Predicting Tidal Currents in the Swinomish Channel
Historically there have been no NOAA predictions for the tidal current speed in the Swinomish Channel flowing past La Conner, WA, but local mariners know it can be strong—over 2 kts at times. Years ago we noted that this…
Introduction to Polar Diagrams and Optimum VMC
A polar diagram is a graphic presentation of a sailboat's sailing performance in various wind conditions. But how do you interpret it? The same information is also often presented in the form of a table or spreadsheet, which…
How to Print NOAA Custom Charts
Traditional paper charts are in the process of being replaced with electronic navigational charts (ENC) and a new form of paper chart called NOAA Custom Chart (NCC), based on these ENC. In this new system, the user designs…
Collision Avoidance Using AIS
As AIS becomes ubiquitous amongst boats of various sizes, it's important to know how to use it to avoid a collision. David Burch of Starpath Navigation offers a good tutorial here. Most navigation programs (electronic…
NOAA Discontinues Tide and Current Books — What Do We Do Now?
The short answer for many mariners is: nothing changes — but that takes some explaining. The official NOAA tide and current books of 1995 to 2020 looked something like these below from one particular printer—but there were…
GRIB School: Explaining this Ubiquitous Component of Modern Marine Weather
What is a GRIB File? Use of numerical weather model forecasts by individual mariners is an ubiquitous component of modern marine weather. Almost any navigation program these days connected to a wireless source on land or at…
How to Report Nautical Chart Corrections
One great virtue of electronic charts is their ability to be updated quickly by both NOAA's Office of Coast Survey (OCS), who makes the charts, and then the mariners who use them on the water. Electronic Navigational Charts…
Detecting Fast Winds Aloft… and Why We Care
The winds aloft at about 500 mb (18,000 ft) play a key role in the winds we ultimately feel on the surface. The relationship is discussed in our textbook Modern Marine Weather. In short, the direction of the winds aloft tell…
Correcting Add East—An Overview of Compass Conversions
A true direction is the bearing of a target from your location measured relative to true north (000 T). True north is the direction on the horizon that is directly below the pole of the sky that all stars rotate about. It is…
Introduction to Vessel Running Lights (Navigation Lights)
Navigation lights help identify vessel type and its direction of motion. The required lights are specified in the Navigation Rules, Part C, Rules 20 to 31. Power vessels in Rule 23 and sailing vessels in Rule 25 are the…