by Allen Mordica, TMLHA
One of the three main aspects of navigation is dead reckoning. Simply put, DR is an estimate of your position, based on course, speed and time from a known, observed point. For example, if you started from a point on a flat open plain, in an automobile with the windows painted over and with a compass on the dashboard, and drove 1 mile north, 1 mile east, 1 mile south, and finally 1 mile west (provided you didn't collide with something along the way), then you should end up right where you started from, whether you could see outside the vehicle or not. If you had no odometer, to measure the distances you would need to know your speed and the elapsed time at that speed in order to determine the distance traveled. In order to determine a ship's DR position, one must be able to observe the ship's course and speed, updated frequently since the last good celestial or visual fix.
In the most ancient times, speed at sea was measured by dropping a piece of driftwood or a small log off of the stern of the moving ship. As the ship moved away from the wood, an approximate speed could be guessed. Of course, one could only do this so many times before exhausting the supply of wood aboard. This was remedied by attaching a length of light twine or line to the log; the same log could then be retrieved and used repeatedly. Marks were added to the line to allow for a more accurate speed reading.
The final refinement of the concept, before propeller-driven logs were invented, consisted of:
-a large reel, with free-turning handles at either end of the reel, capable of holding...
-500' of light line, marked at 33'4" intervals, with...
-6" lengths light white cotton twine,...
-a flat wooden drag or drogue "log"...
-and a small hourglass, with enough sand to measure 28 seconds. The reason for this amount of time will be explained below.
This setup will allow a measurement of up to 15 knots; few sail-powered ships could travel at a higher sustained speed.
A. The drogue is constructed from a 1"x12"x12" board; the type of wood is less important than the shape. A quarter-circle of 12" diameter, measured from one corner, is scribed and cut. From the resulting flat, wedge-shaped piece, 3/8" diameter holes are drilled near each corner. Along the curved edge, about 1/2" in from the edge, 5-7 equally-spaced 7/8"-1" diameter, 5/8" deep holes drilled almost through (see fig. 1).
Melted lead is poured into the equally-spaced holes. The lead ballast weights make the drouge float upright, nearly submerged and point upward, to provide maximum drag.
B. At one end of the 500' line, unlay about 12-14" of the strands and securely seize the line where the strands part company. Take two of the strands and thread them through two of the holes at the corners of the drogue; tie a figure-of-eight or overhand knot in the bitter end of each. From a piece of scrap pine, carve a peg with a 3/8" taper at one end, and a large enough flat area on the other end to drill a 3/8" hole through. Thread the third strand through the hole in the peg, bring the end back to itself and and securely seize an eye, trapping the peg in the eye. Press the peg firmly into the remaining hole in the drogue. When completed, the drogue should lie perpendicular to the axis of the line.
C. Measure 47' 3" feet from the drogue, and force open 2" of the strands
with a marlinespike or small fid. For the first "tag", tie one
figure-of-eight knot at one end of the tag, and weave the other end into the
line, leaving the last 2" of the tag exposed.
-Measure another 47' 3" feet, unlay the line, and insert another tag, but this tag will have two knots tied 1/2" apart, at the exposed end of the tag.
-Continue this process, adding one more knot in each succeeding tag, until the entire line is so marked.
D. The exact construction of the reel is not important, only that the
following features are adhered to;
- that the reel be long and of small diameter, to allow the reel to be held comfortably aloft over the head without the reel touching the head, and
- that the handles, or the axle of the reel be free-spinning, to allow the line to pay out without friction.
E. The glass is made from an egg timer. One end of the glass is carefully opened with a Dremel tool with a dental burr or drill. This is extremely difficult, as too much pressure will break the glass. After opening the glass, get a stopwatch and a piece of clean paper. Being careful to coordinate the watch and glass, invert the timer and pour out 2:32 worth of sand on to the paper. At the 2:32 point, turn the glass horizontal. Reset the stopwatch, and turn the glass back upright as you start the stopwatch again, letting the sand return to the "bottom" half of the glass. Stop the watch as the sand finishes flowing; you should have measured 28 seconds. If the time is too long, pour off a small amount of sand, if it is too short, add sand back into the glass.
To use the log requires two men; one to hold the reel aloft, facing aft at the taffrail, and one to drop the drogue overboard, turn the glass, and stop the line when the sand runs out. The actual time for the reading is 30 seconds; the time required to drop the drogue overboard and turn the glass accounts for about two seconds; this is the reason for the odd amount of sand in the glass. After stopping the line, the number of knots payed out is the speed of the ship; it's as simple as that.
To retrieve the drogue, rather that reeling it in against the dead drag of the upright board, just give the line a quick, very sharp tug, and the peg, snugly fitted into the drogue, will pop out, letting the board flip parallel to the flow of the water and allowing for easy retrieval as it skips across the surface of the sea.