Boatbuilding with Plywood
Chapter 6 - Fastenings

Continued

NAILS

Ordinary nails such as the steel common or finish type may sometimes be used for incidental purposes in plywood boatbuilding, but the main nail fastener is the annular rink shank or threaded type boat nail. This nail is usually made from bronze or Monel. There are also machine driven non-ferrous nails or staples that can be used in lieu of this type, but the average amateur may not have access to these, nor to the pneumatic nor electric powered gun required to drive them. Regardless of the type of nail or staple used, if it is to remain in the completed boat, it should be made from a suitable marine-type metal. The ring shank boat nail has a series of serrated circles around the body of the nail. When these nails are driven into the wood, the rings tend to lock with the wood fibers, giving a positive grip that is hard to pull free, once driven, without destroying the surrounding wood fibers. As a general rule, the ring shank boat nail can be used for fastening virtually all flat member junctions in lieu of screws. However, it must be emphasized that these nails do not have the ability to pull the parts together as screws do, and consequently may not provide the proper clamping pressure required for certain types of glued joints. This makes the use of nails for holding planking in place a questionable practice, especially where bending stresses are severe.

If planking is to be fastened with nails, then a clamp should be moved along the backing member and the plywood to hold the panel down, or another worker may be able to physically hold the panel in place, because it will surely pull free if the tension is too great. In most instances, it will be necessary to have someone underneath the backing member bucking it while the nails are driven from the opposite side to prevent springing of the member.

As noted, the annular ring nail comes into its own on flat members such as gussets-to-frames, decking panels, laminations, cabin sides, and cabin tops. The length of the nail is sometimes governed by the thickness of the member to which the plywood will be fastened, but as a general rule, the nail should have a minimum length of three times the thickness of the plywood or member being fastened, and preferably longer. Always stagger the nails in the grain, and drive the head flush with the surface, preferably driving home with a nail set to prevent damaging the surface if appearance is important.

Usually the wire gage of the nail is smaller than the shank size of a similar length screw, but then a greater number of nails (or closer spacing) are used than for screws. Unlike screws, the larger the wire gage number, the smaller the nail shank. With nail shanks larger than about .100", pilot holes are recommended even though most builders will want to pound them in just like the ordinary type. While this rule can sometimes be violated without incidence, such pre-drilling will prevent splitting and increase holding power.

Naturally the pre-drilled hole must be smaller than the nail diameter, and this will vary between 50% and 70% of the nail depending on the hardness of the wood. Softer woods should be pre-drilled with smaller holes, while harder woods should have larger holes. The depth of the hole measured from the top surface where the nail will set should be about 80% of the length of the nail.

Information on the sizes of the ring shank nails for plywood boatbuilding is given on Plate 6E. These recommendations may need to be varied depending on the type of wood and the pressure required to hold the members in position.

Plate 6E - Annular thread nail recommendation and reference chart for various plywood thicknesses.

BOLTS & SIMILAR FASTENINGS

The common bolt used in plywood boatbuilding is the conventional carriage bolt. The large rounded head of the carriage bolt has a squared portion just below the head, which prevents the bolt from turning in the wood. This means that the nut can be tightened down without the need for wrenching on the head at the same time. This is important in boatbuilding because the heads must frequently be concealed, and yet one must be able to tighten down the bolt both during the construction and perhaps at some time later. No washer is used under the head of a carriage bolt; however, a flat washer of generous size should be used under the nut. Joints where carriage bolts are used include keels to stems, transom knees to transoms and keels, longitudinals like keels and chine logs to frames (especially in larger boats), or other areas where a strong mechanical connection that will not pull free is required.

In some cases, long lengths of carriage bolts are difficult to obtain. If this is the case, it may be possible to cut the joining members down in size somewhat at the heads or nuts to suit a shorter size without weakening the junction appreciably. Or it is possible to substitute lengths of threaded rod, using a punch to jam the nut at one end to serve as a head, and then securing with another nut at the opposite end. Then again, another type of fastening may be substituted in some cases.

Machine bolts with hex, square, or flat heads (called "machine screws" in this form) are sometimes called for, but access to both ends of the fastenings must be possible for tightening. Flat head types are used where the head will be flush with the surface.

Drifts or drift bolts are not commonly used in plywood boatbuilding, but are sometimes called for in larger boats, especially for assembling large deadwood members. A drift is actually just a great big nail, and driven with hard blows of a large hammer. A point is shaped on an anvil, which does not have to be sharp. The hole for the drift is a little smaller than the drift and a clench ring is placed over the hole. The drift is driven into the hole and through the clench ring. When nearly home, the head is shaped and then driven home. Lag bolts or lag screws are basically oversized screws but have hex or square heads for tightening with a wrench. They are used, along with drifts, when it is not possible to use through bolts. The holding power of lag screws is dependent on the proper size hole being provided to suit the size of the lag screw as well as the type of wood being drilled. It must not be too large or too small.

Hanger bolts are like a lag screw without a head, and with one end being threaded for a standard nut. The purpose of a hanger bolt is to allow replacement of the adjoining member by simply removing the nut. The stud of the hanger bolt remains in position for repositioning and refastening the member in place. These fastenings are often used as inboard motor or motor stringer hold-downs since it is impractical to continually remove a lag bolt.

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