STEMS AND STERN POSTS

1. What is the main feature of stem and stern post?
2. What types of stems are used in shipbuilding?
3. What is a bulb provided for?
4. What two posts does the stern frame consist of?

The hull of a vessel is at either end provided with a stem forward and stern post aft for the purpose of obtaining a firm boundary of the vessel.
Stem. The earliest steamships were given stems similar to those of sailing vessels. At present this type of stem, called clipper stem, is still occasionally found with paddle-wheelers and steam and motor yachts. Nowadays, however, straight stems are mostly used, which may be either raking or vertical and often consist of a forging having a rectangular section and smaller dimension than the keel. A stem may be manufactured exclusively from plating either with or without an angle bar, and is therefore called a plate stem. Its bottom part sometimes has a spherical shape, which gives it the name of bulb stem. Raking stems are very often met with nowadays. It frequently occurs that a vessel requires a stem having an extra thickness to protect her from being damaged when navigating ice-bound regions. Modern vessels are very frequently equipped with plate stems.
Stern post. Sailing ships, paddle steamers and twin-screw vessels are provided with stern post of very simple shapes. Like their stems, the stern posts also have rectangular cross sections, though of somewhat larger diameters, while they are vertically erected. Stern frame is a heavy casting, forging or weldment forming the after end of the ship's structure, and supporting the rudder. It also serves as a frame for connecting the ends of the shell plating. The stern post of a single-screw ship or of a triple-screw vessel is more complicated, and is a massive propeller or stern frame consisting of two posts, namely the inner or propeller post and the outer or ruder post.
The propeller shaft passes through the inner post which is enlarged to receive it, and the propeller turns in the aperture between the two posts.

Date: 2014-12-28; view: 1416