Showing posts from November, 2019

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, whic…

Dip Short, Distance Off, and Running Fix

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

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

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…

USCG Deck Exam Questions at

The database of thousands of USCG deck license exam questions is one way to get extra practice with most types of navigation problems. We offer a unique interface to these questions to all students signed up in any of our online courses. This free option is not part of our regular assignments, and not all participants choose to use them,  but the resource is available, and we do offer support on the solutions, so long as they are within the realm of navigation or weather. We do not offer support on the questions for deck general or safety, or things like that.

Our interface is unique in that you can chose random questions from any of the groups (Rules of the Road, Deck General, General Navigation, Safety, Navigation), or you can create custom practice quizzes to your own design.

For example, you can ask for all Nav Rules questions the USCG has ever asked on the subject of Barge Lights; or ask for all cel nav questions on amplitude; and so on.

Below is the part of the first page on a q…

Chart Symbols: Rock or Coral on RNCs and ENCs

Discussion reemerged in class about a problem we have had in Quiz 2, #14, for maybe 20 years or more. The discussion was—as it turns out rightfully—questioned by an attentive student.  We have to admit that the distinction between the unlabeled symbols for rocks and coral can no longer be distinguished by the symbol alone.  Years ago when we wrote this question,  this was more or less discernible in that the symbol for an area of coral often used a bold outline, whereas a similar region of rocks used a normal line width outline.

A possible remnant of the older scheme can be seen in the latest Chart No. 1

which shows the coral segment in bold, but with the label coral or sometimes "CO," and now we see above it that rocky area can be bold or plain, with or without label Rocks or Rks.

The other notes on these symbols in latest Chart No 1 is in a supplement,

where bold is used on the top example with a more ragged symbol similar to the International coral symbol, then below it we se…

Near-live Ship Reports by Email

For many years we have offered a free service to obtain near-live ship reports by email request. The process is described at  Until last week, these reports were only available in text format, but now we have added a GPX file of the reports so they can be imported into a navigation program for direct comparison with the latest model forecast—or more generally, just to see where they are relative to you.

Once we are underway, we have only three ways to evaluate the model forecasts: our own measurements of wind and pressure, ASCAT wind speed and direction, and ship reports of direct observations of wind, pressure, and sea state by participating ships.

Our own data are continuous, and with calibrated wind instruments and careful resolution of true wind from from apparent, remain the primary truth meter. Our only requirement then is to be sure to have a log entry of this data at the synoptic times (00, 06, 12, 18z).

ASCAT wind speed and direction are available to…

Vessel Icing: Resources and References

Icing is a crucial factor in marine navigation and safety at high latitudes in cold weather. High winds in low temperatures cause ice build up on deck structures, called ice accretion. It can be a dramatic effect that can sink a vessel very quickly when readily available warnings are ignored or misunderstood.

The parameters that lead to icing are wind speed, air temp, and sea state, which are taken into account in the NWS warnings and forecasts on icing conditions.  NWS maps as well as custom ice maps mark boundaries as well as the propensity for ice accretion, including rates. In worst conditions, the ice can build up at remarkably fast rate.

The blue dots mark the solid ice edge. The line of red rectangles marks the boundary of likely icing, with a one line symbol being moderate potential, and two bars being heavy potential.

No incident brings the point home more than the recent NTSB's detailed report on the Capsizing and Sinking of Fishing Vessel Destination, with the loss of 6…