Fishing Craft Technology (1+1)

Unit 4 - Boat building materials and construction of boat

4.7. Construction of steel boat
Steel hulls are constructed according to one of three systems viz., Longitudinal, Transverse and Transverse-longitudinal. The longitudinal construction is characterized by member stiffening the plating in the fore and aft direction. In transverse system of construction, the main stiffening of shell and deck plating is arranged in transverse planes. The transverse elements such as floors, frames and beams are connected by brackets. Fishing vessels are constructed mostly by this method. The rigid transverse structure gives a firm and stable basis for shell plating during erection of hull section.

Bottom structure:
The bottom of a steel ship may be constructed as a single or double bottom. The latter is not required vessels over 24 m lengths. The single bottom consists of a centre line girder welded to the keel (generally a bar-keel), side girders, transverse floors (flanged at the top) interconnected by a keelson.

Side shell:
The side shell starts at the upper edge of the bilge strake and ends with the sheer strake. This shell is stiffened with frames connected to the bottom structure and deck beams by brackets.

Deck:
The deck is covered by plates running (like the shell plating) in the longitudinal direction. The deck strake adjoining the sheer strake is called the stringer plate and both strakes form a vital structure from the point of view of longitudinal strength. Deck plating is stiffened by means of deck beams and in way of large openings (engine-room, trunks, and hatch openings) additional longitudinal supported on bulkheads are fitted.

Bulkheads:
Transverse and longitudinal - consists of flat plating with bulkhead stiffeners. The plates are mainly assembled in the transverse direction for transverse bulkheads and in the longitudinal direction for longitudinal bulkheads, as lower strakes of the bulkhead plating have greater thickness than the upper ones (due to the pressures experienced in flooding the compartment).
Keel, stem and stern: The keel of small steel vessels is usually made of a bar connected at the fore end to the stem, which is shaped in accordance with the profile of the vessel. The stern frame is important, as it has to allow the drive shaft and the rudderstock to pass through it and be able to transmit safely the forces due to action of the propeller and the rudder. Its construction may differ depending on the stern arrangement provided in the design.

Deck erections:
Deck erections may be superstructures or deck houses which have lesser breadth than the vessel and take no part in transmitting main stresses of the hull structure. Superstructures are a continuation of the main hull structure.

Engine seating:
The seating for main engine and other machinery must be of sufficient strength to transmit with appropriate safety the forces due to their action such as the propeller thrust, the winch forces at the front power take off, etc. Opening of engine room and hatch- additional longitudinal supports to bulk heads.