§ I. BLOCKS. CHAPTER VI. BLOCKS AND TACKLES. (Plates 31 and 32.) A "Block," in the nautical sense, consists of a frame of wood or metal within which are fitted one or more sheaves or pulleys over which a rope may be led for convenience in applying power. If the block is properly used, it multiplies the power as explained in connection with Tackles. (§ II.) As a rule, blocks are built up of several pieces of wood or metal riveted or bolted together. Sheaves may be of metal or lignum-vitæ. If of the latter, their bearings are boushed with metal. The swallow of the block is the space between the sheave and the frame, through which the rope passes. The side pieces of the frame are the cheeks, and the end of the block opposite the swallow is the breech. A score is cut around the outside of the block to take the strap, which may be of wire rope or of wrought iron or steel. A hook or shackle is usually attached to the strap at one end of the block. The friction of a sheave upon the pin is an important factor in the efficiency of the block, as all power expended in overcoming this friction is wasted. For this reason blocks are often fitted with rollers or balls in the bearings for the pins. The subject of friction will be treated at considerable length under the head of tackles, where it will be shown that a large sheave is essential for efficiency, and that the swallow should be large enough to prevent the rope from touching any part of the block except the sheave. Special types of blocks are made for use with wire-rope. Blocks take their names from the purposes for which they are used, the places which they occupy, or from some peculiarity in their shape or construction. They are further designated as single, double, treble, and four-fold, according to the number of their sheaves. Various types of blocks are illustrated in Plates 31, 32, 33 and 35. Snatch-blocks are single iron-bound blocks, hooking to bolts on the deck to give a fair lead for boat-falls, topsail halliards, etc. The frame and strap are cut and hinged in such a way as to admit of "snatching" the fall, on the bight. Gin-blocks, or gins, are iron pulleys (single) of large diameter, mounted in skeleton frames also of iron. Used chiefly for hoisting cargo, commonly with a wire-rope pendant. Strapping Blocks. Rope is very little used on modern ships for strapping blocks, but wire-rope may sometimes be used with advantage, as being more reliable for a given calculated strength, than the iron work usually fitted. There is an especial advantage in this where very heavy weights are to be dealt with. For such work, a strap fitted with long lashing-eyes is recommended. Plate 35. As to the strength of blocks, see § IV of this chapter. § II. TACKLES. A combination of ropes and blocks for the purpose of multiplying power constitutes a tackle. If we reeve a rope through a fixed block and apply power at one end to lift a weight at the other, we have a "Single whip," which is usually classed as a tackle, but gives no gain of power. If the block instead of being fixed is attached to the weight to be lifted, and one end of the rope made fast while power is applied to the other end, we have a tackle proper in its simplest form. Here (disregarding friction) the power applied to the hauling part is doubled at the movable block because it is transmitted around the sheave and so acts along both parts upon the mass to be moved. In the same way, the tension may be transmitted around any number of sheaves with a gain of power at each sheave of the movable block. It should be noted that the tension is transmitted around the sheave of a fixed block exactly as it is around that of a movable one; but with no other effect in the case of the fixed block than to increase the pull upon the block and its supports. This point is often overlooked, but it is very important. If a weight of 100 lbs. is hanging from a yard-arm by a single whip with both ends made fast to the weight, the pull on the yard-arm is 100 lbs. If now we leave the weight hanging by one part of the whip and man the other part, or hold on to it, or make it fast to some other point than the weight, the pull on the yard is 200 lbs. The same point comes in with all |