Aft End Construction of a Ship
The aft end of a ship terminates the structure and is designed to provide a smooth water flow ioto and away from the propeller. The propeller and rudder are also positioned and supported at the after end and require certain structural arrangements in order to operate satisfactorily. The after end construction involves an amount of overhanging structure to accept the steering gear below deck and mooring equipment higher up on the weather deck. This arrangement 1eads ta large slamming forces in this after region, and an adequately stiffened structure is therefore required.
Two main types of stern construction have been used to date-the cruiser stern and the transom stern. The cruiser stern is rarely used in modern construction but it is still to be seen io a large proportion of the ships at sea. The transom stern, with its straight-line form, 1ends itself well to Current manufacturing techniques. It also provides a greater deck area aft and is currently much used for a variety of ship types.
Cruiser stern
The construction of the cruiser stern ensures adequate resistance to any pounding stresses which may occur. Solid plate floors are fitted at every frame space and a heavy centreline girder is fitted below each of the decks in the stern.
Special frames are radiused around the after end and are known as 'cant frames', since they are set at an angle to the centreline of the ship. These cant frames join cant beams which support the deck at the radiused after end. Horizontal stringers may also be fitted to stiffen up the structure by connecting it to the transverse frames further forward.
Transom stern
Deep solid-plate floors are also a feature of the transom stem construction, together with a centrelioe girder. The flat plate of the transom stem construction, however, allows use of vertical stiffeners around the shell plating. The vertical stiffeners are bracketed to the floor and to the deck beams which run transversely across the stem.
A deep horizontal stringer can pro vide additional stiffening to the sheD.plating if required. A deep centre girder runs beneath each of the decks at the stem and is bracketed to the deep web at the centreline of the after shell plating. This web is likewise bracketcd to the various floors in the stem and finally to the solidplate floor constmction below
Rudder trunk
The rudder trunk is all open section which is left in the stem for the entry of the rudder stock into the steering Bat. A horizontal platform is sometimes fitted midway up the trunk to fit a watertight gland. The trunking above is then constructed 10 be watertight and access to this upper section and the gland is provided by a manhole.
Sternframe
The shell plating at the after end is terminated by the stemframe. This is usually a casting, but fabrications and forgings are sometimes used. ln singlescrew ships the stemframe has a boss on the centreline for the tailshaft to pass through and an adequate aperture is provided for the propeller 10 operate in. If sufficient clearance at the blade tips were not allowed then serious vibrations would be set up in the after end of the ship. The lower part of the stemframe may provide a support for the rudder post or an overhanging section may provide gudgeons for the rudder pintles.
Various sections of the stemframe, particularly abovc the arch, provide connecting points to the individual floors of the after end construction. The transom post and vibration post arc two particular connections. Sound connections at these points ensure that propeller-induced vibrations are kept 10 a minimum. Twinscrew ships have a stemframe which is only required 10 support the rudder pintles and is thus much reduced in size. Larger stemframes, particularly those of cast
construction, are manufactured in two parts with provision made for bolting together and, after careful alignment, welding at the suitably prepared joint.
A-brackets and bossings
Twin-screw vessels with their shafts set away from the centreline require support for the shaft overhang as it leaves the shell. Bossings are oftcn used to increase the vessel's width and allow the shafts to remain within the hull while still retaining a streamlined flow of water to the propellers. The shafting is protected and internal inspection is possible with this arrangement. These bossings are symmetrical about the ship's centreline and give rise to the term 'spectacle frame' because of their appearance from aft of the vessel.
Some modern constructions make use of A-brackets set out from the hull to support the shafts. The final A-bracket in addition to acting as a bearing, must support the weightofthepropeller.
Both bossings and A-frames are led into the stern and solidly built into the structure with additionallocal stiffening where required.
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