Davis Mark 25 Plastic Sextant

Comes with plastic box, same as Mark 15. Sextant weight is 1 lb, shipping weight is 10.5 lbs

All plastic sextants are inspected at Starpath before shipping.

$195.00   ...item# 1840d25



This product when purchased here includes a complimentary Elibra ebook copy of How to Use Plastic Sextants—with Applications to Metal Sextants and a Review of Sextant Piloting by David Burch. The ebook must be read from a Windows PC. A serial number is provided at the time of sextant purchase. More info.


The difference between this model and the Mark 15 are its lighted dials and a full-view mirror. It is also made from a gray plastic, rather than black plastic used on the Mark 15, but we have no evidence that either plastic is superior in any context. Other specifications and achievable accuracy are the same as the Mark 15. These are obviously enhancements that add value to the instrument, but even at the best price it is beginning to approach the price where one might consider the extra investment and go for the metal sextant ($659). Metal sextant prices hold or rise in value with time, and plastic sextants generally do not. For comparison, the Mark 15 is a better buy if you are looking for the minimum price for a sextant that will look and operate like a conventional metal sextant. Extensive tests we have made show the accuracy and stability of the Mark 25 and the Mark 15 are the same.

Compared to the Mark 3, we feel that learning celestial is better done with a Mark 15 or Mark 25 than it is with the Mark 3, since the latter does not feel nor operate like a conventional sextant. (That is not to say we don't recommend the Mark 3. We indeed recommend owning a Mark 3 for coastal piloting, even if you do not plan on celestial navigation, and we have shown many times that it does a surprisingly good job at celestial navigation underway as well.)

On the other hand, one might still want the Mark 25 instrument if one wants a plastic sextant with full-view mirror, and you want plastic over metal in the first place, not because of price, but because it is so lightweight. Both the Mark 15 and the Mark 25 weigh only about 15 oz. The lightest metal sextant will weigh in at some 2.7 lbs (Astra 3b, for example) and some sextant models go up to almost 5 lbs.

A heavy sextant is difficult to use after the first minute or two, even for the strongest person, and the heavier it is, the more difficult it is. And we should not be mislead by what we read on this. Some magazine writing and advertising for the very heavy models go so far as to state something like "professional navigators prefer the stability and feel of a heavy sextant," but that is frankly nonsense. The lighter the sextant, the easier it is to use, when all other factors are the same. And the more tired your arm becomes, the less accurate your sights become. That is why the very makers of the heavy sextants with those claims, fail to mention in the same section that their own top of the line is in fact an aluminum alloy of the same heavy brass model.

So if someone feels overwhelmed by the weight of metal sextants, but still wants to do celestial navigation, then a plastic sextant becomes very attractive. And if this someone also wants the best, then the Mark 25 would fit that need. For most applications, however, we would guess that the Mark 15 would be the more likely choice for a plastic sextant.

Note that the Mark 25 comes with the "full-horizon" mirror as standard. This is not an option on this model, and it is not our first choice for a primary sextant if you will be relying on celestial navigation. See discussion of mirror types. On the other hand, if your goal in practice or underway involves primarily sun sights, then this is not an issue and one might even prefer this type of mirror.

Compare Plastic Sextants.

7/20/17 

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