A block is a portable pulley, made of wood, metal, synthetic-resin bonded fibre (SRBF) or combinations of materials.
General Information
Blocks can be fitted with a hook as an alternative to an eye, andthe hook or eye can be fixed or swivel types. They may have more than one sheave; a single block has one sheave, a double block has two and a triple has three.
Wooden B locks are classified by their size, which is the length from crown to tail measured in millimetres round the shell; and it will take a rope one-tenth of its size. (Rope measured by its diameter in millimetres).
Metal Blocks are classified by the size of rope for which they are designed and this is normally marked on a plate affixed to the cheek.
SRBF Blocks are also classified by the size of rope for which they are designed and this is stamped on the binding of the block.
Study the part of blocks as shown in Figures 
Types of Block:
Internal-bound Block (IB)
This type has a shell partly of wood and partly of metal, and is a version of a wooden block. The metal portion consists of a fork shaped steel fitting called `the binding’ which incorporates both the eye or hook and becket when fitted. The sheave is made of phosphor bronze which is less corrosive and does not create sparks as the pin is made of steel. This type of block can be used for rope or wire. A tally plate is found only on this type of block; it has two practical purposes, to hold in the pin and to supply information about the block. This information will contain the Pattern number (a computerised number to identify the block), the size of the rope for its use and the safe working loads for use. In addition to allow for lost or damaged plates, information about the block will also be stamped on the hook or eye. This will contain the pattern number, the size of the rope to be used, the tested weight (which will be higher than the safe working load) and the test date (month and year). All tests are normally carried out in HM Dockyard Test Centres.Very little maintenance is required for these blocks. Even if the wood is split or chipped it can be repaired by using a wood filler or synthetic resin paste. The swivel hook or eye, whichever is fitted, should be kept free of dirt and given a light oiling to ensure it swivels freely. As most of the metal parts are galvanised only the pin requires a light coating of grease to ensure free running of the sheave. The sheave should be checked for wear periodically, any rough edges should be filed down to prevent damage to rope.
Metal Block
Usually built of steel plates and fittings, their shells have a binding which supplies the strength but the cheeks are of light plating. Some types have their shells cast in one piece. Where possible metal blocks should be used when using wire rope. (Built-up metal blocks which were the main type of block used by the RN for awning tackles make ideal heel tackles for rigging evolutions).A light application of grease should be used on the moving parts. Steel blocks showing signs of rusting should be wire scrubbed, treated with a rust inhibitor and lightly greased. A coating of linseed oil will prevent rust on the shell. Galvanised metal blocks generally require less maintenance, a light oiling normally only required. The pin which is of steel should be given a light coat of grease or oil. Wire brushes should not be used on galvanised metal blocks as this may damage or remove the coating, instead a nylon pad may be used to remove any corrosion.
Synthetic-resin Bonded Fibre (SRBF) Block
This block is built up of steel bindings, and its means of attachment and sheave pins are of steel. The cheek plates and sheave(s) are made of synthetic-resin bonded fibre. These blocks are for use with natural fibre and man-made fibre ropes only and can be single, double, triple or snatch blocks with safe working loads of one, two or four tonnes. They must not be used with wire rope. (This type of block is currently on issue to units. Because of the block’s size it is not often practical to use these as heel tackles, although they make ideal purchases).
Snatch Block
These are single metal, internal bound or SRBF blocks, in which part of the shell is hinged to form a `gate’ which allows a bight of a rope to be inserted into the swallow from one side. They should not be used when a solid block is available for the job and they should NEVER be used when the safety of life depends on them because the gate may open if a sideways pull is exerted. Ideally these blocks should be used for lead blocks during rigging evolutions.The maintenance required depends on the type of block, additionally the gate will require a light greasing or oiling. Avoid applying too much grease to pin springs as this generally attracts dirt and more frequent checks and maintenance may be required.
