Holding power of wood screws
Not the least of many factors aecting the holding power of a wood screw is the angle of its long axis relative to the wood fibres; the holding power of a screw driven into end grain may be as little as half that of one driven into side grain. The species of wood is also significant – screws of equal length and gauge hold better in dense hard woods than in light soft woods. And for a given timber, longer, larger diameter screws hold better than shorter, thinner ones. The metal itself has a significant bearing in the long run: in a saline environment brass screws lose their grip as electrolysis wastes their zinc, while steel screws used in woods rich in tannins not only rust but destroy the wood around them. Technique has also to be taken into account, since every screw must have the right clearance hole for its shank and a pilot hole ensuring optimum bite for its threads.
In light of that, it can only be a rule of thumb which says the length of a wood screw should be at least one-and-a-half times the thickness of the part (such as a garboard) that it’s fastening to another (such as the hog) but it’s a rule that’s held good down the years.