Block and Tackle
 

  BLOCKS & TACKLE

Block A   block  is a portable pulley, made of wood, metal, or wood and metal. It was originally a block of wood with a hole in it for a rope to reeve through. To save friction the hole was enlarged to take a pulley-wheel or sheave. Then the surplus wood was cut away from the outside of the block, leaving a wooden shell. This was grooved to take a rope strop spliced round the block to secure it in place as required. When the wooden shell split the block was useless. The next improvement was the 'iron-bound block' in which the strop or hook was riveted to an iron case carrying the sheave and its pin, the wooden shell (or cheek) acting as a fairing piece. Today the larger modern blocks are made of metal, and friction is further reduced by roller bearings between the sheave and its pin.

Tackle A  tackle  is a portable purchase consisting of a rope rove through two or more blocks in such a way that any pull applied to its hauling part is multiplied by an amount depending upon the number of sheaves in the blocks and the manner in which the rope is rove through them.
The blocks of a tackle are termed the standing block and moving block, and the rope rove through them is called the fall.
The amount by which the pull on the hauling part is multiplied by the tackle is called its mechanical advantage, and, if friction is disregarded, is equal to the number of parts of the fall at the moving block.
The number of parts at the moving block, and therefore the mechanical advantage, is always greater when the hauling part comes away from the moving block, and such a tackle is said to be rove to advantage. Conversely, a tackle in which the hauling part comes away from the standing block is said to be rove to disadvantage.


Double Blackwall Hitch 53k Blackwall Hitch 66k
single whip  is a fall rove through a single standing block. No mechanical advantage is gained. double whip  is the simplest form of purchase where a fall is rove through two single blocks, standing part being secured to the upper block. (a) and (b) above are two variations. Mechanical advantage is 2.

Blackwall Hitch 66k
Blackwall Hitch 66k
Double Blackwall Hitch 53k
A  jigger  is a small tackle (smaller than a luff) used for general purposes. handy billy  is a small tackle for general purpose
luff  is a purchase consisting of a double and a single block with the standing part made fast to the single block. Mechanical advantage is 4 or 3 depending whether it is rove to advantage or disadvantage.

TERMS

Two Blocks: A tackle is said to be "Two Blocks" when both blocks are hauled up and touching.
Overhaul: A purchase is "overhauled" by easing out the fall and separating the blocks further apart.
  RIGGING: shackles & thimbles   

 

There are various rigging fittings used with STANDING and RUNNING rigging of a ship.

STANDING rigging always stands permanently in place to support the mast and spars of a ship and the wire used for this purpose is of the strongest type: it does not bend easily.

RUNNING rigging includes all ropes employed aloft that work through blocks and are used to shift the position of other gear. For this purpose, Flexible Steel Wire Rope (FSWR) is used because it ismore flexible the Steel Wire Rope (SWR)

Shackles: The type and size of shackle depends on the task it has to fulfill. There is a large variety available, ranging from shacles 25mm in length to shackles capable of lifting many tonnes. They are cast steel or wrought iron and used for all joining purposes.

For tasks such as standing rigging, running rigging and cargo work only tested shackles are used. The safe working load will be stamped into the iron, eg: SWL 6T.

Blackwall Hitch 66k Double Blackwall Hitch 53k Double Blackwall Hitch 53k
Parts of a shackle (straight screw type): This is a general purpose shackle and the end of the bolt should be moused. It is also known as a "D" shackle. Bow Shackle with Forelock: Along with the "D" shackle, this is a general purpose shackle.   Securing to Buoy Shackle: This is used to join the bridle to the buoy shackle.

Blackwall Hitch 66k Double Blackwall Hitch 53k
Lugged Joining Shackle: This is used to join two lengths of cable together. Lugless Joining Shackle:  This is used to join two lengths of ANCHOR cable together.

Thimbles: Thimbles are manufactured from mild steel, iron of gunmetal and are classified according to the size of the rope which they are intended. Thimbles are always used in conjunction with shackles.


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Blackwall Hitch 66k
Blackwall Hitch 66k
Parts of a thimble
1. Round Welded Thimble   2. Round Open Thimble:  Used for Boom Fittings and in awnings. Solid thimble:  Used where a close fitting is necessary eg. round a boat or pin so that there is no movement.