The Nautical Sextant

This post was preceded by “C Plath Yachting sextant“ “Making a shades adjusting tool” and “Eighty years of Carl Plath Sextants”. Other posts on C Plath sextants may be found by entering “C Plath” in the search box on the right.

C Plath bought the business of David Filby, Hamburg’s member of Parliament, in 1862 and shortly after he disposed of the book and chart side of the business while retaining the nautical instrument side. Around about the time that he moved into the address known as Stubbenhuk 25, he acquired a dividing machine from Repsold, his former apprentice master, and began to make his own instruments, including sextants. A year or so ago I acquired an early C Plath sextant for a very modest sum on e-bay. It and its case were not in good condition and so I restored them, but other commitments have until now prevented me from writing about it.

Figure 1: The case

Figure 1 shows the case as received, with shrinkage cracks in the top and signs of water damage. Much of the varnish had crumbled, so I stripped off all the old varnish from the outside, filled cracks, made good loose joints, re-lacquered the hardware, re-stained the mahogany and applied several coats of modern varnish to give the result shown in Figure 2. The corner joints are rebate joints, reinforced with steel pins at a time when most makers were using hand-cut corner dovetails. The top, however, is attached with brass screws, while the bottom is glued and pinned on, following the practice of nearly every maker throughout history. The pins tend to rust, being “out of sight, out of mind”.

Figure 2: Case restored.

The interior  as received is shown in Figure 3. Apart from dust and many flakes of paint the green felt lining of the floor and roof of the box had decayed. Fortunately for the instrument, the pocket for the handle and the pads for the legs were attached by screws and glue which had not given way.

Figure 3: Interior as received

Cleaning the interior and fitting new felt  completed the restoration of the case, and restoring the instrument itself presented few problems as everything was present and intact. Figure 4 shows the back of the frame, which is a heavily ridged bronze casting. In the nineteenth and early twentieth century, a very large range of frame patterns was offered by the major makers, but it was not until the early part of the twentieth century that Plath left behind this initial pattern in favour of the ladder pattern, with the occasional three circle or “Dreikreis” pattern being offered (see my post for January 2010). Note the heavy re-inforcement in the areas where the telescope and horizon mirror brackets are mounted.

Figure 4: Frame before re-painting.

The traces of paint that remained were a pale green, but the sextant illustrated on page 50 of Friedrich Jerchow’s history of C Plath, From Sextant to Satellite Navigation, is painted black, so I surmise that the green represents perhaps a primer coat. At any event, I stripped it all off and re-coated the parts black, taking the illustration as a pattern. Figure 5 shows the sextant in its case after restoration.

Figure 5: After restoration

Figure 6 shows the kit of telescopes and other parts supplied. Well into the twentieth century, sextants were often supplied with several telescopes and supplementary eye-pieces. I doubt that there was ever a time when most of them were used at sea. The 6 x 16 Galilean telescope has a tiny field of view and the 10 x 17 inverting telescope is no better. There may have been a time when they were used on land with an artificial horizon to check on the rate of chronometers in distant ports of known longitude, but at sea the 3 x 28 Galilean was probably the one used most, with the ‘zero magnification” sighting tube being substituted in rough weather. The eyepiece shade may have been used to check index error using the sun, but again, most people would probably have used the horizon, as it is easier on the neck to do so.

Figure 6: Telescope kit

The shades and mirror brackets are perfectly conventional and avoid the complications used by French makers in particular and also by Brandis and successors. The tangent screw is of some interest and is shown complete in Figure 7. With slight modification it was also used in the Dreikreis sextant before the micrometer sextant was developed by C Plath around about 1907.

Figure 7: Tangent screw

The spring box, which I have called a “sliding block” in different designs, slides in a close-fitting pocket on the rear of the index arm expansion and can be clamped to the limb using the clamp. A leaf spring keeps the box in place when un-clamped. The end of the tangent screw bears on a tongue projecting from the back of the index arm against the pressure of a coil spring within the spring box. thus, as the screw is turned, the index arm moves along the arc in slow motion. On releasing the clamp, the index arm can be swung rapidly by hand. Figure 8, which shows the mechanism exploded, may help to make this clearer.

Figure 8: Tangent screw mechanism exploded.

The mechanism for raising and lowering the axis of the telescope so that more or less light from the horizon can enter it is shown in Figure 9. It represents an intermediate stage of complexity on its way to the simplicity of the second half of the twentieth century.

Figure 9: Telescope rise and fall.

A telescope bracket having a vee groove and flat machined into it is attached to the frame of the sextant. The rising piece of the telescope has a matching vee and flat to guide it up and down. The lower end of the rising piece has a threaded hole for a screw that is held captive in the telescope bracket, to that when the screw is rotated, the rising piece rises or descends. A clamp holds the rising piece at the selected height.

Figure 10: The arc.

The silver arc, let into the bronze limb, is divided to 10 minutes (Figure 10) and the silver vernier allows readings to ten seconds, though, as with many similar verniers, it is usually impossible to decide which particular pair of lines coincide. It is easier to decide which two pairs of lines just do not coincide and to choose the middle value between them. It bears the C Plath name in flowing copper plate script, a feature of Plath’s earlier sextants.

Figure 11: Serial number and inspection mark.

 Dr Andreas Philipp has kindly provided me with the date of early 1899 for the instrument, or at least, its certification by Deutsche Seewarte. He tells me that both “S” and “D S” were used irrespective of date. He also sent me an illustration from Plath’s number V catalogue of  1906, which I reproduce below (click on this image to enlarge it). Based on the D.S. records, it seems that between 1876 and 1901, Plath produced an average of only 27 sextants per year.

Catalogue entry 1906 (Courtesy of Dr Andreas Philipp)

This post was preceded by “Making a shades adjusting tool” and “Eighty years of Carl Plath Sextants”. Other posts on C Plath sextants may be found by entering “C Plath” in the search box on the right. All figures may be enlarged by clicking on them. Return to the text by using the back arrow.

Several makers, including C Plath, made sextants directed at the yachting market with more or less success. There seems to be a fair number of Freiberger yachting sextants around, but I have only ever seen two Plath Yachtsman sextants. In the years after WWII, many full-size sextants must have flooded the market, especially the USN Mark II sextant and those made by Henry Hughes and Son. The latter also made half size sextants for use in sea planes and presumably they were attractive to yachtsmen, as some have survived. A variety of plastic sextants derived from the Maritime Commission version for lifeboats came on to the market and evolved into instruments that looked like “proper”metal sextants, though few were rigid enough to behave like one. Francis Barker produced a box sextant labelled “Small Craft Precision Sextant” intended for sale to yachtsmen, but despite having been provided with a horizon shade and an eyepiece shade in addition to the usual index shade, I doubt that it found much favour with nautical users. A box sextant is a fiddly instrument at the best of times and it is difficult enough to take sights from a rolling yacht. Ilon industries made an ingenious little micrometer sextant provided with a tiny prismatic monocular (https://sextantbook.com/category/ilon-industries-sextant/) that may have found favour with the well-heeled and Tamaya made a light weight 5/6ths sized micrometer sextant. The French firm of Roger Poulin made an interesting little sextant that was plainly aimed at the yachting market and I have described it here: https://sextantbook.com/?s=Poulin .

It is not clear whether the yachtsman wished a smaller sextant because of lack of space aboard yachts or because a smaller sextant might be cheaper than a full-sized version. At any rate the saving in space and weight must have been insignificant, and the savings made by buying a smaller sextant cannot have been great when compared with the cost of the vessel.

Unlike the Freiberger Yacht Sextant (https://sextantbook.com/?s=Freiberger+yacht), which attempts in a way to echo the full sized instrument, the frame of the C Plath sextant is monolithic and exceptionally rigid. Figure 1 shows a general view of the front. The bases of the index and horizon mirror brackets are identical though the horizon mirror itself is half silvered. Both are circular, presumably because it is easier to seal the mirrors against the intrusion of salt water behind them, but as can be seen in some of the later figures, the index mirror has suffered around the edges. The two index shades and one horizon shade are adequate in most circumstances. Their brackets are simple and no provision is made for adjustment of friction. A notch in the edge of the frame allows the horizon shade to be folded completely out of the line of sight.

The rack in which the micrometer worm engages in machined into the edge of the limb, together with a slot for a keeper to keep correct engagement. The radius of the rack is about 140 mm (5.5 ins) and the instrument weighs 1260G (2lbs 12 oz).

Figure 1: General view of front.

The telescope has a simple draw tube for focusing, and  has an aperture of 25 mm and a power of about 2.5 diameters, giving a field of view of a little over 6 degrees. This is about the same as one gets from a 4 x 40 mm telescope of a full-sized instrument. Though a C Plath leaflet says the aperture is 30 mm with a magnification of x 4, the inside diameter of the tube in front of the objective lens of my sextant is only 27.5 mm and it has to sit on a shoulder, so the aperture behind the lens is only 25.1 mm. The measured magnification is about x 2.5.

The telescope is not demountable, a disadvantage on a small vessel when it is rolling and pitching, as with a standard field of view it can be difficult to acquire the heavenly body and bring it down to the horizon. Removing the telescope altogether makes it much easier to find the body and to bring it down, when the telescope can be replaced and the horizon swept to re-acquire the body. However, the telescope mounting is very robust so that it is not only resistant to knocks, but the sextant can safely be picked up by the telescope without fear of damaging or displacing it. The micrometer mechanism is well protected against knocks and the release catch is simple to operate. Figure 2 shows a rear view of the instrument.

Figure 2: Back view.

The frame is closed off at the back by a back plate, which is attached to the frame by three screws and a leg. The handle, adapted from a full-sized instrument battery handle, is attached to the back plate via pillars by two countersunk screws. Removing the back plate reveals the index arm as shown in Figure 3. Note that if the sextant gets drenched in salt water, it is an easy matter to rinse out the interior with fresh water without necessarily removing the back plate.

Figure 3: Rear view without back plate.

The index arm is in two pieces: a stout rectangular bar attached to the index mirror bearing at the top; and  a plate that I have christened the index arm expansion at the bottom. This plate carries the micrometer mechanism. I have labelled the screw for attaching the horizon mirror and the swing arm keeper in Figure 3 for future reference below. Also seen are the two stout screws that attach the telescope to the frame.

Figure 4: Index arm bearing.

The anatomy of the index arm bearing is revealed in Figure 4. A micro-finished journal runs in a parallel bearing machined directly into the frame, with two PTFE washers acting as spacers and also taking any minor thrust forces that may arise. A flange above the journal carries the index mirror in its bracket, while a spigot below attaches the index arm. Figure 5 shows how the upper end of the index arm is split, with a pinch screw to close it around the spigot. This allows adjustment of the mirror in the horizontal plane as well as axial adjustment to take up any axial movement in the bearing.

Figure 5: Upper end of index arm.

Figure 6 shows how the index mirror is adjusted for perpendicularity and the horizon mirror for side error (the horizon mirror is illustrated) . The mirror bracket is rocked by means of two screws about two ball bearings sitting is depressions to form an axis of rotation.

Figure 6: Mirror bracket adjustment.

As the reflective surface of the index mirror lies a little ahead of the axis of rotation of the index mirror it is necessary to use two vanes to raise the line of sight to somewhere near the centre of the mirror, as otherwise a minor error in perpendicularity may be introduced. Figure 7 shows how two small dominoes have been used, but any two identical objects objects of about the right height may be used, such as pieces cut from aluminium or steel angle, large hexagonal nuts or large rollers from a scrapped roller bearing. One is placed on the limb of the sextant at zero and the other at about 90 degrees. The index arm is then rotated until a reflected view of the second vane is seen alongside a direct view of the first, when the mirror is adjusted to bring their tops into line as shown. In many sextants, including this one, it may be necessary to remove the telescope and/or index shades to obtain the required view.

Figure 7: Adjusting index mirror for perpendicularity.

When adjusting the horizon mirror to remove index error, the screw arrowed in Figure 3 is slackened and a tommy bar used in the hole visible on the right in Figure 7 to rotate the whole base. This is a relatively coarse way of adjusting and may involve much trial and error, but once done, the whole set-up is rigid and not likely to drift out of adjustment in a way that is so annoying with plastic “instruments”.  Removing side error has already been mentioned in the paragraph following Figure 5. Note that index error cannot be removed by using the sun, as the single horizon shade is not dense enough for this method. There is no adjustment available for collimating the telescope, but quite large errors of collimation have relatively little effect on the accuracy of readings, especially for the class of sight likely to be made with this instrument. In any case, this is taken care of at manufacture and would require very rough handling indeed to disturb.

The micrometer mechanism is robust and well-protected. Figure 8 shows it detached from the index arm. The black release catch on the right in fact remains stationary when disengaging the worm and it is the horn extending down and to the left  on the plate that rotates when it and the black catch are squeezed together.

Figure 8: Micrometer mechanism detached from index arm.

In Figure 9, the front plate which carries the fiducial lines for the degrees scale and the micrometer has been removed to show the swing arm chassis. This carries the micrometer worm in a plain parallel bearing, the axial play of which is taken up by a leaf spring. A swing arm extends upwards and to the right to a bearing in the form of a shouldered screw, about which the chassis rotates. A stout helical spring keeps the worm in engagement with the rack machined on the edge of the limb of the sextant.

Figure 9: Front plate removed to show interior of micrometer mechanism.

Figure 10 shows these parts more clearly. In addition, there is a rectangular keeper that guides the index arm expansion and keeps the worm in correct engagement. It slides in a slot machined in the limb below the rack.

Figure 10: Micrometer mechanism exploded.

A further, circular, keeper ensures that the swing arm chassis cannot lift off the face of the index arm expansion. The spigot on the keeper slides in the oval slot and the keeper is retained in the chassis by means of a screw whose tapped hole is shown in Figure 11, centre, which illustrates the bearing surfaces for the swing arm chassis. The keeper can be seen in place in Figure 3, above.

Figure 11: Swing arm bearings.

The sextant frame, being made of aluminium alloy, is inherently resistant to corrosion, but parts that do not run together have a tough coating of blue paint. Other parts are made of bronze and all the screws and springs are of stainless steel. If the sextant should receive a soaking, it is a simple matter to rinse it with fresh water and allow it to dry, as all the parts of the interior are accessible. Nevertheless, at overhaul it would be wise to use waterproof marine grease  for all moving parts except for the rack, which should receive SAE 30 lubricating oil, brushed into the rack with surplus being brushed and wiped off.

The case provided was, like so many other sextant cases over the last fifty years, made of plywood. Quite why the makers did not usually specify marine grade ply is a mystery, as many of them, including those from C Plath, suffered from delamination if stored damp. It was stored face down in the case, leaving the handle ready for use, but as it cannot be set down on a table face down, this is a limited advantage. Perhaps though, it was to discourage users from leaving it in a position on a table to slide onto the floor. The general rule is that a sextant should be in the user’s hand or in its case, relatively easy to follow on a yacht, but more difficult on the bridge of a large ship. All in all, this is a robust sextant, well suited to its task.

Dr Andreas Philipp writes that at least 900 of these sextants were made from 1968, starting with a serial number of 101. They were sold mainly in the USA.

In my post of 18 December 2010 I described the restoration of a Plath micrometer sextant and in Figure 8 of that post I showed how the friction of the shades mountings was adjusted. The adjusting nut needs a special tool to adjust it and recently someone asked me where such a tool could be obtained. As it does not appear in any tools catalogue that I have seen and the C Plath company that made the sextant no longer makes them, the only solution seems to be to make a tool or to have one made.

As the drawing in Figure 1 suggests, the adjusting nut is a pin nut, that is to say it is rotated by a tool that has projecting pins to engage in the holes of the nut. Usually, there are two holes on a diameter, but for some reason, Plath decided to have three holes in their nuts. The drawing shows the pins to be on a pitch circle diameter of 6 mm, but as will be seen, it is much simpler to use one of the nuts as a jig for drilling the holes into which the pins will be fitted.

Figure 1: Drawing of tool.

But first you will need to remove the nut, without having the tool to do so, a Catch 22.  I abused a pair of small spring-bow compasses, inserting the points into only two of the holes and tried to rotate the nut around its centre. It worked, and the dividers survived. If you encounter too much resistance, apply some releasing compound and leave overnight before trying again.

Start by turning a spigot on the end of a piece of 8 mm round bar stock (Figure 2) that closely fits into the 5 mm hole down the middle of the nut. Then mount the nut on the spigot in order to drill the 0.6 mm holes right through the nut and 3 mm into the bar (Figure 3). Drills of this size are both delicate and expensive, so it pays to make sure that the bar is vertical and that the drill enters the hole truly. Anything over 3 or 4 drill diameters counts as a deep hole, small drills have to be run very fast,  and they are particularly liable to jam on the swarf they produce and break. Apart from the expense, it may be impossible to extract the broken fragment, so take things slowly, withdrawing the drill frequently to clear the swarf. Once one hole has been deepened into the bar, insert a short length of 0.6mm piano wire into the hole to anchor the location of the nut for the other two holes.

Figure 2: Turn spigot to fit nut.

Figure 3: Nut used as jig to drill holes.

When the holes have been completed, cross drill the bar (Figure 4) and return the bar to the lathe to turn away the spigot and drill a 5 mm clearance hole (Figure 5).

Figure 4: Cross drilled for tommy bar.

Figure 5: Clearance hole drilled.

Then the bar can be knurled (Figure 6) , parted off, (Figure 7) and the end cleaned up to remove any sharp edges.

Figure 6: Knurling.

Figure 7: Parting off

You then glue three  5 mm lengths of 0.6 mm piano wire into the holes using a smear of industrial adhesive such as Locktite, though superglue would do as well. When piano wire has been cut with side cutters, sharp points are left, so these should be removed by holding the ends of the wires very carefully against a fine grindstone, or by using a diamond file or an old oilstone. Figure 8 shows the completed tool before the ends of the wires have been cleaned up and Figure 9 shows the tool in use.

Figure 8: The completed tool.

Figure 9: The tool in use.

If you are planning to make these tools for sale, it would pay to use a nut to make a hardened steel jig nut, but it would scarcely be worth the trouble, as the true cost of making the tool would probably be more than people are prepared to pay.

See also my posts for 18 December 2010 and 6 December 2012

In this post I will be surveying some C Plath sextants in my possession. I have given detailed coverage already to the Plath Dreikreis or three-circle sextant in January 2010 and to a 1953 ladder pattern sextant in December 2011, so I will be only summarizing details of those instruments. I also looked at Plath’s standard wartime sextant of WWII in the context of detecting fakes in my post of 14 July 2010  and I will give a general overview of that instrument. The last production instrument was Plath’s Navistar Professional and as I have not previously given any cover to that, I will take a detailed look at it. The National Maritime Museum (NMM) at Greenwich has a few Plath nautical sextants  and the excellent photographs of the instruments allows one to follow the evolution of the micrometer sextant. Following this link will take you to their sextant collection: http://collections.rmg.co.uk/collections.html#!csearch;authority=subject-90227;collectionReference=subject-90227;makerFacetLetter=p;makerReference=agent-17323;start=0  ; and I will give the reference numbers to allow more rapid retrieval. Most of the photographs in this post will stand enlarging to 200 percent by right clicking on the figures. Use the back arrow to return.

Carl Christian Plath took over the business of David Filby in 1862. Philby was an instrument maker, but just as with clock and chronometer makers, he did not actually make the sextants he sold. However, Plath bought a dividing engine from Repsold in about 1865 and his own  first sextants probably date from  shortly after that date. As with most early instruments, few of them survive. It seems that they were regarded simply as tools of the trade by their users and most often unsentimentally discarded when their owners retired. No doubt, some were lost at sea and others found their way to attics and basements, only to be thrown out  as unidentifiable junk after the owner’s death. The earliest Plath sextant that I have dates from about 1909.

Figure 1: Front of Dreikreis vernier sextant

The main interest of the Dreikreis (three circle) sextant, apart from its relative rarity, is in the ribbed three circle frame, also adopted by Heath and Co and Hughes and Son, the two leading makers in Britain. That shown in Figure 1 is a vernier instrument. The sliding block arrangement (Figure 2) is rather more complex than usual and is covered in more detail in my post for 24 January 2010. Around about 1907, Plath began to advertise a micrometer sextant whose mechanism  scarcely changed for the remainder of the time that sextants were manufactured. A three circle micrometer sextant by Plath is illustrated in the NMM on line collection, reference NAV1130, and the photographs have sufficient detail to allow one to compare the micrometer mechanism with later ones.

Figure 2: Tangent screw mechanism of Dreikreis vernier sextant

Plath were still producing vernier instruments in 1920, the date of the sextants shown in Figure 3, but by then was using a fine adjustment system that was on its way to being a micrometer, in that the “endless tangent screw” operated on a fine rack cut into the back of the limb, as shown in Figure 4. More details will be found in my post of 4 December 2011. This layout was patented by Heath and Co in 1910 and required only the addition of a micrometer drum and the adoption of a suitable pitch for the rack, which was what Heath and Co did, but Plath had already placed the rack of their micrometer sextant on the edge of the limb and this was the pattern adopted by most other makers.

Figure 3: Plath vernier sextant with “endless tangent screw”.

Figure 4: Rack of sextant shown in Fig 3

A micrometer sextant with a ladder pattern frame of smaller radius dating from 1917 is in the NMM collection, reference number NAV1250 and this design of sextant remained standard until about 1942 when, on a war footing, an instrument that was easier to produce and used less strategic metals was standardised. However, as the magazine cover in Figure 5 shows, many earlier instruments were still in use on the Germany Navy or Kriegsmarine by December 1943. Note that the aperture of the telescope has also increased from 28 to 40 mm, more than doubling the light grasp, with a corresponding increase in the size of the mirrors. This increase in light grasp is important in conditions where contrast of the horizon is poor. When Plath resumed sextant production in the early 1950s it was this pattern that they used, with a bronze frame, rather than the technologically superior wartime sextant which behaved equally as well. Figure 6 gives a closer look at a 1953 sextant of the  type in use in Figure 5. The colour of the drum is different and is divided to whole mnutes rather than half-minutes, but otherwise the two sextants are the same

Fig 5: “Die Kriegsmarine” magazine, December 1943.

Figure 6: Standard ladder pattern sextant.

Figure 7 shows the micrometer mechanism in close-up. The worm is conical so that the end of the shaft that bears the drum is further away from the frame, allowing the use of a larger drum than would have been the case had a cylindrical worm been used. End play in the worm shaft bearings is taken up by the axial preload spring that bears on the rear end of the shaft. A radial pre-load spring keeps the worm in engagement with the rack and a keeper on each end of the expanded lower end of the index arm prevents the mechanism from lifting away from the front of the limb. The worm in its bearings  is attached to a swing arm that rotates about a bearing. When the release catch is operated, the worm swings out of engagement with the rack in the plane of the sextant frame, allowing the index arm to be swung rapidly to any desired position. When the catch is released, the worm swings back into engagement with the rack under the influence of the radial pre-load spring, ready for fine adjustments to be made via the worm. As will be seen, this mechanism was later simplified, probably to reduce production costs, before a final burst of simplification in the Navistar Professional model.

Figure 7: Standard Plath micrometer mechanism

While the standard sextant had a bronze frame with bronze mirror brackets and weighed with its telescope a hefty 1.9 kg (4.19 lbs), the wartime standard sextant had an aluminium alloy triangulated frame (Figure 8) with alloy brackets weighing a mere 1.2 kg (2.65 lbs). Once the very expensive moulds for the frame and brackets had been made, many thousands of the parts could be turned out rapidly by the pressure die-casting process; and the serial numbers increased by about 4,500 between mid 1942 and the end of the war in mid 1945. While aluminium alloy frames were perceived to be inferior to bronze ones by mariners, in fact they are rather more rigid and stable and at least as corrosion resistant. Figure 9 shows the instrument in use during wartime as illustrated by the magazine Die Woche for 14th April 1943. The 0.7 kg reduction in weight makes using this model of sextant a pleasure and it has not been my experience that a lighter sextant is more difficult to use when there is a strong wind.

Figure 8: WWII Kriegsmarine standard sextant

Figure 9: Sextant in use during WWII (1943).

As noted above, when Plath resumed sextant manufacture in the early 1950s, they produced their standard pattern bronze-framed instrument, but by 1975, while retaining the ladder pattern, it was slightly better designed with sharp corners eliminated. I give a general view of the front of this “Navistar” sextant in Figure 10 of this post. A battery handle with scale illumination via light guides had been added and the handle was canted at a  more ergonomic angle. The mirror brackets and shade frames were now of aluminium alloy and the classical tapered index arm bearing was replaced by a plain parallel one of white metal running directly in the frame. Its weight with batteries remained about the same at 1.9 kg. The telescope in older instruments can be dismantled for cleaning or drying out, but in the newer instruments, the objective lens was glued into place in the telescope body and the eye lens was glued into the plastic eyepiece, making any servicing rather problematic. Figure 11 of my post of  18 December 2010  shows how problematic it can be.

Figure 10: Front view of early Plath Navistar sextant

The shades were retained by a non-standard nut requiring a special tool (Figure 11 of this post) to tighten it if the shade friction was insufficient. Such a tool cannot be bought, so any one wishing to dis-assemble the shades for cleaning and greasing had to make one of their own. There was no question of simply sending the shades assembly to the makers for adjustment, as the bracket was now part of the frame (Figure 12), rather than being attached with screws as previously, so the whole instrument had to be sent.. It appeared as though a sextant too expensive to throw away had been made for a throw-away age.

Figure 11: Non-standard pin wrench to adjust shades friction.

Figure 12: Rear view of sextant in Figure 10

Figure 13 gives details of the micrometer, from which it can be seen that its manufacture had been considereably simplified without, one hopes, sacrificing quality. While the release catch itself was an alloy die casting, the fixed part of the release catch was an extension of the plastic of the light guide and was easily broken off.

Figure 13: Plath Navistar micrometer construction

These instruments were no longer supplied in mahogany cases like pre-war sextants, but in heavy, moulded, black bakelite cases with various retention systems that did not always survive heavy handling, though the cases themselves are almost indestructable (Figures 14 and 15).

Figure 14 : Case of Navistar sextant

Figure 15: Retaining latch and pocket of Navistar sextant

In 1977, Plath introduced their Navistar professional sextant, just in time for it to be made obsolete withn a few years by the advent of the Global Positioning System. The frame was a hefty triangle of aluminium alloy 14 mm (0.55 in) thick. While this undoubtedly made it cheaper to manufacture, it had the unfortunate corollary that there was nothing by means of which one could pick it up, except for the handle, so it was provided with a light moulded plastic case in which it sat handle uppermost. This led to a further difficulty: because it was not provided with legs, this is the only place that it can be put down and then only with the index set at – 6 degrees. Figure 16 shows a front  view of the instrument and Figure 17 shows it in its case. What appear to be stubby legs on the rear of the limb serve only to act as stops for the index arm.

Figure 16: Front view of Navistar Professional sextant.

Figure 17: Navistar Professional in its case, rear view.

Extensive use was made of high-impact moulded plastic, in the handle, the telescope body, the shades mountings. the lower end of the index arm and release catch and the micrometer drum.  The latter was a mere 15 mm (0.6 ins) in diameter with figures of a size to tax aging long-sighted eyes (Figure 18). The same slightly inadequate drum was used in the Plath Navistar “Traditional” in which the frame took the form shown in Figure 10. The shades, two each for the horizon and index mirrors, were mounted so that they rotated around their mirrors (Figure 19). They are easy to use, but if the sextant is placed carelessly on a flat surface face downwards it is the shades that will suffer. The two mirrors were identical so only one size was needed for replacement, but because the horizon is viewed directly rather than through glass as in a traditional full glass horizon mirror, overlap of images is rather limited. The mirrors were first surface with overcoating.

Figure 18: Micrometer drum and arc.

Figure 19: Horizon and index shades.

The index arm journal took the form of a bronze bush surrounding a boss extending from the bottom of the index mirror bracket and the bronze-coated journal ran directly in the frame, being secured by means of a C-ring and a nylon washer (Figure 20).

Figure 20: Index arm bearing.

The micrometer mechanism was even simpler than that shown in Figure 12. There was no longer a swing arm. The worm shaft bearing rotated about a boss formed on its base and an arm extended down wards. A spring at the end of the arm abutted against one wall of the plastic enclosure for the mechanism while a U-shaped wire link attached the arm to  the release catch, which formed another wall of the enclosure made springy by slotting the base of the wall. Axial preload for the worm was provided by a spring clip (Figure  21).

Figure 21: Micrometer mechanism of Navistar professional.

The scales were illuminated by a system of light guides within the black plastic covering to the lower end of the index arm. The source of the light was simplicity itself, being simply a wire bulb-holder that also embraced the battery. An orange cap when depressed pushed the positive end of the battery against the central contact of the bulb and thus compeleted the circuit (Figure 22). A ridge on the cap latched against an internal cut-out to prevent the whole falling out, but  slots cut in the cap to allow the ridge to spring into the cut-out had sharp corners which in my specimen have already led to a crack forming, and I anticipate having to machine a brass  replacement some time in the future. Meanwhile, I have drilled a hole at the end of the crack to reduce stress concentration.

Figure 22: Lighting system.

The 4 x 40 Galilean telescope and its integral bracket were of high-impact plastic and there was no provision for servicing the internal surfaces of the lenses as both are cemented into place. An interesting telescope could be supplied which has a front filter that allows normal viewing of extended sources but which acts as an astigmatiser for point sources (Figure 23).  Almost needless to say, this filter cannot be removed for servicing of the lens beneath.

Figure 23: Astigmatising telescope.

The sextant is quite easy to use and is no doubt very rigid for its weight of 1600 G (3.5 lb), but the placing of the handle at 120 degrees means that when it is turned to view the micrometer drum it is out of balance. The telescope has a fairly limited field of view and the micrometer is difficult to read. Instrumental accuracy was given as better than 20 seconds. I doubt that this sextant found much favour with traditonalists, but by the time my specimen was sold (1988) sextants were already falling into disuse.

If you enjoy reading about navigational instruments and technology of the sea, you will probably enjoy reading the book which gave rise to this web site, The Nautical Sextant as well as my more recent The Mariner’s Chronometer, both of which are available via the amazon web sites  in North America and Europe. The Nautical Sextant is also available from the joint publishers, Paracay and Celestaire.