Structure

Figure 2 shows the configuration (schematic diagram) of a sledge net. A sledge net consists of a bag-shaped mesh mouth fixed to a sledge-shaped frame. The frame that holds the opening of the net is made up of a pair of frames connected by metal rods (beams), and the frames and beams can be disassembled for easy transportation and storage. An inner mesh with a smaller mesh size than the outer mesh is attached at the tail end (cod-end) of the mesh where the collected samples are gathered to prevent the loss of samples collected inside the outer mesh.

The sledge net is towed by cables (bridle wires) connected to the left and right frames. Weights are suspended from the tail of the net to prevent the net from floating up and getting entangled in the sledge (frame) sections and ropes while being lowered toward the seafloor before being towed, or to keep the net in proper shape during towing. A steel chain is connected to the end of the fuse wire to prevent the main wire from rubbing against the seabed and damaging it. The weight of the chain also keeps the frame from lifting during towing.

Fig. 2 Illustration of sledge net structure

重錘: weight ビーム: beams チェーン: chain メインワイヤ: main wire

内網: inner mesh 外網: outer mesh フレーム: frame ブライドルワイヤ: bridle wires

When towing a sledge net, it may get caught on a reef or other obstacle on the seafloor. If the net continues to be towed while caught, not only will the net be wrecked, but in the worst case, the main wire of the winch may break, causing the entire net to be lost.

Therefore, in actual observations, a device is used to avoid the risk as shown in Fig. 3. A fuse wire is attached between the bridle wire and the chain. The fuse wire is a wire rope thinner than the main wire, and it breaks before the main wire when excessive tension is likely to be applied. The net is then towed by the “lifeline” connected to the tail of the net, and at the same time the direction of the sledge changes, releasing the sledge net from the obstacle and allowing it to be retrieved.

If the net contains a larger-than-expected amount of collected material, there is a risk that the fuse wire may be loaded to the point of breaking when the net is pulled on board. To avoid such a risk, a “squeeze rope” is installed in the middle of the net to squeeze the cod-end and lift it up at two places to collect it on board.

Fig.3 Manrope of sledge net

絞り綱: squeeze rope 命綱: lifeline ヒューズワイヤ: fuse wire