(Updated May 2017)
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 total 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 periodically 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 bow of the moving ship. As the ship moved along, the time required for the ship to pass by the "chip" would be measured, and 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 retreived and used repeatedly. Marks were added to the line to allow for a more accurate speed reading.
2. Units of Measure-
- A Nautical Mile is defined NOT as one minute (1/60th of a Degree) of an arc, but is defined internationally as 1852 meters, about 6,076 feet, or 1.1508 statute miles. Historically, it was defined as one minute of latitude, which is equivalent to one sixtieth of a degree of latitude exactly.
-The "Sea Mile" defined as 6000 feet is an archaic unit of distance quoted by Richard Norwood in his extraordinarily popular book, "Seaman's Practice" (1637). He calculated that a degree of latitude was 367,196 English feet, but was rounded down to 360,000 feet, or 6000 feet/arc-minute. Commentators until the mid 19th century noted that 6000 feet was a rounded value. I use this unit in my construction because it was in common use through the age of sail; because using this results in an even measurement of 50ft between "knots"; and all calculations based on this unit come out to even numbers (probably why this was done in the first place).
The final refinement of the concept, before propellor-driven logs were invented, consisted of:
-A large reel, with free-turning handles at either end of the reel (or free-turning axle), capable of holding...
-A bridle of 20 to 30 feet, with an attachment point at the end, to allow the drogue to enter and stabilize in the water before the measurement proper begins (I use a toggle and spliced eye)... -750' of light line, marked at 50' intervals, with...
-6" lengths light sail twine,...
-a flat wooden drouge "Chip"...
-and a 30-second glass.
(This setup allows measurement of up to 15 knots; few sail-powered ships could travel at a higher sustained speed without "sailing under").
A. The drouge is constructed from a 1"x12", 12" 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 7 5/8" deep holes drilled almost through, or a trough of the same depth carved into the surface, about 1/2" in from the edge. The number of holes, or the length/width of the trough, should allow just enough melted lead to be poured in to result in the drouge floating just awash, point upward (see fig. 1).
B. At one end of the bridle, unlay about 12-14" of the strands and securely sieze 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 drouge; tie a figure-of-eight or overhand knot in the bitter end of each. From a piece of scrap pine (or preferably, bone/antler), carve a tapered peg, and leave 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 sieze an eye, trapping the peg in the eye. Press the peg firmly into the remaining hole in the drouge. When completed, the drouge should lie perpendicular to the axis of the line.
C. Attach a toggle to the bridle bitter end. The measuring line will be attached here.
D. Splice an eye to the beginning of the measuring line, for allow attachment of the bridle. This will enable the drogue to enter and stabilize in the water,
and provide a reference point to begin the speed measurement.
- Measure out 50 feet from the eye, and force open 2" of the strands with a marlinespike or small fid. Measure out about four inches of sail twine for the first "tag", tie one figure-of-eight knot near one end of the tag, and weave the other end into the line, leaving the last 2" of the tag exposed.
-Measure another 50 feet, unlay the line and insert another tag as above; add an additional knot, tied 1/2" apart from the first, 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.
E. The exact construction details of the reel are not important, only that the following features are adhered to;
- that the reel be long and of small diameter, to hold the entire length of line, but allow the reel to be held comfortably aloft without touching the head, and
- that the handles and/or axle of the reel be free-spinning, to allow the line to pay out without friction.
F. The glass is altered from an egg timer. To perform this task:
- Disassemble the glass from the frame carefully.
- Open one end of the glass very carefully with a Dremel dental burr, grinder or drill. This is very difficult, too much pressure will break the glass (If you find an inexpensive source, buy more than one timer; it took me three tries/timers to get it right).
- After opening a hole in the glass, prepare a stopwatch and a piece of clean paper.
- Being careful to coordinate the watch and glass, pour out 2:30 worth of sand on to the paper.
- At the 2:30 point, turn the glass horizontal to stop the flow of sand.
- Reset the stopwatch, and turn the lass 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 30 seconds. If the time is too long/too short, pour out/add sand to the glass to reach 30 seconds.
- Seal the end with a small drop of hot glue, and reassemble the timer.
To use the log requires three men;
- One to hold the reel aloft, facing aft near the taffrail;
- One to turn the timer and call out as the sand runs out;
- One to drop the drouge over the taffrail, call out as the toggle clears the rail, observe/count marks as they pass, and stop the line when the sand runs out.
The procedure is as such:
-The reel is held aloft;
-The drogue is dropped over the rail; when the toggle passes over the rail,
-The observer calls "Turn!";
-The timer turns the glass;
-The line runs free over the rail; the observer lets the line pass over his hand, and notes the knots as they pass through; when the glass runs out,
-The timer calls out "Hold!"; the observer stops the line in his hand.
After stopping the line, the number of knots payed out indicates the speed of the ship, including estimated final half knot as well; it's as simple as that.
To retreive the drogue, rather that reeling it in against the drag of the upright drogue, just give the line a quick, very sharp tug, and the peg fitted into the drogue pops out, letting the drouge lie parallel to the flow of the water, skip across the surface of the sea and allowing for easy retrieval.
The procedure is repeated at every bell (half hour) through the day (as describes in another treatise on this site).