Plastic Sextants: Which One Is Best?
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When it comes to plastic sextants, the only manufacturer making reliable instruments is Davis Instruments.
They produce the Mk3 “Lifeboat” sextant, the Mk15 “Standard” sextant, and the Mk25 “Delux” sextant.
Each has its advantages and disadvantages. If you are just looking for my quick assessment, however, I will always tell you the same thing.
The best plastic sextant is the Davis Mk15. It is the best value plastic sextant, offering the same mirrors, shades and adjustments as metal models. Despite its plastic construction, it still offers sufficient accuracy for most situations in which you would consider using a sextant.
Although the Davis Mk3 is the most affordable one in the range, it is hard to get sufficient accuracy to take regular sights. For example, it does not have the micrometer that is included on both the Mk15 and the Mk25.
At the other end of the range, the Mk25 is usually more expensive than the Mk15 and offers a few upgraded options. Its horizon mirror is a “Beam Converger”, or semi-silvered mirror, and the drum has integrated LEDs.
My personal preference, however, is for the split mirror offered by the Mk15.
In terms of price, the Mk15 is usually priced lower than the Mk25, although there are always different prices and offers so you can often find the Mk25 priced lower than Mk15 depending on where you look.
Pro Tip: Selecting the right sextant is an important decision. Before committing, you should read my full guide: Choosing The Perfect Sextant: Which Is Best?
The full range of plastic sextants from Davis Instruments
Davis Instruments manufacture three types of plastic sextant, the Mk3, Mk15 and the Mk25.
Each has advantages and disadvantages, so it is best to look at each one in turn.
Davis Mk3 “Lifeboat” Sextant
Marketed as a “Lifeboat Sextant”, the Mk3 is the most affordable of the range.
It can produce moderately accurate readings by taking measurements along its arc, however, it lacks the fine adjustments of a micrometer drum.
The arm itself has a vernier scale, giving a theoretical accuracy of around 1 second of arc.
For sights, there is only a viewfinder, meaning there is no magnification from a telescope. This is fine for taking measurements of the sun, but you would find it hard to take readings of dim stars.
The horizon mirror is half-sized, meaning that it behaves in the same way as a split-mirror sextant. You line up the reflection of a celestial body with the true image of the horizon beside the mirror.
It is still possible to perform basic corrections on the Mk3, using the adjustment screws on the index mirror, however, there are no such controls on the horizon mirror.
Overall, the Mk3 is well marketed as a “Lifeboat Sextant” because it is inexpensive and does offer a way of measuring the altitudes of celestial bodies.
For training or learning, however, the Mk3 does fall short of what I would deem essential in a training instrument.
It is too different to a “normal” sextant to allow you to practise transferable skills like performing corrections or making fine micrometer adjustments that you would need on another model.
I recommend the Mk3 for experienced sextant users that want an affordable tool they can keep in their grab bag for an emergency.
Davis Mk15 “Standard” Sextant
The Davis Mk15 is the standard plastic sextant manufactured by Davis Instruments. As my current favourite sextant, I have written a complete review which you can read here: Davis Mk15 Sextant Review.
The Mk15 features all the same controls that you find on metal sextants, like the micrometer drum, horizon mirror adjustments, and telescope.
The vernier scale on the micrometer drum offers a theoretical accuracy of 0.2 seconds of arc, a vast improvement on the 1-second accuracy of the Mk3.
Its telescope offers 3X magnification, which gives ample magnification for slightly dimmer stars while still maintaining a wide field of view to assist in taking sights.
The main thing that I like about the Mk15 is that it performs in a similar way as expensive metal sextants, giving beginners an ideal training instrument.
Its accuracy, while not as great as metal sextants, is still perfectly adequate for almost every situation in which you would consider using a sextant.
Davis Mk25 “Delux” Sextant
At the upper end of the Davis Instruments range is the Mk25.
It is broadly similar to the Mk15, with the addition of a full semi-silvered horizon mirror and a long-life LED for illuminating its readings.
Davis calls their full mirror a “Beam Covnerger” mirror, but it performs in the same way as any semi-silvered mirror on other sextants.
The reason that I don’t prefer the Mk25 is that I actually prefer to use a split mirror, especially when using a sextant for training purposes.
I find that split mirrors reflect dim stars better than semi-silvered mirrors.
In addition, I do not use LEDs on a sextant. My reason is that when it is so dark that I cannot read the measurements on my sextant, it will also be too dark to write down the readings. As I need a torch to write down my readings, I may as well use the same torch to read the sextant.
Are plastic sextants any good?
Having established the full range on offer from Davis Instruments, the next most common question I get asked is whether plastic sextants are worth buying at all.
A plastic sextant is a great, cost-effective, alternative to a metal sextant. Plastic sextants offer sufficient accuracy for most purposes while still remaining affordable for first-time users. The reputation of plastic sextants for being less accurate and durable is often misplaced.
Plastic sextants do have a reputation for being less accurate than their metal counterparts. The reason for this is that the plastic frame is more susceptible to wear, meaning things like the mirrors are more likely to become misaligned.
Provided you complete a full set of adjustments before every use, you should be able to counter most of the reduced accuracy.
The rest of the reduced accuracy can be accounted for when you consider the true purpose of the sextant.
A sextant is designed to be used when a boat is away from land and unable to navigate using terrestrial features. As such, a position with an accuracy of a few miles is considered perfectly acceptable.
If you add a little extra uncertainty because you are using a plastic sextant, that is fine.
Whenever you are navigating so close to hazards that a few miles will make a difference, you should not be navigating with a sextant anyway.
In terms of durability, a well maintained plastic sextant will last as long as you need. My only caution would be that they may be a little more susceptible to extremes of temperature than their metal counterparts.
For regular use, on a boat that is sailing in comfortable temperatures, a plastic sextant will be plenty durable enough.
Why I recommend a plastic sextant for beginners
For beginners, I recommend the Davis Mk15 plastic sextant for a number of reasons.
Firstly, as a plastic sextant, it is one of the most affordable options. I think that it is important that no one should feel pressure to spend thousands of dollars on an instrument while still learning.
A plastic sextant provides plenty of accuracy for any use case that will be experienced by a beginner.
Secondly, the Mk15 has all the same controls as traditional metal sextants. You will still be able to practise correcting for perpendicularity and side error, and removing index error.
Additionally, the vernier scale on the micrometer drum allows you to practise taking readings in the same way that you would on a more expensive model.
Despite its plastic construction, I find that the Davis Mk15 allows you to learn how to operate a sextant and will give you accurate enough results to obtain a fix within a few miles.
Once you master the sextant, you can then decide for yourself whether it is worth investing ten times as much in a top-of-the-range metal sextant.
If you need further details about why I recommend the Mk15 for a beginner, you should check out this article: Which Sextant Is Best For A Beginner?
Why I recommend plastic sextants for small boats
On a small boat, I recommend a plastic sextant primarily because of its weight.
Larger vessels provide a more stable platform, allowing you to make better use of a heavier metal sextant.
When you are being thrown around on a small boat, however, it is more important to be able to keep yourself balanced.
I find that it is easier to balance when looking at the stars if my sextant weighs less.
The main disadvantage of a plastic sextant is that it is perceived to be less accurate than a metal sextant. Considering that I am only using the sextant when far from land, many miles away from navigational dangers, I am not too concerned about whether my error is 3M or 6M.
As such, I recommend a plastic model just because it makes taking sights on a rolling yacht so much easier.
For more details about the best sextant for a yacht, you should read: Which Sextant Is Best For A Yacht?