Chapter 9- Search and Light Salvage Search and Light Salvage 225 Requirements for Safe Light Salvage • Knot tying and rigging - You must be able to properly tie the following knots when rigging objects to be lifted: Bowline, sheet bend, clove hitch, and a round turn and two half hitches. • Lift bag use - You must be able to estimate the correct size of the lift bag required, take it down, rig it, inflate it using an independent air source, and maintain complete control of the lift as it ascends. • Thinking - You must be able to remain calm, think, analyze, and act to minimize or correct errors. • Training - You must have the proper training for the lift you intend to make. Types of Lift Bags There are a number of types of commercially made lift bags in a variety of styles, sizes, and shapes for specific uses (figure 9-7). For lightweight objects, divers have used a simple inverted goody bag with a trash bag plastic liner. An extra buoyancy compensator works nicely for lifting things like weight belts or small anchors. Steel drums have also been utilized in salvage operations for larger objects. The most popular type of lift bag is the open bottom style. These are shaped like a parachute, with a manual dump valve on the top and an opening on the bottom through which air is added and perhaps vented. They have straps and clips, or shackles, for rigging and are very versatile. They usually roll up into a compact bundle for easy handling underwater. Lift bags are rated by their lifting capacity. The open bottom lift bags come in sizes ranging from 25 to 5,500 kg (50 to 12,000 lbs). Large bags are for use by commercial divers and must obviously be handled with care. Pillow bags are totally enclosed lift bags that have overpressure relief valves to vent expanding air. They are often used for raising sunken boats and for towing, as they can be distributed inside spaces without respect to the orientation of the bag. Once filled, they do not have to be manually dumped on ascent. They come in sizes FIGURE 9-7. LIFT BAG ranging from 100 to 3,000 kg (250 to 6,000 lbs). Lift bags can be in any number of shapes. Ocean salvage tubes are long cylindrical bags used for pipe-laying and other construction projects. They are often used to raise sunken boats, by suspending them underneath the boat, or alongside like pontoons. They are available in sizes up to 1,500 kg (3,000 lbs). Controlling a Lift The amount of air placed and kept in a lift bag must not exceed the weight of an object to be lifted by a great deal, because the air in a lift bag expands during ascent, so lift capacity must be near lift requirement. If a 100 kg (200 lb) lift bag is used to raise a 25 kg (45 lb) object from a depth of 20 m (66 ft), the lift will gain 50 kg (100 lb) of buoyancy during ascent. This extra air, if not vented, will cause the lift to rise rapidly and dangerously out of control. It can come completely out of the water when it reaches the surface, spill its contents and drop the lift back down on the divers below. The first important step in light salvage is to closely match the capacity of the lift bag to the weight of the object to be lifted. A lift bag should be inflated with air from an inde-
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