At some point, whether professional boat builder, Weekend Warrior or DIY guy, you’re more than likely going to need to attach something to something else. Especially in a marine environment which is known for being a not so friendly place for things to live together. Meaning corrosion can and does take it’s toll. In an assuredly accelerated fashion to say the least.
If longevity is your goal and if you’re even the least bit interested in making as few repairs as possible, as well as the least amount of times necessary, then the following should help you to pick the correct part for the job.
Marine grade fasteners come in a variety of shapes and styles, from wing nuts to slating nails and cotter pins. All of these categories have numerous types of metal and sizes available.
Stainless Steel, Silicon Bronze, Brass, Galvanized, and Copper, from a #2 up through 1 inch and some metric sizes as well.
Most fasteners are available by the ‘100’ or per pound. Larger sizes are available in smaller quantities and all are available in larger quantities.
Stainless Steel screws are the work horse for modern boat building or exterior woodworking applications. They provide great corrosion resistance while remaining affordable.
Hardened steel screws are more common in the woodworking environment but cannot be used in a corrosive environment.
Stainless steel is available in a wide range of engineered grades, each with its own designation number and specific properties. Only a few grades of Stainless Steel are suitable for fasteners–those that can be cold headed easily and have adequate torsional and shear strengths.
The 300 series stainless steels are the most popular — 18-8 grades contain very little carbon, about 18% chromium and 8% nickel — giving them good corrosion resistance. In our opinion, 304- and 305-series stainless steel is not that much of a step-up from 18-8. But, 316 series stainless contains molybdenum, which significantly increases corrosion resistance, strength and of course, the cost.
Use Stainless Steel screws cautiously below the waterline. Stainless Steel Screws cannot be in an anaerobic environment. If the screw is immersed in what is called still water, with no oxygen the corrosion-resistant film, chromium oxide, will not be allowed to form.
Without the chromium oxide film the screw will suffer from galvanic corrosion and eventual failure.
When using stainless steel nuts and bolts especially when they are larger in size always remember to lubricate with an anti-seize compound. This will prevent the nut from binding on the thread (known as galling or cold-welding).
Silicon Bronze is the ideal metal in the marine industry and for personal boat building projects. It has moderate strength, which makes it a much better choice than brass, which is extremely soft.
It’s corrosion resistance makes it a much better choice than stainless steel and hot dipped galvanized parts.
Furthermore, with the high number of bronze based hardware replacements on traditional boats, silicon bronze should be your number one choice for marine fasteners (screws, nuts, washers, bolts, etc.) and hardware (hinges, hatch lifts, rudder hardware, etc.).
Silicon bronze fasteners are primarily used for marine boatbuilding, although many people appreciate the look they can provide with Western Red Cedar or Redwood because the screws will eventually blend into the color of the wood. Silicon bronze screws provide excellent corrosion resistance but are soft and require a carefully sized pilot hole to avoid breakage.
Pre-threading the hole with a steel screw will certainly help as will drilling twice once for the root diameter and once for the shank. Using a Step drill is another alternative. In all cases you should lubricate the screw and be conscious of the torque you are exerting.
Silicon bronze fasteners come in a variety of combinations of alloys, but most commonly a compound of copper, tin and silicon. When replacing copper hardware on boats, traditionalists turn to silicon bronze more than 90% of the time.
Again, it is stronger than brass, so when rebuilding a cat boat, the students and experts at the Newport International Yacht Restoration School choose silicon bronze fasteners and hardware on a daily basis.
When working with pressure treated wood, it is important to remember that pressure treated wood now contains six times the level of copper as it did before 2004, making it much more corrosive to common steel.
Fasteners used with pressure-treated lumber should always be double hot dip galvanized steel, Stainless steel, silicon bronze, or copper.
Brass or aluminum fasteners should not be used with waterborne preservatives.
In structural applications where a long service life is required, stainless steel, silicon bronze, or copper fasteners are recommended. In long term corrosion testing conducted by the Forest Service division of USDA on fasteners used in CCA treated wood, only stainless steel nails and screws exhibited virtually no visual signs of corrosion and negligible weight loss over a fourteen year period.