Figure 2-8.-Some other tackles.Many combinations of single-, double-, and triple-sheave blocks are possible. Two of these combinationsare shown in figure 2-8.You can secure the dead end of the fall to themovable block. The advantage is increased by 1. Noticethat this is done in figure 2-7. That is a good point toremember. Remember, also, that the strength of yourfall—rope—is a limiting factor in any tackle. Be sureyour fall will carry the load. There is no point in rigginga 6-fold purchase that carries a 5-ton load with two tripleblocks on a 3-inch manila rope attached to a winch. Thewinch could take it, but the rope couldn’t.Now for a review of the points you have learnedabout blocks, and then to some practical applicationsaboard ship:With a single fixed block the only advantage is thechange of direction of the pull. The mechanicaladvantage is still 1.A single movable block gives a mechanicaladvantage of 2.Figure 2-9.-A yard and stay tackle.Many combinations of single, double, and tripleblocks can be rigged to give greater advantages.Remember that the number of parts of the fall goingto and from the movable block tells you the approximatemechanical advantage of the tackle.If you fix the dead end of the fall to the movableblock you increase the mechanical advantage by one 1.APPLICATIONS AFLOAT AND ASHOREWe use blocks and tackle for lifting and moving jobsafloat and ashore. The five or six basic combinations areused over and over in many situations. Cargo is loadedaboard, and depth charges are stored in their racks. Youlower lifeboats over the side with this machine. We canswing heavy machinery, guns, and gun mounts intoposition with blocks and tackle. In a thousand situations,sailors find this machine useful and efficient.We use yard and stay tackles aboard ship to pick upa load from the hold and swing it onto the deck. We useyard and stay tackles to shift any load a short distance.Figure 2-9 shows you how to pick a load by the yardtackle. The stay tackle is left slack. After raising the loadto the height necessary to clear obstructions, you takeup on the stay tackle and ease off on the yard fall. Aglance at the rig tells you that the mechanical advantageof each of these tackles is only 2. You may think it’s hardwork to rig a yard and stay tackle when the smalladvantage is to move a 400-pound crate along the deck.However, a few minutes spent in rigging may save manyunpleasant hours with a sprained back.If you want a high mechanical advantage, a luffupon luff is a good rig for you. You can raise heavy loadswith this setup. Figure 2-10 shows you what a luff upon2-4
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