Engine Lift Through a Deck Hatch: Rig Mechanical Advantage and an Off-Center Load
A working party must lift a 900-pound diesel engine out of an engine room and through a deck hatch. The rig on hand is a double purchase: a fixed block with two sheaves and a moving block with two sheaves, giving four parts of the fall, with the hauling part led through a fairlead to an electric winch on deck rated for 500 pounds of pull. Experience with this rig shows it delivers about 75 percent of its theoretical mechanical advantage because of sheave friction and line stiffness. The hatch is 6 feet above the engine's lifting eyes. The single lifting eye is not over the engine's center of gravity, which sits about 6 inches aft of the eye and about 18 inches above the base pan. The deck pad eye that anchors the fixed block carries a working load limit of 1,000 pounds, and the rigger has a short length of chain, two tag lines, and a spare snatch block available.
1. What is the ideal mechanical advantage of this rig, what pull will the winch actually have to make, and is the winch adequate?
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Ideal mechanical advantage. Count the parts of the fall supporting the moving block: four. Ideal MA = 4, so the ideal pull is 900 / 4 = 225 pounds.
Real pull. Friction and line stiffness cost about 25 percent, so the rig delivers 0.75 x 4 = 3 effective parts. Pull = 900 / 3 = 300 pounds.
Winch check. 300 of 500 pounds is 60 percent of rating, which is acceptable but not generous. The winch reaches its rating at a load of 500 x 3 = 1,500 pounds, and shock loading from a snatch or a hung load can briefly exceed the static figure. The pad eye is the tighter item: it carries essentially the engine's weight plus rigging, roughly 900 to 950 pounds against a 1,000-pound limit. Hoist slowly and keep hands clear; do not jerk the load.
2. How much line must pass over the winch to raise the engine 6 feet, and what changes if the rig is re-reeved for six parts of fall?
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Four parts. The line hauled equals parts of fall times distance raised: 4 x 6 = 24 feet, plus tail for the winch drum and handling. The fairlead changes direction but not the amount of line.
Six parts. 6 x 6 = 36 feet of line, and the pull drops to 900 / (6 x 0.75) = 200 pounds.
What changes is where the limit lies. With six parts the winch is comfortable, but the fixed block and pad eye still carry the same load, and 36 feet of line means a longer haul and more chance of a jam at the fairlead. Re-reeving helps the winch; it does not raise the pad eye's working load limit.
3. What will happen the moment the engine comes off the deck, and how should the rigger use the chain, tag lines, and snatch block to control it?
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A freely suspended load rotates until its center of gravity is directly under the hook. Because the center of gravity is 6 inches aft of the eye, the aft end swings down and the eye is loaded off-axis, putting a side load and some bending on the eye and the block. At the hatch the low aft end catches the coaming, and the winch keeps pulling - the classic recipe for a shock load on a pad eye that has only about 50 to 100 pounds of margin.
Controls available: attach two tag lines before lifting and hold the engine from swinging; use the chain to fashion an adjustable two-leg sling so the hook sits nearer the center of gravity; use the snatch block to reposition the lead so the fall pulls vertically over the hatch. Raise it a few inches, stop, let it settle and re-trim, then hoist. The permanent fix is a lifting beam or a second lifting point over the center of gravity.