§ I. CHAPTER VII. HANDLING HEAVY WEIGHTS. Where masts and yards are available, as in sailing ships and steamers with auxiliary sail power, the problem of handling weights presents no great difficulty. The lower yards are used as derricks, with suitable support from the mast by lifts and tackles. The principles which govern the rigging of these are identical with those discussed theoretically in Chapter V, and practically in §2 of the present Chapter. Unfortunately no such facilities as are here assumed exist on modern ships. Yards are non-existent and masts are not always so placed as to be available. When they are available, they are in some ways more useful than the masts of sailing ships, because so much stiffer. A span can often be used between the masts to furnish a point for hooking a tackle just where it is wanted (Plate 29); and the place of the old fashioned yard, for carrying a tackle that shall hang clear of the ship's side, may in many cases be supplied by a cargo boom or a boat-crane. In spite of the fact that masts and yards of the type shown on Plate 37 are not found on modern ships, the plate has been retained because it illustrates principles which are as significant to-day as they were half a century ago and shows them more clearly than they could be shown by almost any other sketch that could be devised. Every tackle and every spar shown in the plate represent a force either of action or of re-action. The downward pull of the weight W is met by the upward reaction of the tackles, a, from the mast head, assisted by the spar, S, whose heel rests upon the deck, supported by shores from the decks below. The tendency of the spar to buckle is met by a brace B from its midship point to the mast. The inward thrust of the yard resulting from the horizontal component of the forces represented by W and a is met by the rolling tackles cc, the lower one of these contributing at the same time, through the leverage of the yard, to the support of W. The inward thrust of the yard is of course still further met by the cross-lashing which binds the yard to the mast. Of especial significance is the hawser OLD-FASHIONED MAST AND YARD RIGGED FOR HANDLING Note. The material here shown is obsolete, but the principles involved are clearly indicated and of universal application. § I. CHAPTER VII. HANDLING HEAVY WEIGHTS. Where masts and yards are available, as in sailing ships and steamers with auxiliary sail power, the problem of handling weights presents no great difficulty. The lower yards are used as derricks, with suitable support from the mast by lifts and tackles. The principles which govern the rigging of these are identical with those discussed theoretically in Chapter V, and practically in §2 of the present Chapter. Unfortunately no such facilities as are here assumed exist on modern ships. Yards are non-existent and masts are not always so placed as to be available. When they are available, they are in some ways more useful than the masts of sailing ships, because so much stiffer. A span can often be used between the masts to furnish a point for hooking a tackle just where it is wanted (Plate 29); and the place of the old fashioned yard, for carrying a tackle that shall hang clear of the ship's side, may in many cases be supplied by a cargo boom or a boat-crane. In spite of the fact that masts and yards of the type shown on Plate 37 are not found on modern ships, the plate has been retained because it illustrates principles which are as significant to-day as they were half a century ago and shows them more clearly than they could be shown by almost any other sketch that could be devised. Every tackle and every spar shown in the plate represent a force either of action or of re-action. The downward pull of the weight W is met by the upward reaction of the tackles, a, from the mast head, assisted by the spar, S, whose heel rests upon the deck, supported by shores from the decks below. The tendency of the spar to buckle is met by a brace B from its midship point to the mast. The inward thrust of the yard resulting from the horizontal component of the forces represented by W and a is met by the rolling tackles cc, the lower one of these contributing at the same time, through the leverage of the yard, to the support of W. The inward thrust of the yard is of course still further met by the cross-lashing which binds the yard to the mast. Of especial significance is the hawser on the off-side, leading from the topmast-head to the side of the ship and in through a port to the mast between decks, and acting against the tendency of w and a to pull the mast to their side. The shores which brace the bulwarks against the thrust of this hawser are also to be noted. An application to this case of the rules and examples given in § II of this Chapter will be found extremely instructive; and while the materials here shown may never be duplicated in practical experience, the principles (or some of them) will be duplicated in every problem that presents itself where heavy weights are to be handled. Parbuckling (Plate 38). Where the object to be handled is of such a nature as to admit of parbuckling, this is a simple and convenient method to use, especially on a ship of low free-board. This is a characteristically naval method, and comes in conveniently in any case where an object of generally cylindrical shape is to be hoisted or dragged. A typical example is that of landing guns on a shelving beach, the guns being carried into shallow water by boats and rolled overboard onto hawsers which have been laid off at right angles to the beach, the ends of the hawsers being then brought ashore, over the gun, leaving the gun in the bight. The operation will be much simplified if the chase of the gun is built up by some convenient method to the diameter of the breech. Cargo Booms (Plate 39) are secured to a mast by heavy pivotbolts and supported by topping lifts and guys which admit of plumbing any point within a considerable range on deck and alongside. There is no difference in principle between such a boom and a yard rigged and used as in Plate 38, except that the yard has the characteristics of a long boom used nearly level. The cargo booms of merchant vessels and the boat-booms or cranes of men-of-war, are usually heavy enough for weights up to three or four tons, while the cranes of a Dreadnought are designed to handle boats weighing as much as 25 tons. The range of power of such a boom may be considerably extended by the use of a spar as in Plate 39 to take the direct downward thrust, the deck under the shoe of the spar being well shored up. This takes a large part of the tension from the topping-lift. For very heavy weights, it is well to block up under the pivot-bolt, or to unkey the boom and step the heel upon |