Saturday, June 16, 2018

Davis Mark 3 Sextant Part 1 — How to read the Angle Scales

This is one of several notes with associated videos on the use of the Davis Mark 3 sextant. We have a more general book on How to Use Plastic Sextants, but now we are focusing in on the Mark 3.  The reason for this focus is a bigger challenge we set for ourselves in our new booklet that teaches mariners how to use a sextant and find position at sea with no previous training at all. In fact, to the extent we succeed, you do not need this article or video! Just get the book and open it when you need it. To that end, we put together a kit that includes one of these sextants, this new booklet, and a few other things to serve as a GPS Backup Kit.

But for now, however, we address those who, for whatever reason, wish to use a Mark 3 sextant. There is a manual that comes with the Mark 3, rather detailed even, but it is our experience from teaching cel nav for so many years to so many thousands of students that the stock manual is not enough. So our GPS Backup Kit includes the book below, which explains how to use the Mark 3, which must start with how to read the scales, the subject at hand.

Back and front covers of our new book, available in print or ebook format.

Later we address how to calibrate the sextant and take sun and star sights, but now we just look at how to read the scales.  Below is the angle the sextant measures, called sextant height (Hs).

Below is a picture of the Mark 3 with parts identified. There are two adjustment screws (#1 and #2) which we discuss later. The angle we measure Hs will be in the form 39º 20', which is about what the one below is set to. 

We read the degrees part of the angle from the arc scale, and the minutes part of the angle from the vernier scale at the bottom of the index arm that slides along the arc. A vernier scale is a way to estimate fractional positions between two lines. The linear version we use today was invented by Pierre Vernier in the 1630s, likely based on a circular version used by Portuguese navigators in the late 1500s. The vernier scale has interval separations slightly larger than those on the scale it is interpolating. In the picture below we see that 30 intervals on the vernier scale span 31 intervals on the arc scale.

Start by looking at sample A below (click the pic for a better view). This is a reference showing what 0º 0' looks like. The checks we make for sextant calibration at each sight (index error) will be just a slight variation of this alignment, as discussed below.

Click the image for a bigger view.
The degrees part of any sight is read from the arc scale relative to the 0' mark on the vernier scale. In sample B we see from the degrees scale that the angle is bigger than 32º, but somewhat less than halfway to 33º. In other words the minutes part of the angle will be less than 30' It is always valuable to estimate what the minutes are before actually checking to see what they are. In this case, for example, you might decide is this just a bit bigger than 32 or is it almost 33 or is it near halfway, just below or above halfway, and so on.

Use the vernier scale to to get a better measure by finding which of the tic marks on the vernier scale most closely lines up with any of the degree marks above it. Zooming in on the image (which we do with a small magnifying glass when underway) we see that 20' or 22' could be considered aligned, with 18' or 24' definitely not aligned. Note too that the out of alignment marks will be off in the opposite direction on either side of the best aligned one... or maybe two, as in this case.  We called this one 22, but you could argue in this case that 21' might be best, since 20 and 22 were pretty close.

In Sample C this is a little easier with the degrees being almost 32, but not quite, so degrees part is 31º and the minutes alignment is best at 46'. Again, notice that the 45' and 57' are off in opposite directions.

A possible blunder to make in these measurements is to count the degrees scale backwards. In sample C that would be reading the angle as just bigger than 28º. It pays to double check we are doing that right. In other configurations it could be more misleading.

Another challenge we face is when it looks like the degrees line up exactly as in sample D. It would be a mistake to call this 28º 00' and go on.  When the degrees line up very closely (as they will with all of the index error measurements) then we must turn to the vernier scale to see if it is large minutes or small minutes that line up. Small minutes alignment means you are just over 28º; large minutes alignment means you are just under 28º and the actual degrees part is 27º, not 28º. Zooming in on D we see that 4' is the best alignment, so the angle is 28º 4'.

The next three samples are what we see when measuring the index correction, discussed later. In sample E we can see from the degrees scale alone that we are just above 0º and checking the vernier we see the amount above is 6', with 4' and 8' off in opposite directions. This value of Hs would be 0º 6'. When doing an index correction measurement we would call this 6' "On the scale."

In sample F we have similar case, but in this one it is easier to see that both 4' and 6' are equally unaligned but they are better than all the others, so this would be called 5' On the scale.

In sample G we cannot tell from the 0º alignment if this is above or below 0, so we check the vernier to find that the 52' mark is best aligned, and again we check that the alignment on either side is off in the opposite directions.  With these large minutes aligned, we have effectively -1º + 52' = -8'.  In other words, this alignment is just 8' Off the scale, which is how we would record it.

For index corrections we have then either small minutes aligned which are called "On the scale" or we have large minutes aligned and we subtract that from 60' and call the result "Off the scale."

That is how the scales are read. This must be done carefully if we want to get out the full potential of the Mark 3 sextant. A small magnifier helps.  Also we stress multiple places that whenever possible we should not rely on just one measurement. For good work we should take 3 or 4 sights each time so we can average the results.

We will add more articles and videos on the use of this sextant, but they will now assume we know how to read it.

Friday, June 15, 2018

Race to Alaska (R2AK) Navigation

A question came up in our online nav course about R2AK navigation. As it turns out we have worked on this route in great detail a couple years ago as we assisted the team MADDOG in preparation for their record setting race. I started to answer this longish question in our class discussion forum, but decided it could be of broader interest, so I put these notes here.

All such planning starts with the waypoints. We made this set for the full race that the team then transferred into their two handheld GPS units and also into Navionics "Boat US and Canada" app on two iPhones and one Android phone. This app for about $50 includes all Canadian charts. They are not as good as the official Canadian echarts, but those cost $200 or $300 from EC... although there is a very good set from RosePoint Navigation for $99 the last I Checked, but these may only work on Coastal Explorer.

PS. There are some tricks to getting an external gpx file into a mobile app version of Navionics. On the other hand, it is relatively easy to get them into a Garmin handheld.

R2AK-1.gpx is the full route Port Townsend to Ketchikan, with logical waypoints numbered and named from start to finish.  R2AK-2.gpx and R2AK-3.gpx are two short alternative legs if the main route has bad weather in these regions. [ Might have to right click and choose save, else you might look directly at the xml files.]

These files can be loaded into OpenCPN or other enav program and then studied in on the screen and on the routes manager display. Without Canadian charts, however, the details are hard to discern. The WA and AK charts are free downloads.

You can also drag these gpx files onto Google Earth to see what the routes look like in detail. There is some thought that there could be very many routes to Ketchikan, and probably so, but there is a logical direct route if weather permits.

Next we used current predictions from the Canadian Current Atlas that we customized and made into ebooks that could be loaded into their phones. Then each day and hour is a book mark, easy to find. Recall that MADDOG was all navigated from the trampoline of an open catamaran. No nav station and no other electronics.

Once into AK waters, we used our own publication called Southeast Alaska Current Atlas, and did the same ebook layout with that data. Both of these convenient current presentations paid off.

This was an unassisted race, so they had to get their weather info underway themselves, but we could run a day or so out at least as starting points. We considered the key was getting to Seymour Narrows on time, so this was focused upon.  They hit the Narrows, exactly as the current turned in their favor, flying by at 20 kts as the fishing boats hanging out for the right time were pulling their anchors.

Now we have better wind data than we had at that time.  For US waters WA and AK you can use the HRRR (our 19 hr, updated hourly) or 3-km NAM for longer runs.  The best data once into Canada would be HRDPS - High Resolution Deterministic Prediction System, which is 1.3 km run every 6h out to 2 days.  You can get that from LuckGrib on an iPad and transfer it to Expedition for routing... or just look at it on the luckgrib app. OpenCPN and other popular nav programs may not be able to read it.

Our new book Modern Marine Weather 3rd edition includes detailed discussions of latest high resolution models.

If questions come up on this navigation, then post a question here or in the classroom.

We have other more general weather notes on the inside passage at