Sometimes referred to as a “lifeboat” sextant, the Davis Mark 3 sextant will enable the beginner to learn and practice the rudiments of celestial navigation at the lowest possible cost. It is made of heavy gauge plastic with an adjustable half mirror (which is equivalent to a traditional half silvered mirror). Although it is without magnification or illumination, it still works just fine. The vernier scale is a little harder to read closely than the micrometer scale on other sextants. It can also serve reasonably well as a back-up emergency sextant.
A forerunner of this instrument was developed for emergency use during WWII, called the “Lifeboat Sextant.” This is the least expensive sextant in the world that will provide dependable accuracy adequate for actual ocean navigation. With good procedures you can obtain some 5 to 10 miles position accuracy routinely, and better with practice. All replacement parts (even the screws!) are readily available. Note that there are no optics nor telescope and the vernier can only be read to the nearest 2′, so you need to take multiple sights and average them to obtain good LOPs and the fixes themselves must be made from 3 bodies properly selected. This sextant is popular as a back-up or learning sextant, but aside from these celestial applications, it is actually the sextant of choice for inland and coastal piloting. Using the sextant horizontally, you can find your position on a chart much more accurately than you can by other conventional piloting techniques such as compass bearing fixes.