Every powered vessel, whether single- or twin-screw, must be fitted with a shaft log or tube, for each shaft, so, arranged as to provide support for the shaft bearings and secure watertightness where the shaft passes through the hull or deadwood. In a single-screw vessel the log will be on the centerline and will fit against or mortise into the stern post. Such a log is shown in Fig. 26. In a twin-screw vessel the log will pass through the hull some distance away from the centerline, but the construction of the log will be the same as that shown.

Figure 27. Proportions Of Splines.

Figure 26. Shaft Log.

These logs, where not necessary to use four timbers, are built in halves, each half having exactly one-half of the hole cut in it. The joint between the two halves is arranged as a calking seam with an opening of about 3/32inch outside. In addition, it is customary to fit a soft wood spline on each side of the hole and inside of the line of bolts. If a good fit is' secured these splines will make the log watertight without the calking. These logs are afterwards bored out perfectly true and fitted with lead or iron sleeves, which are watertight, but this does not relieve the workman from the necessity of making the log as tight as possible without the sleeve.

Logs may or may not be made with a tenon for mortising into the stern post. With careful workmanship the mortise makes the best job.

The bolting, or fastening, of the log varies with different ships. Logs made for insertion through the skin of the ship, as for twin-screws, are invariably bolted up as shown in the figure. Where possible, this should be done with centerline logs for single-screw ships. Where it is not possible to drive bolts from the top of the log, clear of the hole and into the deadwood below, then the lower half of the log is fastened in place first, bolts being driven through it and well into the deadwood below. Then the top half is fastened to the first half with bolts arranged as shown in the figure. These bolts cannot be clinched. In the majority of cases, however, the log can be bolted up before being placed in the ship. The two halves must be clamped together very tight before holes are bored, and bolts driven and clinched. Clinch rings should be set in flush.