CHAPTER II. BLOCKS. BLOCKS are divided broadly into two varieties, wood and iron, the former being used when reeving falls or tackles of Manila, and the latter for wire rope. Wood blocks are either "made" or "mortised," and may have metal or lignum-vitæ sheaves. The space in the block between the wood and the sheave is called the "swallow," the opposite end of the block being named the "" breech," and the sides the "cheeks." The frame of the block may be strapped with iron or rope, a score being cut to form a housing for same. All good blocks should be fitted with patent roller sheaves, especially for halliards and sheets, or for any heavy work. For topsail, sheet, throat and peak halliard purchases, etc., ash blocks, rope stopped, should be used. For derricks on freighters, where wire rope is used for heavy loads, iron blocks are best; where Manila falls and topping lifts are fitted, wood blocks are most suitable. It will be evident that a good deal of power can be wasted by friction of the sheave on pin, and also by the rope chafing, through insufficient "swallow." To minimize the loss due to friction through the former cause, the pins should be bushed. Various bushings are employed for this purpose, probably the most efficient being a gunmetal or bronze sheave with spotted graphite next the pin. The loss due to friction is 10 per cent for each sheave. Blocks are designated "single," "double, or "treble," in accordance with the number of sheaves fitted, and are variously named to denote either a particular shape or as indicating the purpose for which they are intended. Some of the more common bnes are: Snatch Blocks are used to divert the lead on the hauling part of a fall or tackle, having for this purpose a hinged part on one of the cheeks, to permit of placing the rope in, which would otherfvise require reeving a tedious and often impracticable process. They are usually fitted at heels of derricks, and on deck, to take varping and other leads, and are mostly made of iron, the oldaashioned wood snatch block being clumsy and cumbersome. Fiddle Blocks take the name from their resemblance to the nstrument, being constructed with two sheaves placed tandem, to ermit of reeving separate halliards leading in opposite directions. They are to be found on peak-halliards, at preventer stay tackles, etc., and are made in wood where Manila is rove, and in iron for wire rope. Gin Blocks are used on derrick heads and spans in conjunction with a whip for handling cargo, and comprise a skeleton frame and sheave of iron. Cat and Fish Blocks are fitted to the anchor davit, or crane, and consist of a pair of blocks with double or treble sheaves, having a large swallow. The fish (or lower) block has a large hook, sometimes made to trip, for fishing the anchor by the gravity band on the stock. These blocks are made in both wood and iron, the latter being often fitted with Manila falls. Clump Blocks are made short and thick, as their name implies. They are used for tacks and sheets, and for this 'reason are extra large in the swallow. Made in wood and iron. Wrecking Blocks are large, extra heavy iron strapped blocks, with lashing shackles, and are used for rigging up special derricks for temporary use with heavy loads. Cheek-Blocks have only one side, the other cheek being formed by fitting against a spar. The size of a block is designated by the length of the shell, and this is determined from the circumference of the rope which it reeves, as a unit. For most purposes three times the size of rope gives a suitable block, but in a few cases, where the minimum of friction and extra ease is desired in the swallow, as with blocks for boat davit tackles, three and one half times should be taken, e.g., a block for ordinary purposes to reeve three-inch Manila would be 9 inches, but if required for davit falls, the size would be increased to 10 inches. The diameter of sheave is usually about two thirds of the size of block, a 12-inch block having an 8-inch diameter sheave. In ordering blocks it is necessary to prepare a list, giving a concise but full and exact description of each individual block, embracing the following points: Sheaves. -The number of sheaves to be indicated by "S," "D," or "T," and whether of lignum-vitæ, brass, or iron sheaves, bushed or patent roller bushed. Name. The purpose for which the block is intended should be given, as, jib-sheets," "derrick falls," etc. Shackles should be very clearly specified where they are for special fittings. Ordinarily the shackle is fitted with its pin at right angles to the axis of the sheave, this being the most natural way to engage the strap of block, therefore when the word "shackle," without further description, is used, it is always fitted in this manner. Where, however, it is essential to have it with the shackle pin running parallel with sheave pin (as is often necessary to get the falls of a tackle to lead in line with hauling part) the words "reverse shackle "must be used. If the shackle be required with its jaw uppermost, "reverse upset shackle" should be specified. It often happens that a block is required with an eye to engage a shackle, which the blockmaker is not required to furnish. In such cases it is well to state whether the eye should be "worked " or a "shackle-eye" wanted. A "worked eye," of course, is one having its edge worked round like a ring, the “shackle-eye" being drilled straight through, so that the inserted pin bears along its entire length. For a given diameter of pin, that in a shackle-eye would be twice as strong as the one bearing on a worked eye, so that where other considerations do not count, it is economy to fit a shackle eye. Beckets are small eyes fastened at the breech end of blocks to take the thimble on the standing part of a tackle. They are useful to have on all spare tackle blocks. Strops. When blocks are intended for brace or guy pendants, they should be specified as having a score cut to receive the rope strop. 66 Hooks should not be used on blocks where heavy loads are dealt with. For loads under ten tons they are equally reliable with shackles, besides being handier. They should be specified as stiff front," "loose," side," or "swivel" hook, as required, and the working load given in all cases, as many of the hooks on low grade blocks are considerably inferior in strength to the other parts of the fitting. Sister, or Match Hooks are used for a variety of purposes, and consist of two hooks on a common eye, arranged to open, and when closed, to form a seemingly solid eye. Lashing Shackles are especially large in the bow, and wider at the jaws, than ordinary shackles, being fitted to the heavier classes of double and treble blocks, to permit of their taking a Manila or wire rope lashing. Swivel Jaws are sometimes fitted to the upper block in davit tackles. Appended is a table giving actual weights of blocks, fitted with shackles and beckets complete, which will be of use in estimating rigging and outfit weights. 9.84 11.07 12.30 13.78 15.5 17.22 15.75 17.7 19.69 19.55 22.00 24.44 7055 10,582 12,015 13,558 Wt. of gun, tons. 0.66 Wt. of st. proj. in lbs.. S 11.5 2388 2500 0.76 0.86 1.67 1.86 2.11 2.45 2661 2822 2539 2677 2841 2556 2697 2858 |