Got a Screw Loose? Time To Brush Up On Your Marine Fasteners

Apparently, it all started with olive oil.

In the 4th century B.C., the principle behind screw threads came into use for pressing the oil out of olives. Two millennia later, we have far more options. Every boat owner eventually runs into choices between fine and coarse threads; self-tapping and wood screws; and lock washers and lock nuts.

Sometimes, we need a compound to make sure we can take it apart later. Sometimes, we want a compound to make sure it won’t come apart later.

Let’s look at the numerous options so that you will be prepared the next time you need to hold it together.

SCREWS

Screws work well for noncritical fastening situations, such as attaching trim, securing a Bimini frame, or installing a flag socket. If you are depending on the installation for safety or reliability (with things such as handrails, mechanical systems or critical components), then bolts should be used.

A self-tapping screw has a uniform width (no taper), and the threads run the entire length of the fastener. Self-tapping screws require a pilot hole and a clearance hole.

The size of the pilot hole varies depending upon the material being fastened, and sometimes requires experimentation. Drill the hole too small, and the screw might wring off before drawing up tight. Drill the hole too big, and the screw won’t hold.

For most applications, you will be using No. 8, 10, 12 or 14 self-tapping screws. (See chart for recommended pilot hole sizes.)

Keep in mind that the pilot hole allows the screw to thread into the material. If you are attaching a piece of trim to a cabin side, for example, then the trim needs a clearance hole, which is a hole large enough for the screw to pass through without binding, and a pilot hole in the cabin side.

Wood screws differ from tapping screws in two ways. Wood screws have a tapered thread, and the threads do not run the full length. Unless you are building a wooden boat or fine cabinetry, self-tapping screws will be easier to source and use.

BOLTS AND MACHINE SCREWS

For all practical purposes, the terms bolts and machine screws refer to the same fastener, but in different applications. Generally, we call it a bolt if it will have a nut attached. We call it a machine screw if it will thread into a part. For this discussion, we will use these terms interchangeably.

With bolts, their diameter is either fractional or metric (½” or M10); their number of threads is defined per inch or, if metric, by the distance between threads in millimeters (½” x 18 means a bolt with a half-inch diameter and 18 threads per inch, while an M10-1.5 would mean a 10-millimeter diameter with 1.5 millimeters between threads); they are sized by length; their most common heads are hex, round or pan; and their material can be stainless, bronze or something else.

For hardware installations (other than below the waterline), a coarse thread stainless bolt will work well. Some situations will call for a fine thread, which provides greater strength in sheer and tension. If threading into a relatively thin material, fine threads offer more thread engagement.

Thread engagement matters, and the magic number happens to be six. After the sixth thread is engaged, additional threads don’t add any strength. Once you have engaged six threads, if you overtighten, the bolt will break before the nut threads strip. Generally speaking, after the first thread is engaged, you’ve covered 34 percent of the load. With the second thread engaged, you’ve covered 57 percent of the load. The third thread gets you to 73 percent; the fourth, to 84 percent; the fifth, to 93 percent; and the sixth thread to 100 percent.

Some critical applications call for higher strength than stainless can offer in any thread type. Shaft coupling bolts must withstand high sheer and tension loads, and this application calls for Grade 8 carbon steel. Alloys should not be mixed; if you use a Grade 8 carbon steel bolt, then use the same alloy for the nut and washer.

HOLDING IT TOGETHER

Now that we have selected the proper thread, length, head and material, we have to decide how to secure the attachment. The greatest risk occurs when the part vibrates in a direction perpendicular to the bolt. A test machine known as a Junker provides the vibration and quantifies the results. (To watch a video showing how well, or poorly, each option performs, look up “Junker vibration test” online. You’ll be surprised.)

The hardware choices include the common spring-type lock washer, a nut with a nylon insert (nyloc), and specialized washers such as Nord-Lock. Then there are flat washers: Although these washers do not contribute to securing the connection, they do have value. Flat washers prevent marring of the surface beneath the head of the bolt or nut, and they reduce friction while tightening. For nonmechanical installations where strength is important (a stanchion base, for example), a flat washer helps spread the load onto a backing plate. Where backing plates are not possible, a fender washer can be used, offering a much more effective way to spread the load than a simple flat washer can provide.

With lock washers, varieties abound, but split washers are the most common choice and are widely used on boats. The concept seems intuitive enough: The washer has an uneven shape that acts like a spring when compressed, and the sharp ends bite into the nut and the mating surface, preventing them, in theory, from turning.

Their effectiveness, however, is highly questionable. Here is what a NASA fastener design manual concludes: “The typical helical spring washer … serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lock washer of this type is useless for locking.”

I thought nuts and bolts were not rocket science, but I was wrong.

With nylon insert nuts (nyloc), the lower portion of the nut is no different from a standard nut, but the upper or outer end contains a nylon insert. As you tighten the nut, the bolt threads engage normally with the lower portion. But when they pass through, they deform the nylon, which then compresses around the threads and prevents the nut from turning and loosening. Locknuts can be reused several times without a ­significant loss of effectiveness. Keep in mind that the nut only goes on one way: with the nylon insert facing away from the head of the bolt. Nylocs outperform spring-type lock washers.

Nord-Lock washers utilize a notched wedge shape. As the bolt starts to loosen, the orientation of the wedge increases tension, keeping the bolt from turning. Nord-Lock washers outperform nyloc nuts. You’ll pay considerably more for a Nord-Lock, but in critical high-vibration applications, such as alternators, it is worth the added cost.

Not all applications allow for the use of a nut, such as a bolt threaded into a metal plate. For those applications, you can choose from several liquid compounds to prevent loosening.

Loctite is the most widely recognized name brand, and it offers two products of relevance: Threadlocker Blue 242 and Red 271. Blue will prevent the parts from loosening but can be undone with hand tools. Red is considered permanent and can only be removed by heating to 500 degrees Fahrenheit. Red is the fastener equivalent of 3M 5200 marine adhesive sealant; use it only if you never expect to take it apart again.

And, Loctite Threadlocker is not limited to machine screws. It also can be used with a nut and bolt.

HOW TIGHT?

After choosing all the right parts, you still have to put them together. That means deciding how much pressure to apply when you tighten them up.

In general, proper tightening consists of applying enough pressure to elongate the fastener slightly. This technique is known as preloading. Too much pressure, and the bolt will break; too little pressure, and the connection will allow for movement.

How do you know when your technique is just right? For boat applications, we can talk about three different methods: by feel, by torque wrench and by angle.

Other than during engine repairs, few attachments that you will ­encounter require anything more than tightening by feel. Unfortunately, no clear guidelines can be offered, as the idea of feel varies according to the application and the fastener size. In the absence of any torque specification, you are left to feel, and in most cases, that works just fine. Tighten by hand until you can’t turn any more, then use your wrench to make it snug—perhaps a half turn. You should not feel like you are applying excessive force or struggling to get it tight enough.

In some situations, a torque specification will be required. For those cases, you will need a torque wrench or instructions for how many degrees to turn the nut. While torque wrenches are commonly used, they are not as accurate as you might expect. Variations in friction skew the results, which is why measuring the turn of the nut is considered more accurate.

After initial tightening just to the point where a wrench is required to loosen the nut, the position is marked. The installation instructions will then instruct you to turn a specific number of degrees (say, 180 degrees) with a wrench.

We’d like to have it both ways—making sure the connection doesn’t come apart unless we want to take it apart—but threads in the marine environment have a way of seizing in place, making removal difficult at best, and requiring destruction at worst. Compounds such as Permatex Anti-Seize Lubricant can protect the threads from corrosion and facilitate removal.

For critical mechanical attachments in high-vibration situations (such as engine mounts and alternators), consider creating a witness mark. By applying a mark along the bolt head and another mark onto the mounting surface, you create a reference indicator to reveal any movement. Dykem Cross-Check Torque Seal works well for this purpose.

In the end, to paraphrase Mark Twain, the difference between the right fastener and the wrong fastener can be like the difference between lightning and a lightning bug. Remember that any fastener subjected to a high vibration rate will be especially at risk of loosening. In every case, think through what would happen if the attachment failed, and then choose your hardware accordingly.

Steve Zimmerman is the president of Zimmerman Marine, which operates five boatyards in Maryland, Virginia, North Carolina and South Carolina. Zimmerman has been building and repairing boats for more than four decades.

Passagemaker Magazine, September 2020