In single-screw vessels having a wood rudder there will be a rudder post abaft the stern post. The space between the two posts is called the propeller aperture. A typical construction for this post is shown in Fig. 23. The layout method for this post is the same as that described later for. Fig. 25, although the construction of the trunk is different. Since the rabbet, in Fig. 23, crosses the trunk in a straight line and does not turn down onto the post, as shown in Fig. 25, the trunk can be built up, as shown in Bection A-A. This saves a great deal of labor and material, as the post can then be ordered to the siding of the lower end instead of to the siding of the trunk, and there will be no excess material on the lower end to be cut away. All of the joints in this trunk, as well as that shown in Fig. 25, should be well packed with white lead and cotton before bolting up. Very often, in addition to this, a seam is left, which is calked with oakum after bolting up.

The entire trunk must be clamped together hard before the holes are bored and the bolts driven and clinched.

Figure 24.

Figure 33.

Rudder Post As In Single Screw Steamers.

Very often the main piece of this post is made of hardwood. A fairwater is fitted on the forward side of all rudder posts, as shown in Fig. 23. This is spiked in place with ship spikes, after the arch knees have been fitted.

In twin-screw vessels with a wood rudder there will be a stern post, as shown in Fig. 25. An outline of the rudder blade and stock is also shown in this figure. Here, as shown in sections A-A and B-B, the main piece and forward part of the trunk are cut from a single timber. The afterpiece of the trunk is much the same as that shown in Fig. 23. Before the stern frame is complete, in this as in the propeller posts shown in Figs. 21 and 22, the rabbets are merely roughed out, or left uncut, the final cutting being done during the dubbing operation. The turn on the rabbet at the tuck should be roughed out quite full, as indicated in the figure.

To lay out the trunk and clearance line one must know the diameters of the stock and pintles and the location of the uppermost gudgeon. A detail drawing of the braces should also be available. A set of braces, consisting of gudgeon and pintle, are shown in Fig. 24.

The distance from the after side of the post to the center line of the stock and pintles is usually shown on the plans or can be taken from the detail drawing of the braces. However, in the absence of this information, it is usually made equal to one-half the diameter of the pintle plus ⅜ of an inch to 1 inch for clearance.

The lap of the stock onto the blade should be, at least, from one-fourth to one-third the length of the blade measured in the same direction as the stock. This lap is often called the "plug." Sometimes the entire stock is so-called. The diameter of hole in the trunk, athwart-ships, should be equal to diameter of stock plus about 1 inch.

Then, to obtain the clearance, locate and draw outline of top gudgeon. From a point at the top of the post about 3 inches from the forward face draw a straight line N-N passing about « inch outside the outline of the gudgeon. This line is the clearance line, and represents the foremost extent of the hole in the trunk required to clear the stock past the top gudgeon when shipping the rudder.

Stern Post On Sailing Vessels Or Twin Screw Steamers.

Then measure square out from the line N-N the distance, Dia. of stock plus 1 inch, and draw the short line 0 parallel to N-N. The line 0 then indicates the amount that must be removed from the lower after side of the trunk to clear the stock when shipping the rudder. The remainder of the hole in the trunk is obtained by measuring out each side from the centerline, the distance, « Dia. of stock plus « inch, and drawing parallel lines until they intersect the lines N-N and 0. The forward one of these lines must be carried down on the main piece far enough to clear the plug. All of these lines are shown on the molds for the post and are readily transferred to the timbers.

The after face of the post below the trunk is bevelled off each side of the centerline to an angle of about 22 degrees, as shown in section D-D. Since the forward edge of the rudder is cut to the same bevels, and there is about % of an inch clearance between the rudder and the post, this permits the rudder to go over about 45 degrees each side of the centerline.

The travel of the rudder as given above is not necessarily the rule in all ships, and the plans should be carefully followed should other degrees of travel be specified.