion resistance.

Note that each material has a different strength. They are often not interchangeable. For instance, you would never want to use stainless steel bolts or nuts in an engine shaft coupling where they are not strong enough for the application.

The heads of most bolts have a marking system that denotes the grade and strength of the steel alloy. This is different for SAE and metric systems. Most engines use steel fasteners. If kept painted, these will last the life of the engine. Just be sure if you are replacing them that you pay attention to the grade.

Torque

A standard box wrench set has wrenches of different lengths. The leverage applied by an average person to each length is loosely calibrated to the torque an average steel fastener should require. For noncritical loads, this is acceptable.

But for important, high-vibration or safety-related objects, there is nothing like a calibrated torque wrench and a torque value from the manufacturer.

After you’ve spent the time to align your engine to its prop shaft down to .004 of an inch, the last thing you want is for the coupling bolts to loosen. The first step is to torque them. For fasteners that don’t have specific manufacturer’s torque specs, there are plenty of tables online for each alloy and size of bolt. It’s also not a bad idea to put a dab of Cross Check Torque Seal on the nuts so you can visually see if they’ve loosened.

Engine manufacturers in particular want you to use a torque wrench when assembling their parts. Engines today are composed of multiple alloys, some much softer than others. It can be easy to crack some castings or strip the threads in others.

Note that the torque specs are usually intended for the fasteners to be installed dry without oil, lubricants or thread lockers. Just like with the size of fasteners, there are different measurements of torque. Be careful when reading torque tables. Set your torque wrench to the proper measurement.

With the proliferation of excellent battery-powered drills and ratchet guns, there is a tendency to rely on them not only to install fasteners, but also to torque them. This can be dangerous. The torque is typically not calibrated for each size of fastener, and it is easy to strip the head, break off the fastener, or leave the fastener loose.

When using an electric tool, if it has a slip setting, set it low and use a hand tool to do the final tightening. If you do start to cam out the head during installation, stop immediately. You may be able to back out the fastener and replace it with a fresh screw.

Leaving the stripped fastener is a mechanical no-no. And while it may look nice to line up all the screw heads on a cosmetic piece on the boat, it is not recommended for anything where the torque is important for safety or water-tightness.

Assembling the Puzzle

There will be many different sizes and shapes of fasteners on your boat. Making sure you have the correct tools for repairs is just part of getting ready to go on a voyage.

Often, a loose, poorly performing or failed piece of equipment only requires a quick snugging or simple disassembly to find the problem and make a repair. The length of the time spent on the repair can expand dramatically if you use the wrong tool for the disassembly.

Take a moment to look around your critical systems and commune with your favorite tool catalog to make sure you’re ready to fix whatever the sea throws at you.  

This article was originally published in the November/December 2024 issue.