TACKLE

Tackles are used in a ship to raise, remove, or secure weighty bodies; to support the masts; or to extend the sails and rigging. They are either move-able, as communicating with a runner; or fixed, as being hooked in an immoveable station; and they are more or less complicated, in proportion to the effects which they are intended to produce.

Plate XI

If a b d e, fig. 3. plate XI. be a single block, upon which are suspended the weights f g, then since the nearest distance of the ropes f g, from the center of motion c, and a c equal to d c, the block will be reduced to the lever or balance a d with respect to its power: Since a c is then equal to d c, it is apparent that f g will always be in equilibrium. As no advantage therefore can be acquired, in raising a weight by an immoveable single block, it is only rendered useful by changing the direction of the moving power. This circumstance is extremely convenient to the labourers, and often absolutely necessary; particularly in raising bodies to a higher station; as from the hold to the upper decks, or from the deck to the masts or yards, &c. which would otherwise be difficult or irnpracticable to perform. See also the articles BLOCK and WHIP.

When a single block is moveable along with the body to which it is attached, fig. 4. plate XI. as the blocks of the brace-pendents, reef-tackle, pendents, jigers, &c. the momentum of the power is doubled; because it moves twice as fast as the weight, or body to which it is attached. For in the same time that any part of the ropef, moves upward from f to g equal in length to the two equal ropes d and c, the block, and consequently the weight annexed, will be drawn through the space e h, whole length is equal to one of the ropes only.

When a tackle consists of two or more fixed and moveable blocks, wherein one rope communicates with the whole; if one end of the rope be fixed, as in fig. 5. 6. and 7. in order to proportion the weight to the resistance, the power applied must be to the weight, as one, to twice the number of sheaves in the moveable blocks: because, in the efforts of a tackle, the velocity of the moving power is, to the velocity of the rising or moving body, as twice the number of moveable sheaves to unity, as appears in fig. 5. which consists of one fixed block a, and another moveable as e. For since one rope operates on all the sheaves from g to f, the part at f, lying beyond the fixed block, and called the fall, cannot be drawn down and lengthened, unless the two parts d and c, on each side of the moveable block, be at the same time equally drawn up and shortened. Hence it is evident, that the part a f will be lengthened twice as much as either d or c is shortened, because whatever is taken from each of those parts is added to the length of a f; but the point f, to which the power is applied, descends as fast as a f is lengthened; and the point e, to which the weight is fastened, ascends as fast as d or c is shortened. If therefore, a weight suspended at f, be to a weight suspended at e, as one to two, they will balance each other, as being in the reciprocal ratio of their velocities.

Whatever has been observed with regard to the tackles above mentioned, is equally applicable to all others, and is in the same manner demonstrable, viz. that the velocity with which the mechanical force moves, in raising a weight, is to the velocity where with the weight rises, as twice the number of moveable sheaves to unity.

A tackle wherein both the blocks are moveable, and communicate with a runner, is represented by fig. 10. plate VIII. That part of the tackle which is fixed to one of the blocks, &c. is called the standing part; all the rest are called running parts; and that whereon the men pull when employing the tackle, is called the fall. The application of the tackle to mechanical purposes is termed hoisting or howling. See those articles.

Reference Works