The Shell Plating:
This forms the watertight skin of the ship contributes to the longitudinal strength of the structure and resists vertical shear forces. The bottom and side shell plating consist of several flat and curved steel plates are butt welded together. They are of greater length than breadth.
Insulation:
A steel hull structure is an excellent conductor of heat. Some form of insulation must be provided at the boundaries where there is a requirement to maintain desired temperatures, such as the refrigeration compartments.
Cork, glass fibre, and different kinds of foam plastics in sheet or granulated form may be used for insulating purposes. Air spaces, which are less efficient, may be provided. Glass fibre is widely used in modern ships as it has several advantages over the other materials. It is light in weight, tends to be vermin-proof, does not absorb moisture and is fire-resistant.
Superstructures:
The introduction of aluminium alloy superstructures has provided increased passenger accommodation on the same draft, and/or a lowering of the lightweight centre of gravity with improved stability. These are hence used on passenger ships and cruise ships. It is possible to accept more significant deformation in these superstructures than would be possible with steel. This is brought about by the lighter weight of the aluminium structure.
Watertight Doors:
In some instances, it is necessary to provide access between compartments on either side of a watertight bulkhead. Hence watertight doors are fitted for this purpose. For example, in a cargo ship, direct means of access is required between the engine room and the shaft tunnel. In passenger ships, watertight doors are found where passengers are allowed to pass between one point of the accommodation and another. Mild steel or cast steel watertight doors are fitted below the water line, which prevents flooding of the compartments when shut while providing adequate strength in the case of emergencies.
The Stern Frame:
This structure supports the rudder and the propeller. The stern itself may be cast, forged, or fabricated from steel plate and sections. Modern rudders are also fabricated from steel plates, with plate sides that are stiffened by internal webs. To prevent corrosion, the internal surfaces are suitably coated, and the rudder may be filled with inert plastic foam.
Rudder Pintles:
The rudder pintle is a bolt or pin which is inserted into a gudgeon to attach the rudder to the ship. Older ships may have a brass liner or bronze liner shrunk on the pintles which turn in hardwood (Lignum Vitae) bearings, fitted in the gudgeons. In these days, the industry practice is to use synthetic materials like Tufnol for the bearings, and in some cases stainless steels for the liners. In either, the water which immerses the bearing is used to lubricate it.
Rudder Stock:
The stock may be of cast or forged steel, with its diameter as determined by the torque and any bending moment it is to withstand.
Propellers:
As they have to withstand the corrosive effects of saltwater, ship propellers are constructed from copper alloys such as brass. These are designed to minimize cavitation, which happens when a propeller working under heavy load creates a region of low pressure. Bubbles of water vapour form suddenly and then burst next to the propeller blades, blasting little pits into the surface and wearing it away.
The fitting of zinc plates in the way of bronze propellers and other immersed fittings being used as sacrificial anodes is common practice in shipbuilding. These anodes are metals or alloys attached to the hull, which have more anodic potential than steel when immersed in sea water.
Hence these anodes supply cathodic protection current and get consumed in doing so. Regular maintenance and replacement are hence required in such systems for protection.
Modern anodes are based on alloys of zinc, aluminium, or magnesium which have undergone many tests to examine their suitability; high purity zinc anodes are also used. Sacrificial anodes are fitted with the hull and also often in ballast tanks as well.
Should any part of the anode fall and strike the tank structure where gaseous conditions exist, an explosion could result and hence magnesium anodes are not used in the cargo-ballast tanks of oil carriers owing to spark hazards. Aluminium anode system is employed in tankers, and they are only fitted in locations where the potential energy is less than 28 kg.m.
Paints:
Maintenance of a ship requires that its hull and parts be painted regularly to avoid corrosion and provide resistance to other natural elements. Paints consist of a pigment dispersed in a liquid which is referred to as the ‘vehicle’. It is spread out thinly and overtime the vehicle changes to form an adherent dry film.
Paints which inhibit corrosion of steel have the following vehicle types:
(1) Bitumen or pitch solutions available in naphtha or white spirit solvent.
(2) Oil-based: These consist mainly of vegetable drying oils, such as linseed oil and tung oil. To accelerate the drying by the natural reaction with oxygen, driers are added.
(3) Oleo-resin: In this case, the vehicle consists of natural or artificial resins incorporated into drying oils. Some of these resins may react with the oil to give a faster drying vehicle. Other resins do not react with the oil but heat is applied to dissolve the resin and cause the oil to stick to the body
(4) Alkyd resin: These vehicles provide a shorter drying time and improved film forming properties of drying oils. The name alkyd arises from alcohols and acids, which are the chemical ingredients. These may not be made from oil, as an oil fatty acid or an oil-free acid may be used. Vehicle types (2) and (4) are not suitable for underwater service, and only certain kinds of (c) are ideal for such service.
(5) Chemical-resistant: Vehicles of this type show high resistance to severe conditions of exposure. A number of important vehicle types come under this category which includes: Epoxy resins Chemicals, coal tar/epoxy resin, Chlorinated rubber and isomerized rubber, polyurethane resins, Vinyl resins, and Zinc-rich paints.
(6) Anti-fouling paints: These are a category of underwater hull paints (known as bottom paints) which are specialized coatings applied to slow the growth and/or facilitate detachment of subaquatic organisms that attach to the hull of the ship and affect a vessel’s performance and durability. Copper oxides and biocides are commonly used in anti-fouling paints. Another type of hard bottom paint includes Teflon and silicone coatings, which are too slippery for growth to stick.
