Page 292 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed
P. 292

Chapter 9: Excavation 271
 Figure 9.23 Transportation of Batavia timbers from the Abrolhos Islands by sea to Fremantle. (Courtesy of Patrick Baker, Department of Maritime Archaeology, Western Aus- tralian Maritime Museum.)
Before undertaking any heavy lifting, the vessel should be surveyed by a qualified marine surveyor to determine the stability parameters for safe working of heavy lifts. When using wire and shackles for lifting, it is essen- tial to provide a method of ditching the lift in an emergency. It is almost impossible to undo a shackle while it is under tension. A simple solution is to include a rope loop between the object and the shackle. This can be made up of several loops of light rope so that it is easy to cut through one strand and free the object in an emergency.
If resources are limited, some quite simple and ingenious systems of lifts can be devised. One particular system utilizes an inflatable boat with a strong board mounted amidships across the pontoons. A simple, hand-oper- ated trailer winch is attached to the center of the board; the wire is run over a roller at the end of one side of the board, down to a snatch block, and then back up and attached to the other side of the board. This forms a pulley system with a mechanical advantage and, as the weight is well below the center of gravity of the craft, gives a stable situation. Using such a system, an object can be raised under the boat and the boat brought into shallow water under power.
Similarly, a vertical hollow tube can be welded inside the hull of the boat, amidships, near the keel. The hull is cut away from around the inside of the tube and, provided the tube rises above the water line, the boat will not sink. A tripod is mounted over the tube, from which an endless chain is sup-






























































































   290   291   292   293   294