The dredge is lowered from the derrick boom, which has been previously trained over to windward so that its end is well clear of the ship, while the ship is slowly moving forward. The rope is checked until the net is seen to be towing clear, and then lowered rapidly. Where a weight is used in front of the trawl Captain Calver successfully adopted the plan of backing after sufficient line had been paid out: the part of the rope from weight to surface thus became more vertical, while the shorter remainder, previously in line with it, sank to the bottom without change of relative position of weight and dredge. The ship was then ready for towing. When no front weight is used the manoeuvre is unnecessary. There should be a relation maintained between speed of vessel onward and of rope downward, or a foul haul may result owing to the gear capsizing (in the case of a trawl), or getting the net over the mouth (in a dredge). The most satisfactory method of ensuring this relation seems to be so to manage the two speeds that the angle made by the dredge rope is fairly constant. This angle can be observed with a simple clinometer. The following table abridged from Tanner most usefully brings together the requisite angles with other useful quantities. +----------+----------------+---------+---------------+---------------+ | Depth of | Speed of ship |Length of|Angle of dredge|Angle of dredge| | water. | while shooting | rope | rope while | rope while | | |dredge or trawl.|required.|lowering trawl.|dragging trawl.| +----------+----------------+---------+---------------+---------------+ | Fathoms. | Knots. | Fathoms.| | | | 100 | 3 | 200 | 60 | 55 | | 200 | 3 | 400 | 60 | 55 | | 400 | 3 | 700 | 60 | 52 | | 600 | 2¾ | 1000 | 55 | 50 | | 800 | 2½ | 1200 | 50 | 44 | | 1000 | 2½ | 1500 | 50 | 40 | | 1500 | 2¼ | 2166 | 50 | 40 | | 2000 | 2 | 2670 | 45 | 35 | | 3000 | 2 | 4000 | 40 | 35 | +----------+----------------+---------+---------------+---------------+ The speed of towing, always slow, may be assumed to be approximately correct if the appropriate angle is maintained. Hauling should at first be slow from great depths, but may increase in speed as the gear rises.