Showing posts from November, 2019

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…

Finding Longitude From the Time of Sunset

With accurate time available, there are several ways to find your Lat and Lon without a sextant if you ever lose GPS data. We cover each of these in the book Emergency Navigation. In this note we work through one example, which is just noting the time of sunrise or sunset. The principle behind this sunset method is easy enough to understand, but as we show here there are details to executing it, and a couple different approaches.

The measurement was kindly provided by meteorologist Angeline Pendergrass  during a research voyage on the RV Thomas G Thompson in the Strait of Juan de Fuca. She took the data from the aft deck at a height of eye estimated to be 10 ft above the water.

This type of sight requires seeing the top of the sun (upper limb, UL) disappear below the visible sea horizon. This observation is not quite as common as we might guess, even at sea. More often than not, there is a low layer of clouds on the horizon so we do not get to see a nice clean crossing of true sea hor…

Using Buoy Data and BuoyCAMs to Study Passing Storms

When we learned that Hurricane Florence was headed toward NDBC BuoyCAM 41049, we set up a convenient graphic way to monitor the relative positions of storms to this and other buoys in its track so we could know when to look at the pictures.  The pictures update every hour, so we need to catch them at the right time to learn what we can.

Finally a working system emerged to view all the BuoyCAMs in the form of a KML file that loads into Google Earth (GE) superimposed onto the NHC storm track chart that automatically updated every hour.  We even eventually got it working in smart phones. Then when all was in place and working, sure enough, Florence went by Buoy 41049 in the middle of the night when the cameras could not see it!

For those unfamiliar with the BuoyCAM program, here is a short video overview.

Missing the storm's closest point of approach was a disappointment, but not the end of the world.  The storm had to approach several more data buoys and one more BouyCAM on the way…