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 water. | Speed of ship while shooting dredge or trawl. | Length of rope required. | Angle of dredge rope while lowering trawl. | Angle of dredge rope while 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.
For further details of deep-sea dredging, especially of the hauling machinery and management of the gear, the special reports of the various expeditions must be consulted. Commander Tanner, U.S.N., has given in Deep Sea Exploration (1897) a very full and good account of the equipment of an exploring ship; and to this book the present article is much indebted.
| Fig. 15.—Deep-sea Dredge, with Tangle Bar. |
Modifications and Additions to the Dredge.—From 1818, when Sir John Ross brought up a fine Astrophyton from over 800 fathoms on a sounding line in Baffin’s Bay, instances gradually accumulated of specimens being obtained from great depths without nets or traps. The naturalists of the “Porcupine” and other expeditions found that echinoderms, corals and sponges were often carried up adhering to the outer surface of the dredge and the last few fathoms of dredge rope. In order to increase the effectiveness of this method of capture a bar was fastened to the bottom of the dredge, to which bunches of teased-out hemp were tied. In this way specimens of the greatest interest, and frequently of equal importance with those in the dredge bag, were obtained. The tangle bar was at first attached to the back of the net. From the “Challenger” expedition onward it has been fixed behind the net by iron bars stretching back from the short sides of the dredge frame which pass through eyes in their first ends (fig. 15). The swabs are thus unable to fold over the mouth of the dredge. Rope lashings to the lips of the dredge are sometimes added, and a weight is tied to the larger bar to keep it down.
Occasionally the tangle bar is used alone (Agassiz), and one form (Tanner) has two bars, stretching back like the side strokes of the letter A from a strong steel spring in the form of an almost complete circle. The whole is pulled forward from a spherical sinker fastened in front of the spring apex; and should the apex enter a crevice between rock masses, the side bars are closed by the pressure instead of catching and bringing up. This is said to be a very useful instrument among corals.
The Blake Dredge.—In the soft ooze which forms the bottom of deep seas the common dredge sinks and digs much too deeply for its ordinary purpose, owing partly to its chief weight bearing on the frame only, partly to its everted lips. To obviate these defects Lieutenant Commander Sigsbee of the “Blake” devised the Blake dredge. Its novel features were the frame and lips. The former was in the form of a skeleton box; that is, a rectangle of iron bars was placed at the back as well as the front or mouth of the net and four more iron bars connected the two rectangles. The lips instead of being everted were in parallel planes—those, namely, of the top and bottom of the net. The effect of this was to minimize digging and somewhat spread the incidences of the weight. Another advantage was that the net being constantly distended by its frame, and, moreover, protected top and bottom by an external shield of canvas, quite delicate specimens reached the surface uninjured. The dredge weighed 80 ℔ and was 4 ft. square and 9 in. deep.
Rake Dredges.—These are devices for collecting burrowing creatures without filling the dredge with the soil in which they live. Holt used, at Plymouth, a dredge whose side bars and lower lip were of iron, the latter armed with forward and downward pointing teeth which stirred up the sand and its denizens in front of the dredge mouth. The upper lip of the dredge was replaced by a bar of wood. The bag was of cheese-cloth or light open canvas, and the whole was of light construction. The apparatus was very useful in capturing small burrowing crustacea. The Chester rake dredge is a Blake dredge in front of which is secured a heavy iron rectangle with teeth placed almost at right angles to its long sides and in the plane of the rectangle. Each of these instruments has a width along the scraping edge of about 3 ft.