Boatbuilding with Plywood
Chapter 6 - Fastenings

Continued

SCREWS

Screws come in three main types: round head, oval head, and flat head. The flat head wood screw is the most commonly used in plywood boatbuilding. All head types are available as a slot head or with a crosshead type such as the patented Phillips or Read-Prince varieties. These latter types are usually easier for machine driving, however, with the hot dipped galvanized variety, these do not work out so well since the cross head tends to load up with the molten zinc thereby making them harder to drive than the ordinary slot head.

Screw lengths of the standard flat head type range from 1/4" to 4" long, however, screws less than 3/4" and over 3 1/2" are seldom used in plywood boatbuilding. A number is used to designate the size (diameter) of the screw shank, ranging most commonly from a #2 to a #18. The bigger the number, the larger the screw shank diameter. Relative sizes of the various screws are indicated on the chart shown by Plate 6B. Screws are sold either by the hundred (1C), or by the gross (144); so when comparing prices make sure you are comparing like amounts.

When driving any screw, it is imperative that the shank and pilot holes are of the proper diameter for the wood being used. Over-drilling or under-drilling will provide poor holding power for the screw or possibly cause the wood to split. Although tables are given recommending the proper sizes, these may have to be varied to suit the work. Obviously, the threads will not cut into oak as well as into a soft wood such as spruce. There are many types of tools available that will drill, countersink, and/or counterbore for screws in one operation, and are well worth the small amount that they cost.

Plate 6C - The "right" way (at left) to drive a screw in plywood, and the "wrong" way (at right). The screw should compress all veneers in the plywood panel.

When driving screws through plywood, it is essential that compression wood be left underneath the head (see Plate 6-C). By this term, it is meant that the screw should not be countersunk so far that it would eliminate the top layer of plywood. The top layer of the plywood should be compressed under the screw head. This gives much better holding power. Screws in plywood that is under tension, such as in the application of curved planking panels, should never be counterbored. They should be driven just below the surface of the wood. Putty is used to cover the head of the screw. If the hull will be covered later with fiberglass or equal sheathing, such putty should be an oil-free type compatible with resin for a proper bond.

Often when driving screws in hardwoods such as oak, the screws have a tendency to bind and are very difficult to drive. When this is the case, rubbing them on a bar of soap or paraffin can lubricate the screws. This will facilitate driving considerably. Avoid driving screws along the very edge of the cross grain on a plywood panel as it can cause the grain to split at the edges. It is better to leave a panel oversize when possible, drive the screws, and then trim to size, such as can be done on the side or bottom planking of a boat at the transom. In extremely fast powerboats or racing boats, it is considered excellent practice to dip each screw in a hard-setting glue before driving it home. When fastening along a length of wood, the screws should be well staggered not only for good holding power, but to avoid splitting the wood. When driving a screw, especially by machine, keep pressure on the screw until it is driven home. If the screw hits bottom but continues to turn freely, it means that the screw has stripped out the hole and has no holding power. When this occurs and the screw is necessary at this point, remove the screw and replace with one of a larger shank size or longer length, or both.

The chart in this chapter (see Plate 6-D) indicates the recommended sizes of screws for various planking and decking thick nesses. However, this may vary considerably depending on the particular boat and the backing member sizes. Quite often screws may require closer spacing or the use of longer screws, or may require larger spacing and smaller screws. This chart should serve as a general guide, but minor variations will not be detrimental.

Plate 6D - Screw size and spacing recommendation for fastening plywood planking and decking of various thicknesses.

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