The shell may be of stainless steel or an aluminium alloy and the pin or mandrel is made from a copper alloy wire. The flanges come in two sizes, the smaller size is satisfactory for metal but for wood or frp the larger diameter flange is necessary to spread the load.
Cut Nails
Occasionally the marine surveyor will come across so-called cut nails. These are stamped out of 2 mm or 3 mm thick mild steel plate and are sometimes used in securing deck planking as hidden fastenings. They rust badly, disintegrate and are not at all suitable for the marine environment. Cut nails are sometimes used in conversions and new construction to fasten down interior cabin sole boards but are, nevertheless, rare in boat building and should be avoided.
Designation of Fastenings
Boat nails and spikes are designated by their length and dumps and coach screws by their length and diameter. Coach bolts are designated by their length and diameter. Wire and cut nails are usually designated by their length in the United Kingdom but, in a practice dating back to Colonial times, in the United States they are as noted above rather curiously designated by the number of pennies by which they used to be purchased.
In the days when the
author was serving his time all bolts had Whitworth threads but these days they are more likely to have metric threads.
Fastening Withdrawal Resistance
One point that has to be considered is the security of the fastening i.e., its withdrawal resistance. Wire nails driven into green oak or hemlock are almost impossible to pull once the timber has seasoned and dried whereas the same nails driven into woods with low tannin content become progressively easier to pull as the wood dries. Corrosion of the metal of the nail can temporarily improve the resistance to withdrawal but it is often accompanied by hydrolysis of the wood (nail sickness) which negates any increase in withdrawal resistance due to that cause. The primary factors that control the resistance of a nail to withdrawal are the density or hardness of the wood, the nail diameter and depth of penetration. Other factors which have a lesser effect are the nail point shape, type of shank (smooth or ring shank), the presence and type of surface coatings and the length of time the nail has remained in the timber. Nails driven into end grain have a considerably lower resistance to withdrawal than those driven across the grain. Differences also exist depending upon whether the timber is plain or quarter (rift) sawn. Some timbers, particularly cedar, split more easily radially than tangentially and cedar used for planking is probably best quarter sawn to prevent the ends of the plank splitting when fastening off. Pine, larch and oak on the other hand, from the author’s experience are best plain sawn for the same reason. The resistance of the nail can be improved by using coatings. Cement, for example, does increase resistance in soft woods but not hard wood as the coating is usually lost in driving the nail. The use of Chinese or Swedish galvanizing also improves resistance to withdrawal.
Zinc galvanizing, if smooth,can improve the resistance but if rough, as is more common, can have the opposite effect. A nail with a sharp tapering point can penetrate the wood more easily and does less damage. It results in a more strongly bound nail but, because they tend to have a wedge effect, such nails tend to make the wood split.
The holding power of a nail is determined by its length, diameter and the shape of the shank whether round, grooved, square or threaded. Smooth shank nails give the least holding power. The common smooth shank nail as used in general carpentry and wood framing is often, in Britain, called a French wire nail. Threaded nails are used in wood construction because of their superior performance but relatively few tests have been conducted on nails larger than three inches (12d.). Experience has suggested that threaded nail withdrawal design values are too conservative although the average withdrawal strength of threaded nails has proven to be greater than that of smooth shank nails of the same diameter. In comparison of experimental withdrawal strength to existing design procedures for assigning allowable withdrawal strength design values, annular shank nails show the greatest difference. Threaded nails - annular, spiral and knurled - provide the best holding power and performance. Barbed nails which have horizontal or herringbone indentations in the shank hold better than smooth nails but far less than threaded nails of equal size. Nails with twisted or fluted shanks equal or exceed the barbed nail in holding power but provide less hold than nails with rolled on threads.
SURVEYING TIP: When driving French wire nails, it is good practice to turn the nail head down onto a hard surface and to slightly blunt the point with a hammer. That enables the nail to be more easily driven and also increases its holding power. It also reduces the possibility of the wood splitting.
The Report • September 2020 • Issue 93 | 45
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