Anywhere you see wood inside a yacht, you’ll find screws holding it together. These screws don’t have an easy life – the moist, salty conditions would quickly turn a plain steel screw into a pile of rust. But getting the right screws – the correct material and the correct size – requires some care.
Back on dry land, steel is a commonly used material. It’s cheap and strong, and you can pick up steel wood screws in any DIY store. But steel corrodes quickly when exposed to salty sea water and turns into a red rusty mess. Coating the steel with zinc (“galvanizing”) protects the underlying steel, so long as the outer coating isn’t scratched or damaged. Alternatively, adding chromium to the steel makes so-called “stainless” steel which greatly reduces the tendency to corrode.
Aside from steel, brass and bronze are two other materials used for woodscrews. Both are copper-based metals, with the addition of zinc giving brass, and the addition of tin giving bronze. Both are corrosion resistant and have a colour which blends well with wood. However, brass is a fairly soft metal – requiring you to predrill holes and take care to avoid breaking small screws. Bronze is stronger than brass which makes it a good choice for screws on a boat. There is a cost impact though – a bronze screw of a given size will cost twice as much as the same sized brass screw.
What size is that screw?
Wood screws are sold in sizes such as 4×1″ and 6×1/2″. At first glance this looks like a crazy hybrid between metric and imperial, but that’d be a big mistake!
Back when modern metal screws were invented, there was a wide range of measurement systems. Gradually, this got whittled down and around 1880 the American Society of Mechanical Engineers was formed with the idea that all these arbitrary choices were wasteful and it’d be better if everyone standardised on a single measuring system. For woodscrews, they decided to have a series of sizes (gauge 1, gauge 2, etc) whose sizes are written down in a document with the catchy name of “B18.6.1”. Bigger numbers mean bigger sizes, but the gauge numbers don’t correspond nicely to any simple size in inches of millimeters.
Sadly, it costs $40 to get a copy of the B18.6.1 document from ASME. Most engineers use a reference book like The Machinery’s Handbook which has all the same measurements in it. The standard details everything about the wood screw – the size of the head, the shank, the threads etc. It also gives tolerances (no real-world screw will be exactly the nominal size, and often it costs more to make a very exact screw and often it’s not that important).
According to the standard, a gauge 4 wood screw (aka “G4″) has a shank diameter of 0.112” = 2.8mm. A gauge 6 wood screw (“G6″) has a shank diameter of 0.138” = 3.5mm. You can find tables online with the rest of the sizes. There are also some rough “rules of thumb” to remember what size each gauge is, but they’re crude approximations – the size is defined by a table of numbers in the standard, and that table isn’t derived from a simple relationship.
If you need to replace an existing screw on your boat, you have two options to determine what “gauge” it is. One is to get a Thread Gauge Checker plate which had holes of the appropriate size so you can just offer up your screw and see which hole it fits in. The other method is to measure the diameter of the unthreaded shank using a vernier gauge, and look up the table. Or you could just use a steel rule to get a rough estimate; most suppliers only stock the even “gauges” anyway.
The length of a screw is measured in inches, from the top of the head to the bottom of the point. Putting this all together: A 6 x 1/2″ screw has a 3.5mm shank and is about 12mm long. Just remember that the ‘6’ does not refer to a metric/millimetres measurement (that’s why metric bolts are written as “M6”).
There are several shapes of screwhead. Countersunk means the underside of the head is sloping so it can fit into a chamfered hole. They normally have flat tops so that the screw lies flush with the surface. But you can also get “raised” (aka “oval”) countersunk heads which sit proud of the surface – just an aesthetic thing. Finally, ‘pan’ headed screws aren’t countersunk – they have a round top but a flat underside.
Using a washer avoids having all the load concentrated on a small region of the wood, which can lead to damage. Screw “cups” are slightly raised washers, so that a countersunk screw fits neatly into the middle. Screw cups are sized based on the shank size – eg. a “G6” or “gauge six” cup will fit a G6 screw. With pan-headed screws you can use plain flat washers since they’re not countersunk.