Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1900 Excerpt: ...x No. of parts at the moveable block..'. Power gained by a purchase = No. of parts at the moveable block. One quarter of this should be subtracted to make allowance for friction, and if the purchase is 3-fold, one third should be subtracted for friction. To find what weight a rope will lift when rove as a purchase. Multiply the No. of parts at the moveable block by the weight the rope can lift, and subtract one quarter of this for friction, or if the purchase is 3-fold, subtract one third for friction. To find what sized rope, you will have to reeve as a tackle to lift a certain weight. Divide the weight by the number of parts at the moveable block, this gives the strain on a single part, add one quarter, or one third if a 3-fold purchase, for friction, and reeve your rope of a size giving a good margin of safety. When two or more purchases are put on one another the total power gained is found by multiplying the power gained by the different purchases together. When using a purchase besides gaining power, i.e., having to use fewer men jerking strains are greatly reduced and also you lose very little of the rope you have been heaving on when you turn up with the purchase. The following is a rough list of the uses of the more important purchases. Single whip, used for light work when one only wishes to alter the direction of the pull. Double vjhip, used for clearing a boat where more power is required. Runner, used with a tackle for staying lower masts, with a top burton for setting up topmast rigging. Luff tackle, used where a heavy pull is required. Up and down tackle, used for setting up lower rigging Gun tackle, formerly used as side tackles for guns. Quarter tackle, used for hoisting in stores,-fee. Sail tackle, used for staying topmasts, shifting or be...
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