Posts

Free Barometer App Designed for Mariners

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Not many mariners realize that the most accurate barometer on the boat might be in their pocket... in their cellphones. There are not any native functions in iOS or Android phones that show the pressure reading, so you would not know it is there by just surfing around the functions of your phone.  It is not listed anywhere.  A third party app is needed to test that you have one.  There are many free versions of such apps, which brings up point one. Why do we make another one when there are so many already?

We want to let folks know about this tremendous resource, but then we are asked for an app recommendation, and the problem begins. Until now, the apps have all been too complicated. They want to do more, so they end up doing things like getting your elevation from the GPS, which even with a WAAS satellite connected is not accurate enough for barometer work adaptable to marine navigation.  In these days of remarkable weather services via model forecasts, we want the pressure accurat…

Weems & Plath Expanding Square SAR Course Identifier #113

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A common search pattern used in search and rescue (SAR) operations is called the expanding square pattern, which is just what it says it is. You travel along calculated route legs in a pattern that expands every 3rd leg so you systematically cover the the search area in an expanding pattern, as shown below.




To plan out such a pattern you need to know your (1) initial heading, (2) the distance you want between the squares, (3) your boat speed, and (4) the time you plan to start. 
In one sense, the next logical step is turn to your electronic charting program, put in the way points, select an average speed and start time, and look at the resulting route plan, which will tell you the headings and times to turn onto each leg.  Then print that plan or take a picture of it with your cellphone.  In fact, the route itself will be laid out on the chart plotter and you can just follow it.
Or you could simply start off with the boat's track showing, and create the route from your displayed a…

Race to Alaska (R2AK) Navigation

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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 MAD Dog 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 ot…

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

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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, whic…

Dip Short, Distance Off, and Running Fix

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We use dip short primarily for practicing with cel nav sights on land using a shoreline of a lake or bay that happens to be too close to use, so the shoreline is blocking out the true sea horizon.  We can determine if a shore is far enough away to ignore this issue using the square root of our height of eye (HE) above the water surface.  The minimum distance for a good sea horizon is D in nmi must be greater than the Sqrt (HE in feet).  Standing on a dock with HE = 16 ft, if the shoreline I use for a horizon is 4 nmi or more away, then I can use that shoreline as if an open ocean horizon.  Nothing special would be required for cel nav sight reductions.
Indeed, for D > Sqrt (HE) we are not actually seeing the shoreline; the curvature of the earth is blocking it and we are seeing a true horizon.
Beyond its use in sextant practice on land, dip short remains a practical tool for the navigator's bag of tricks. It was more important to large vessels that relied on cel nav than it is…

Finding Watch Rate

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For celestial navigation we need to know UTC, formerly called GMT. We have many ways to get accurate UTC on land with internet connections. Offshore we need either an HF radio or satphone or we need to have a watch whose rate we know.  All watches gain or lose time at some rate. A chronometer is a watch whose rate (gaining or losing) is constant. The cheapest quartz watch has a rate of a few seconds every 10 days, and it is constant, which qualifies it as a "chronometer." A very expensive quartz watch might half that rate.

In another note and video we show various ways to get accurate time and demonstrate that these various sources do indeed give the same time, which took some coordination of sources. That article includes a 2015 rate measurement of a Timex quartz watch, done in a way similar to what is described here.

For now we illustrate the process of measuring the rate of a watch, in part to support our GPS Backup Kit that includes a rated watch. This shows the method w…

Note on USCG Exam Questions

USCG deck license exams are made up from their database of questions. In the past, every few years they have made up a dozen or so exams for each license or upgrade, covering a list of specific topics, with different numerical values in each of the exams.  For those who need to pass one of these exams, the traditional method of study is to get access to the data base and work through enough examples that any random sample of that type of problem can be solved. The test time is limited, so this type of practice is crucial. There is not enough time to figure out what exactly they want, and how to best solve it.

The deck questions are in these groups. There are a couple thousand questions in each set.

•Rules of the Road
•Deck General
•General Navigation
•Safety
•Navigation

As a historical note, all mariners owe a debt of gratitude to Richard Block of Marine Education Textbooks in Houma, LA, a company that remains Central Headquarters for license preparation training materials. In the late 70s…