The Nautical Sextant

 This post was preceded by  “A Russian Naval Dip Meter”; and  “An Improvised Dip Meter”

When sailing in company with other ships, as for example, in a convoy, or when maintaining a safe distance when rounding a danger, it was useful to know the distance of one’s ship from the other objects. Until the advent of radar, a variety of distance meters was used. The most obvious is perhaps the sextant, likely to be found aboard every ship of any size. If some dimension of the object is known, like height of the mast above the waterline, and the vertical angle subtended by the dimension is measured, its distance can be calculated, but every self-respecting set of nautical tables had a table of “Distance by vertical sextant angle” to obviate calculation. A variety of distance meters was invented in the late nineteenth century to eliminate even the incovenience of looking up tables by giving a direct reading of the distance once the mast height had been set.

Most, like that of Fiske, in effect used a modified sextant that read through a relatively small angle while having a scale that gave the distance directly in yards. Recently I came into the possession of a Stuart distance meter that uses a different measurement principle, somewhat similar to that of the N5 dip meter described in the preceding post. The instrument was very dirty and the ivorine scales had shrunk and torn away from their screws, but happily no parts were missing and I anyway paid very little for it. Figure 1 shows the meter after cleaning and restoration.

Figure 1 : General view of distance meter.

The height of the object, say, a ship, up to 200 feet, is first set against the left edge of the transverse height scale. This need not necessarily be mast head to waterline. The note pad on the other side of the meter has provision for noting also the distance from the mast head to the “lower top” and “Upper speed(?) to stern lt.” The ship is then viewed through the telescope, when a field split vertically is seen. The image of the head of the mast in one half is brought alongside the image of the waterline in the other half by rotating the knob, when the distance in cables (a cable is one tenth of a nautical mile) can be read against the index on the distance scale. In Figure 1, the height is set to 60 feet and the distance is one cable.

Figure 2 shows the somewhat shrunken note pad on the front of the instrument and Figure 3, showing it with the telescope removed, begins to reveal some of its workings. In front of the left half of the telescope objective is a fixed slice of a negative lens of about -2.3 dioptres (about -440 mm) and a similar but longer slice is in front of the right half of the field. This latter lens is attached to a slide that carries the scale and as the slide moves through a usable distance of about 60 mm, the images separate as shown in Figure 3. Note that in Figure 2 “Patt 498” is probably a naval designation and cetainly not a reference to a patent. The telescope is about x 3 power and has an interrupted thread that allows it to be fitted in its bracket with just one sixth of a turn

Figure 2 : Front of distance meter

Figure 3 : To show split image.

Figure 4 shows the effect of time and sunshine on ivorine. I replaced the scale with a sheet of brass 1.6 mm thick and glued to it a paper scale copied from the original scale. It does not allow for shrinkage and the meter is probably no longer accurate, but it does allow the principle of the meter to be illustrated.

Figure 4: New scale for old.

Figure 5 shows the relative complexity of a Stuart distance meter’s competitor in the form of a Fiske-type distance meter or “stadimeter”, invented at  the same time in about 1895. While the Stuart instrument has a single slide machined in an aluminium casting requiring no great precision of manufacture, in the Fiske instrument the distance screw and scale are carried in a close fitting bronze carriage running in a precisely machined bronze frame. There are two mirrors, each needing means of adjustment, two lead screws and a bearing for the height scale, which corresponds to the index arm or alidade of a nautical sextant. It may well be that the Fiske is capable of greater accuracy of measurement, but no great accuracy is required in station keeping in a convoy, while one would err on the side of caution in rounding a danger. It may be that the instruments were originally envisaged as a range-finder for gunnnery or as a rangefinder during a “creeping attack” by two ships hunting U-boats. In this, one attacking ship remained at 1000 yards  astern of the submarine, where the latter was in its asdic cone and guided another ship moving from astern at slow speed so that its approach was masked by the submarine’s own propellor noise, until the distance of the sub by asdic and the distance of the ship by rangefinder coincide.

Figure 5: Fiske-type stadimeter or distance meter (Mark II, Mod 0).