De-MYTH-tifying Sextants  

 

From time to time, a navigator will call our shop seeking parts for his marine compass without knowing its make or model. With the instrument miles away-and with no one onboard to check things for him-we get regaled with such comments as:

"It's just your regular globe-type compass; you know, four or five inches across." Well, that does narrow it down to about 100 models, but. . .  Then, there's the old, "Hell if I know what kind it is. It's whatever they put on a 1947 XYZ runabout. You're the expert; you tell me!"

Of course, this is much like calling a wrecking yard and asking for a rebuilt engine without supplying the salesperson with make and model of car into which the engine is supposed to go.

As confusing as all that might be, even greater problems can arise when trying to address sextant lore with individuals who have spent most of their lives as professional-or near professional- sailors.

Through my experience as a Navy Opticalman, and repair department manager for Captain's Nautical Supplies, I have repaired or restored hundreds of sextants. In so doing, I have seen the same bits of misinformation concerning them crop up again and again. Hopefully, the following will dispel some of the myths surrounding their use and maintenance.

Myth #1: The Accuracy Certificate Remains Valid Unless The Sextants Sustains Major Damage.

After each new sextant has been finished and tested, imperfections noticed in the arc, along with information concerning the time and place of manufacture, are documented and placed inside the top off the case. This information allows the navigator to take these imperfections into consideration when taking sights in the affected sections of the arc.

Unfortunately, many mariners seem to feel this data remains consistent throughout the life of the sextant-even if it's several decades old. If, however, the instrument has served as more than a paperweight during all that time, the errors will have changed many times. Substantially? No, but the perfectionist navigator should know-at least for the sake of conversation.

Myth #2: Local Instrument Shops Can Certify My Sextant.

It is often assumed that each time a sextant is repaired a new performance certificate should be issued. This would be so in a perfect world. Unfortunately, it is not practical for local repair shops to provide this service. Most facilities have neither the personnel nor equipment to perform the necessary tests, and those capable of doing the work will often charge more for running the tests than for executing all the other repairs combined.

"So, what do I have to do to get a new certificate, bend the frame?" Yes, that will work just fine. Unfortunately, it will come with a brand new sextant attached because it is usually not cost effective to bother with a sextant once the frame has been warped, dented or bowed.

New certificates come from the factory and there are only a couple of reasons to assume that the sextant warrants exhaustive repair and re-certification. The first reason would be that the teeth of the rack or worm gear have been damaged by an impact or, secondly, if there has been a shift in the pivot bearing and pivot.

[Many navigators worry unduly about the latter happening. If it should occur, accuracy would be destroyed all along the arc. However, in 34 years as a repair tech, I have only seen this situation four times; three of the four were due to over-confident navigators owning a screwdriver.]