Beyond the South End

The Traverse Board

This interesting tidbit of historical navigation devices was included in the latest newsletter from the Sail, Power and Steam Museum, “Reading Both Pages.”

The traverse board is a memory aid formerly used in dead reckoning navigation to easily record the speeds and directions sailed during a watch. Even an illiterate crew member could reckon with this simple device.

The traverse board is a wooden board with peg-holes and attached pegs. It is divided into two parts. The top part is for recording direction sailed. It has a compass rose with its 32 compass points, just as it would on the face of the ship’s compass. Eight concentric rings are inscribed on the compass rose, each with one peg hole at each point of the compass. Eight pegs are attached to the center of the compass rose with strings.

The bottom part is for recording speed. It has 4 rows of holes. Each column represents a certain speed, measured in knots. Eight pegs are attached to this part of the board.

Each half-hour during the watch, a crew member would insert a peg in the top portion of the board to represent the heading sailed during that half-hour, as is shown on the ship’s compass. The innermost ring of peg-holes is used for the first half hour, the second for the second half-hour, and so on until all 8 rings were use.

Each hour during the watch, a crew member would insert a peg in the bottom portion of the board to represent the speed sailed during the hour. The speed would have been measured using a chip log. If the speed for the first hour of the watch were 101⁄2 knots, the crew member would count over 10 holes in the first row and place one peg, then place another peg in the column marked “1/2”. In the second hour of the watch, the crew member would use the second row of pegs, and so on until all 4 rows were used.

At the end of the watch, the navigator would collect the information about the speeds and directions sailed during the watch, clear the pegs from the board, and use the information to figure the vessel’s dead reckoning track.

Meanwhile, the helm of the new watch would begin recording the new sailing headings and speeds on the traverse board. Remember, dead reckoning was the only method the navigator had to determine longitude and the calculations were fraught with errors until the invention of the Harrison chronometer (1737).