German WW II Gyrosextant (See-Kreisel-Sextant)

23 01 2019

Figures 5, 6, 12, and 13 may be enlarged by clicking on them.

During the Battle of the Atlantic, which for Britain began on 2 September 1939 and ended on 8 May 1945, without a break, Britain was nearly brought to its knees by submarine warfare. The battle began to turn away from Germany’s favour in mid-April, 1943, when for the first time convoys could receive continuous air cover between Britain and North America. It soon became apparent from increasing submarine losses that German submarines could no longer safely remain surfaced during daytime. While the Schnorkel ventilating tube mitigated this problem to some extent, by early 1943, the protection given by darkness was removed by the Allied development of airborne centimetric radar and the Leigh Light which illuminated the sea for attack once an aircraft had been guided to the submarine by radar.

In spite of having a Schnorkel, a submarine still had to surface to make observations of the stars, moon and planets at night, but as the horizon is generally not visible at night, an instrument with an artificial horizon had to be developed. I have described the SOLD KM2 bubble sextant in my post of 5 November, 2013. Unlike a large aircraft, which has at least partly predictable oscillations in flight, anyone who has taken a bubble sextant to sea will know that the accelerations as revealed by the bubble vary wildly and unpredictably.  If the instrument is provided with a read-out that integrates observations over two or three minutes, as is the C Plath SOLD, it may be that results will be better than with instruments that simply average many observations at an interval that may coincide with the frequency of, say, rolling of the submarine. This potentially can lead to very large errors indeed.

When a bubble sextant is subject to an acceleration, all the fluid in the spirit level is affected. As you will see when I describe the gyro unit, when the Kreiselsextant is subject to an acceleration, the only connection between the sextant and the gyro is via its low friction, small area bearing.  Effectively, the gyro is almost detached from the sextant and retains the direction of its axis of spin in space.

Figures 1 and 2 show the left and right hand sides of the Kreisel sextant. Apart from the gyro unit and minor changes to the light path, it is almost identical to the SOLD sextant, so I will describe only the gyro unit and the consequent light path changes in what follows. Readers interested in the interior of the instrument may consult my post of 5 November 2013.

ga left side

Figure 1: Left hand side.

ga rhs

Figure 2: Right hand side

Figure 3 shows the gyro rotor sitting on it bearing in the bearing housing. The upper part is cross bored on a diameter, one end of which carries a collimating lens and the other a graticule at the focus of the lens, so that light rays emerging from the lens are parallel. When viewed, the image of the graticule thus appears to be at infinity. The lower part has 35 crescents, or buckets machined into its edge, so that when air is blown into them the rotor is caused to rotate. As the gyro rotates at hundreds of time a minute, the image projected into the sextant flickers only a little.

gyro in situ

Figure 3: Gyro rotor on its bearing.

Figure 4 shows the image obtained of the graticule when viewed through the collimating lens. The central pair of lines are intended to be used for star observation, while the outer pair are used for moon and sun observations. The out-of focus vertical dark line is of course the central bearing spindle.


Figure 4: Graticule seen through collimating lens.

Figure 5 is a cross sectional drawing of the rotor and its housing, copied from a British analysis of the sextant reported in August, 1945. I have added the light path, The rotor has a  hard steel spindle through its centre coming to a point of 0.13 mm radius.  Together with a concave artificial sapphire of 3.7 mm radius, it forms a low friction, self-centring  bearing for the rotor. The carrier for the sapphire is spring loaded within a lifting tube that can be raised to lock the rotor against the top of its housing.

gyro section labelled

Figure 5: Sectional drawing of gyro.

Figure 6 shows details of the gyro bearing and the lifting tube used to lock the rotor.

gyro bearing detail

Figure 6: Gyro bearing detail.

Figure 7 shows the window through which rays exit the rotor into the body of the sextant. Circled in white are two of the six tiny holes or nozzles through which air is projected into the buckets at high speed at about 40 degrees to tangential to make the rotor rotate at high speed. They are 1 mm in diameter.

gyro window to sext

Figure 7: Detail of interior of housing.

Air enters the nozzles from a gallery which is connected to an air inlet shown in Figure 8 below. The air leaves the gyro housing through 8 holes drilled in the circumference of the bearing housing. A flap covers a viewing window through which the motion of the gyro may be viewed to check when it has settled into a steady motion.

gyro extterior labelled

Figure 9: Exterior of gyro unit.

Figure 10 shows the lamp carrier for the gyro unit. It screws into the gyro unit. When I bought the sextant it came with three 3 volt bubs with the miniature bayonet base shown, all unfortunately defective. In this voltage they now seem to be unobtainable, but I was able to find some 6 volt versions, perhaps the last dozen on the planet, and had to make up a makeshift 6 volt battery pack that would fit into the sextant in order to try it out (Figure 11).

gyro lamp carrier closeup

Figure 10: Detail of gyro lamp carrier.

battery compartment

Figure 11: 6 volt battery pack in place

Figure 12 is of another drawing from the British 1945 report, showing the full light path. After being collimated at the rotor the image of the graticule is, I think, reverted  by a pair of lenses, one applied to the face of a 90 degree prism and the second beyond a fixed mirror. This second lens appears to be at the focus of a further collimating lens that brings the rays parallel again, to be viewed in a Galilean telescope. (I am not confident that I have correctly described the function of the lenses between the two collimating lenses. If any  reader can enlighten me further I should be glad to hear from them in the “Comments” section.)

lightpath 2

Figure 12: Light path through sextant.

Figure 13 shows the instrument in its case. At top left is a carrying handle used to carry the instrument up through narrow confines of the conning tower hatches. Although it would not normally be used in daylight with a natural horizon, it is nevertheless provided with a set of four shades so that daylight observations of the sun or moon could be carried out on the uncommon occasions when the bodies are visible but not the horizon, e.g. in ice. A light shade would be needed for night observations of the moon when near full.

1 case inside

Figure 12: Sextant in its case, with accessories.

Below the shades is a charging adapter and blue bulb used as a dropping resistance to allow the white nickel-iron-alkali battery to be recharged from a 110 volt direct current supply. Proceeding clockwise, there is a spare gyro bearing and a bank of four spare bulbs. The sextant is held in the case by a bracket that folds down from above, which is then locked in position  by a transverse bar. The sextant itself weighs just over 3 kg, but with its case it is a hefty 8.6 kg.


Figure:13  Instructions

The instructions, pasted to the inside of the lid, are of course in German, and there appears to have been at least two versions. It seems that the instrument in its case was to be set down on a table with the lid open and horizontal. An air supply probably from a foot pump, was then attached to the air inlet and pumped rapidly until the whirring of the rotor reached a high pitch, when the rotor was then allowed to settle down for three minutes, though this appears later to have been altered to five minutes. During this time, precession of the rotor settled down so that its vertical axis was aligned with local gravity, and the light path through it horizontal. The integrator was then wound up, the instrument eased carefully out of its case and the carrying handle clipped into place (Figure 14), all the time keeping the sextant upright and avoiding knocks or sudden movements.


Figure 14: handle in place.

Even removing it from its case needs great care, as it is a narrow fit and it is all too easy to catch it on some projection. The sextant is then passed from hand to hand up the conning tower, all the time avoiding sudden movements and knocks. On reaching the top, the handle is then unclipped. The spring is strong and again it is quite difficult to do this without upsetting the gyro. The main switch on top of the left handle was then switched on and the gyro lighting control in the left handle (Figure 1) adjusted and a view of the graticule obtained. The gyro lighting comes on only when the integrator is wound up and goes out when the integrator runs down, signalling the end of the observation period to the observer. The lighting of the integrator read out then comes on and its intensity can be adjusted using the scale lighting control (Figure 2). Pressing the button switch on the scale lighting cover (Figure 2) causes the integrator lighting to go out and the lighting to the remainder of the scales to come on.

The cone of visibility of the graticule is quite small and it needs a little practice simply to obtain a sight of  it on dry land. I imagine it would need much practice to see it and then make it coincide with a star on a submarine at sea, but we know from at least one voyage report that this was done successfully, though we do not know with what degree of accuracy. Hand held, on dry land, the mean error of 50 observations of the sun was 10.9 arc minutes, with a standard deviation of 8.86 minutes.

As spinning tops as toys have given way to electronic games at all ages, some readers might wonder how it is that the gyro comes to define the vertical with its axis and hence provide an artificial horizontal via the graticule and collimating lens. Another way of putting the question might be: why does a spinning top stand up, but this is somewhat complicated because at rest in this gyro, the rotor is stable, as its centre of mass is 3.5 mm below the bearing.

The law of conservation of angular momentum decrees that undisturbed, a rotating body will continue to maintain the direction of its axis of rotation unless a force acts upon it. The only available forces are gravity and friction in the bearing and, as I have noted above, the end of the spindle is spherical, with a radius of 0.13 mm, so that when leaning, the centre of mass does not coincide with the centre of the spindle. This with gravity creates a couple, which leads to the axis describing a cone, or precessing until the centre of mass is coincident with the centre of bearing, in which position it is upright.

Readers of a mathematical bent (which I am not), will find a more lengthy and satisfying explanation in most university level textbooks of physics and mechanics. In 1890, J Perry published an entertaining little account of a popular lecture he had delivered, “Spinning Tops,” in which the words, “vector,” “angular velocity” and “torque” do not appear. General readers may find in this a more accessible account.




A French Hydrographic Sextant

13 01 2019
2 a case inside

Figure 1: Sextant in its case.

I recently acquired for a relatively modest sum the three-circle vernier sextant shown in Figure 1. Attached at the front corner of the frame is a plate engraved with the letters “S.H.” or “Service Hydrographique (de la Marine)” or French Naval Hydrographical Service, formed in 1886 as successor to the “Dépôt des cartes et plans de la Marine”, founded in 1720. The plate seems to serve no other purpose that I can think of  than as an identifier.

3 a e bouty name

Figure 2: Front of the tangent screw mechanism.

Engraved on the front of the tangent screw mechanism is the name “E. Bouty”. Edmond Bouty (1845 – 1922) was a physicist in the Science Faculty at Paris, but I cannot find that he was an instrument maker, nor is there any other name on the sextant. It may be that his contribution was the design of the scale lighting system, about which more later. It is not even clear that the sextant is of  French manufacture, as at the left end of the limb are the letters “D.S.” indicating Deutsche Seewarte, the German Hydrographical Service, but the frame, of about 180 mm radius, differs in detail from that of C Plath’s Dreikreis sextant.

2 b frame turning marks

Figure 3: Turning marks on front of frame.

The bronze frame is of no particular interest except that when clearing old and perished paint from the frame during restoration I noticed marks (Figure 3) that showed that it had been faced in a lathe, giving a small clue to the manufacturing process.

3 c spring nut

Figure 4: Spring box detail.

Returning to the tangent screw mechanism, the spring box is shown exploded in Figure 4. A tongue on the sliding block is trapped between the end of the tangent screw and a long spring mounted on a guide and retained by a nut. The end of the guide can be seen on the right of Figure 2.

3 b clamp

Figure 5: Exploded view of index arm clamp.

The sliding block is retained in its slide in the lower end of the index arm by the retaining spring on the upper right of Figure 5, while the clamp screw and its leaf spring bears on the back of the limb. In use, the clamp is slackened and the index arm moved approximately into position, when the clamp is tightened, thus fixing the sliding block to the limb. Turning the tangent screw thus moves the index arm about the sliding block against the pre-load of the helical spring as a means of fine adjustment. In truth, it is the index arm that slides rather than the sliding block, but as no one else had given it a name, I decided to do so when writing “The Nautical Sextant.” This system of applying pre-load was used in many vernier instruments such as vernier theodolites and gun aiming systems. as well as in several makes of sextant.

4 perp adjust

Figure 6: Index mirror bracket.

The index mirror is held against a vertical bracket by means of a clip which is tightened against the bracket by means of a screw bearing on the back of the bracket. The mirror is made perpendicular to the arc of the sextant by a system that seems  to have been used only by French makers. Two screws attach the radiused feet of the bracket to the upper end of the index arm and the end of a screw held captive in the base of the bracket can then rock the bracket to bring the mirror square to the plane of the arc..

5 side error

Figure 7: Horizon mirror bracket.

Figure 7 shows a somewhat similar method of adjusting out side error of the horizon mirror, but in this case a deep slot cut nearly through the base of the bracket gives flexibility to the the adjustment by means of another captive screw.

7 horizon mirror

Figure 8: Horizon mirror detail.

The detail shown in Figure 8, as well as making clearer how the mirrors are held against their brackets, shows that the horizon mirror bracket can be adjustably rotated about an axis vertical to the plane of the sextant, in order to adjust out index error. Note that the mirror is fully silvered, which means that the direct view of the horizon does not pass through glass and that the edge of the silvering of the mirror can be given better protection against corrosion. It does however result in a smaller area of overlap of the direct image of the horizon and the  reflected  image of the observed body when using a Galilean telescope. Enter “Freiberger yacht sextant” in the search box at the top of the page for a discussion of why this is so.

6 index error

Figure 9: Detail of index error adjustment.

Figure 9 gives more detail on the index error adjustment. There is a boss as an axis on the underside of the horizon mirror bracket that passes through the frame and is held by a retaining screw. A further boss passes through a clearance hole in the frame  and has an internal thread tapped in it as a nut. The index error adjusting screw, held captive in the frame by a screw and clamp, engages with the “nut”, so that when the adjusting screw is turned, the whole mirror bracket rotates. When adjustment is complete, the bracket is locked in place by a  clamp screw..

This is a rather complex means of adjustment of the horizon mirror, which had long been achieved much more simply by means of   a pair of screws bearing against the back of the mirror, while lugs on the mirror clamp provided spring loading. Elegant though it may have seemed to its (?) French inventor, it is unnecessarily complex., though perhaps no more complex than the solution adopted by Brandis and its US successors.

9 battery handle

Figure 10: Interior of battery handle.

This sextant represents perhaps one of the earliest ones to light the scale in poor light. Scale lighting had to wait for the development of suitable dry batteries in the 1890s and of miniature flashlight bulbs with robust tungsten filaments in about 1904.

Figure 9 shows the interior of the Bakelite handle which accepts a 3 volt 2R10 battery.  A screw at the lower end holds the negative pole of the battery firmly in electrical contact with the frame of the sextant and at the upper end a spring loaded switch plunger makes contact with the positive pole. The top end of the lid is bevelled and the lid itself is slightly bowed, so that when rotated closed it remains in place.

10 b handle to bearing

Figure 11: Wire from handle to foot.

A wire passes from the body of the switch to the foot (Figure 11), inside which is a spring loaded brass plunger (Figure 12).

10 a switch to contact

Figure 12: Inside of foot.

The index arm journal is hollow and a wire passes up its centre to an insulated contact on the end, to make electrical contact with the contact inside the foot (Figure 13).

11 a journal contact

Figure 13: Insulated index arm contact.

The other end of the insulated wire passes down the index arm in a machined groove to a clip held on an insulator block (Figure 14).

12 lighting system

Figure 14: Lighting bulb holder.

The clip makes contact with the outside of the bulb holder and thence to the central contact on the bulb. The outside of the holder is insulated from the brass interior, which is threaded for the bulb. The brass interior fits snugly in the cylindrical shade which is attached to the index arm and hence the frame, thus completing the electrical circuit. Most subsequent makers contented themselves with a simple loop of insulated wire to conduct electricity to the bulb, but this more complex and no doubt more expensive system has the merit of not flexing any wire. Like most complex systems, however, there is more to go wrong.

13 rising piece in situ

Figure 15: Rising piece.

The telescope rising piece (Figure 15) is simpler than that of many of its early 20th century competitors and it has a rectangular mortice machined in its face to engage closely with a tenon on the telescope bracket, so that it can be slid up or down to vary the amount of light from the horizon entering the telescope. Collimation is standard, by means of a tilting telescope ring held in place by two screws.

8 index shades

Figure 16: Shades mounting.

The shades make none of the usual provisions to prevent movement of one being transmitted to its neighbours. Resistance to rotation is given by means of a Belleville washer, a conical washer with the characteristics of a short, stiff spring. Since these date from about 1870, they add no clues to the age of this sextant.

15 telescopes

Figure 17: Telescope kit.

The kit of telescopes shown in Figure 16 is for the most part standard, with a 4 x 24 mm Galilean “star” telescope for general use and a 6 x 16mm Keplerian “inverting”  telescope. By the twentieth century, this latter probably received little use except for artificial horizon sights to rate chronometers in out-of-the-way places of known longitude. The large 3 x 36mm Keplerian telescope is of interest as it has a wide angle eyepiece with an eye lens of 25 mm aperture. This gives an image nearly as bright as the 4 x 24mm telescope (the extra lens in the eyepiece causes some loss of light) and with a field of view about four to five times wider.

1 a case exterior

Figure 17: Case exterior.

The mahogany case was much battered and stained, and with several shrinkage cracks, so it was gratifying to be able to restore it to the state shown in Figure 17. It looks decidedly English and placing the handle on the side follows Henry Hughes and Son’s practice, but neither the sextant frame nor the mirror mountings  are consistent with this.

If you enjoyed reading about this sextant, you may also enjoy reading my “The Mariner’s Chronometer“, also available via