Deck beams for weather decks must always have a round up, or to use another term, camber. The standard amount of this camber, or spring from a straight line, is one-quarter of one inch for each foot of length of the beam. In wood ships, however, the amount of camber is very often reduced to about three-fourths of the standard camber.

Figure 107. Beam Mold.

Figure 108.

Figure 109.

Figure 110.

Only a part of the camber is cut on the beam. The balance of the required amount is obtained by springing the center of the beam upward after the ends have been fastened down in the ship. This operation, while performed with heavy jacks, is generally called "pumping the beams".

The relative amounts of camber to be sawn and sprung into the beam depend largely upon the difference between the molding of the beam at the center and that at the ends, and the length of the beam.

Beams of ordinary proportions may generally be sprung about one-eighth of one inch for each foot of length although this must be regarded as very near the limit, and it is often best to hold the spring to about a tenth of one inch per foot of length. Thus a beam forty feet in length may be sprung from four to five inches. The balance of whatever camber may be required will have to be cut in the beam.

As an example let us suppose that we have a beam forty feet in length as above and that the required camber is eight inches, and that of this amount it is considered safe to spring five inches. This leaves three inches of camber to be cut in the beam. Now, if the required molding at the center is fourteen inches, and at the ends, eleven inches, the difference between the two moldings is three inches and we may cut the camber on this beam on one side only, thus leaving the underside straight as shown in Fig. 108. (The molding in this type of beam at the end is measured at the bottom of the hook. In the type shown in Fig. 109 it is measured at the end of the beam).

Again, let us suppose, that with the same length of beam and required camber, the required molding at the center is fourteen inches and at the ends thirteen inches. Now, the amount of camber to be cut in the beam is three inches and the difference between the moldings is but one inch, therefore it will be seen with a little study that we must cut two inches of camber on the under side of this beam, in order to be able to cut three inches on the top and still maintain the required moldings. We will then have a beam of the type shown in Fig. 109.

After it has been determined what camber to cut in the beam, a beam mold is made for the midship beam, having this amount of camber. This may be easily laid out by what is known as the "one, four, nine rule," as shown in Fig. 107. This is the same rule that has been previously given for the laying out of spring points on the keel and need not be further explained here.

This same mold is then used for scribing the tops of all beams except those near the after end of the ship, where the beams must be flattened out in order to fair the center-line of the deck into the knuckle or rim. If the beams are of the type shown in Fig. 109 then the mold may be made with both edges cambered as required, so that the beam may be scribed from the one mold.

The amount that the beams must flatten out at the stern to avoid a hump in the deck depends upon the shape of the deck at that point and the amount of sheer. Very often the deck line inside the frames is dropped slightly just forward of the stern so that the beams will not have to be flattened so much. No rule can be given for this operation and experience is about the only teacher that will enable the workman to obtain uniformly good results. However it may be stated that in many cases, all of the beams may be molded and cut to the same camber, the flattening out at the stern being gauged by pumping the beams to a stiff fairing batten set in the centerline.