Page 76 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 3: Search and Survey 55 C. CIRCULAR SEARCH
The circular search is well known to recreational divers as a simple and effective method of searching. For archaeological work it is probably only really useful in low visibility where a diver can conduct reasonably efficient searches around a marked point or where a quick identification of a known site is required. The real problem exists where one wishes to be systematic, because several overlapping circular searches are quite inefficient. As a technique it is very useful where one has low visibility and often two, or possibly three, divers can operate on a circular search, either feeling for sites or objects, or with limited visibility, maintaining visual contact with the adjacent diver. Obviously, lots of planning is required and underwater communications will increase the efficiency of this type of search system enormously.
D. TOWED SEARCH
The swimline requires a great deal of organization and preparation and proceeds slowly. An alternative to this is the towed search whereby one or two divers are towed on a board or sled behind a boat at a speed comfort- able for the diver (about 2–3 knots). By means of diver-adjustable depres- sors and controls on the sled, the diver is able to adjust the height so that the seabed is kept in view. In this way, considerable distances can be sur- veyed quite quickly and the only real concerns are the diver’s ability to withstand the cold, as almost no energy is expended sitting on the sled; the ability to keep the seabed in view over undulating topography; and to main- tain a reasonable speed for the comfort of the diver. The sleds and boards vary in complexity, but it is advisable to avoid the so-called manta boards unless the diver has some form of support. The most comfortable system is one where the diver lies supported in a relaxed prone position on a form of saddle. If depressors are used they should be balanced so that strong depression or (more important) elevation can be achieved with only a small amount of effort. The sled should be arranged so that in a free condition it automatically rises and, for obvious reasons, the diver must be able to abandon the sled easily. Voice communication can be an advantage, but again it should be easy to ditch. The depth that the sled can reach will depend on its drag, which is a function of the speed of the tow and the length and the diameter of the tow rope. The cable equations (see Green, 1970) are quite complex, but in general it is preferable to keep the cable diameter as small as possible, because, for a given velocity, it has a signifi- cant effect on the drag. For this reason it is worth considering using wire,