A later Shackman sextant: a guest blog post.

14 03 2018

John Triplett recently wrote to me about a Shackman sextant that he had acquired and kindly agreed to write a guest blog post. In what follows, comments that I make are shown in blue.

It wasn’t very long after I first read Bill’s blog entry on Shackman sextants (https://sextantbook.com/category/shackman-sextant-and-link-to-ramsden/) that I had the opportunity to acquire one, and at an extremely reasonable price.  I found the design to be unique and interesting, and wanted to see it up close.  I found this sextant in a recent eBay auction that was listed referencing its reseller, Kelvin & Wilfrid O. White of Boston, and not its maker, D. Shackman & Sons (Figure 1).


Figure 1: The sextant sitting on its case.

In making some needed repairs (the index arm was bent away from the limb and had to be straightened), it soon became apparent that there are some design differences between Bill’s sextant and mine.  My sextant has a later serial number (No. 3236) than Bill’s (No. 2262), and that does seem speak to some of the differences between the two pieces.  As of this writing, there is another Shackman sextant being offered on eBay (again, resold by Kelvin & Wilfrid O. White) with a serial number (No. 2097) even lower than Bill’s, and having characteristics of Bill’s earlier number, specifically the short tail and leaf spring on the index.    No. 2097 also has a 30” micrometer with a single index line, not a 10” micrometer with a vernier, so, either different precisions were made by Shackman, perhaps some for survey purposes and others for higher precision needs in navigation, or the more precise vernier was a later revision. Most non-British makers had by this time begun to abandon the vernier, perhaps recognising that errors due to uncertainty about the dip of the horizon tend to swamp instrumental errors, which in any case are sometimes of the order of 30 seconds. If greater precision is needed, it is easy to estimate to 0,2 degrees, but such precision is likely to be devoid of meaning, given that the instruments were calibrated at only 30, 60, 90 and 120 degrees.

It generally appeared to be in fine condition, very clean, and showing very little use.  As for the needed repairs, my best guess is that this sextant was dropped early in its career at the US Merchant Marine Academy (as the property label on the box suggests), and, once the owners discovered the nature of the damage, it was simply shelved and forgotten.  (This long convalescence is further evidenced by the light damage to the lid of the box:  papers left on the top for a long period resulted in a stark bleaching/darkening pattern from a masked exposure (See Fig 1.)  Some disassembly was required, and this process uncovered some of the aforementioned differences.

The noted differences to my No. 3236 are:

  • ‘Paddle’ or ‘spoon’ grip extensions on the index and worm that improve manipulation and placement of the micrometer movement. One of the ‘spoons’ is deformed from the drop impact. See Figure 2 for close up view.
  • Copy of IMG_3295_preview

    Figure 2: Damage to release catch “paddle”.

  • The leaf spring has been replaced with a coil spring and plunger system that applies and maintains the pressure of the worm against the rack. The spring slides in and bears on a curved groove on the underside of the opposite paddle grip (Figure 3).
  • Copy of IMG_3294_preview

    Figure 3: Radial helical pre-load spring.

  • The index journal bearing system has seen a design change from the keyed washer to a spring and washer. (This was discovered when the index arm was removed from the frame for repair.) This is such a strange arrangement that I think it must be a later and rather clumsy repair. The washer under the head of the screw does not seem to have a square hole, so there is nothing to stop the washer from working the screw loose as the latter rotates with the journal (Figure 4).
  • Resulting in part from some of the above changes, the later sextant is positioned differently in the case to allow room for the elongated worm assembly.

Ironically, the strength of the ribbed arm casting Bill references in his original blog either worked well or not at all, depending on how you look at it.  It certainly preserved the alignment of the ribbed section of the arm, but fully transferred the bending moment into the weakest part of the arm at the hole for the journal mount.  In agreement with Bill’s assessment, this inherent weakness certainly makes the arm rib a questionable design point, and an overkill in manufacturing.

As for the spring-and-plunger system, this could be considered the Achilles’ Heel of this design.  The spring seems, to me, to be undersized and understrength. Once the spring becomes weakened to a critical point, there is no longer sufficient pressure to keep the worm against the rack; the user has to consciously add the needed force by hand. (Perhaps this too was a later repair. The spring seen in Figure 3 should perhaps be a larger one that fits outside the fitting in which the foot of the spring presently sits.)  It is especially annoying in that it occurs mostly when you are trying to use it normally.  The worm tends to fall away from the rack rather than onto it as does, say, a Hezzanith Endless Tangent Screw.  When turned ‘backwards’, the worm tends to ‘bite’ the rack and assist the spring in holding against it; when turned ‘forward’, the worm throws itself away from the rack and works against the spring.  This tendency was especially pronounced when I first received the sextant and the rack and worm were dry.  After the repair, a proper oiling to the contacting parts greatly mitigated this effect, but it is still noticeable

Another observation on an avoidable issue is the cork padding on the case top braces that contact the ends of the limb and the scope.  In addition to being glued down, they are actually nailed in place.  My example has some shoddy factory work in this aspect as one nailed head was bent over, leaving the edge higher than the cork.  The bronze surface at the left end of the limb is fairly mangled up from years of contact.  Was this really necessary?  Simply gluing it would have been just as effective and far more maintenance friendly for replacement.  Happily, the DS&S logo on the other end has fared much better. Shackman’s were manufacturing jewellers pre-war and probably did not make the case. It may well be that more careful woodworkers were doing war work deemed to be more important, e.g. making the all-wood Mospquito aircraft.


Figure 4: Shoddy workmanship.

The Shackman sextant is, aesthetically, a very attractive piece.  The simple and elegant all-black frame is highlighted only by the bronze of the limb and truly tangent micrometer (and maybe some exposed scope slide), which, while giving a striking appearance, is also quite functional and uniform for usage – the eye is naturally drawn to the scales that need to be read, and in somewhat of a progressive manner.  Also, for a non-engineering, non-optical firm, Shackman’s optics are very good (although they could have been subcontracted).  The telescope is very bright and clear, noticeably more so than those of my instrument by Buff & Buff, a maker known for excellent optics.  It is not a perfect design, as Bill points out, and suffers from some over complexity and confusing ideas that needlessly reduce its functionality in other ways and, quite possibly, its accuracy.  While the solid bronze casting is strong and rigid, its significant weight is felt rather quickly when trying to make an observation.  Still, I find it to be, overall, a rare, unique, and lovely design that makes for a desirable addition to a collection, and I am enjoying becoming more familiar with it.

Thank you, John.




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