Diagonal iron strapping generally consists of a wide band, set at or slightly below the sheer line, extending from approximately abreast the stern post to a short distance abaft the stem. As shown in Fig. 151 the diagonal straps are attached to this band and so arranged that they cross in the frame bays, where they are riveted to each other. Both the straps and the wide plate at the top are dapped into the frames so that their outer surfaces do not extend outside the fair surface of the frames. Each strap is fastened at each frame with at least one countersunk head drift bolt, the best arrangement being to stagger these on the frames as shown in the figure. If two bolts are driven to each frame the stagger rule will of course not hold good.

The diagonal straps should extend downward around the bilge and onto the ends, or heads, of the floors at least 24 inches, where they should be extra fastened.

Around the bilge they are often set or dapped in deep enough to permit dubbing the frame flat under each strake of planking which saves the expense of hollowing out the faying surface of the planking to fit a rounded bilge. As has been mentioned before, it is best where possible to do the dubbing before the strapping is fitted, as then there is no uncertainty about how deep the strapping must be dapped into the frame to clear the planking. The daps should not be deeper than absolutely necessary as they in any case tend to weaken the frame.

Figure 151. Showing Arrangememt Of Diagonal Strapping.

Diagonal planking, or sheathing as it is sometimes called, is laid in two courses, each course being at an angle of about 45 degrees to the keel and about square with the other course. It covers the entire frame of the vessel from stem to stern. The frame bays at the tops and along the keel are chocked to provide continuous fastening surface for the upper and lower ends of the diagonal stuff. This chocking has been shown in previous figures.

At the ends of the ship, separate rabbets are cut for the diagonal planking where possible so that the end fastening of the diagonals will not land in way of the hood end fastening of the outside planking. Along the keel, and above the shaft log at the stern there is generally but one rabbet for both the diagonal and outside planking.

As before explained, the entire frame of the vessel must be dubbed fair before diagonal planking can be applied.

It is customary to start the diagonal planking at a point near midship, working thence each way to the ends of the ship. While the planking as started should be at an angle of 45 degrees with the keel, it will be found as the work progresses fore and aft, that one edge will buck up, or refuse to fay to the frame, around the bilge when the plank is bent to place. This is due to the shape of the ship and the greater distance from end to end of the vessel around the frame at the top than along the keel, which soon causes the upper ends of the diagonals to fall behind the lower ends. When this point is reached it is necessary to take off what is known as a spiling. This is done by springing a thin wood batten about ⅜ inch thick and 6 inches wide, around the frames in the position of the next strake of planking to be fitted. This batten, known as a spiling batten, is allowed to lay in its natural position, and the amount that it springs away from the edge of the last strake of planking will indicate the taper that must be cut on the next strake to correct the work. At some points in the ship it may be necessary to take a spiling every fifth or sixth strake. The operation of spiling is explained in detail further on in this chapter.

The diagonal planking is fastened with common galvanized wire nails just sufficient to hold it up against the frames until the outside planking is put on. The nails for the first course are generally shorter than those driven in the second course, each being of such length as to penetrate the frame for about the same distance. They are driven at the extreme outer edge of the frames so as not to interfere with the planking fastening. This arrangement of the fastening also serves to mark out the location of the frames which have now been completely covered up. In addition to this, however, the locations of the edges of all timbers in the frames should be accurately scribed on the outside of the diagonal planking, so that the outside plank fastening may be properly located.