From the Chart Table

Calculating Speed Distance and Time

Often called “Ded Reckoning”, a shortening of the term “Computed Reckoning”, if we know two things, we can calculate the third. A useful diagram to help us do the math is shown below.

Speed (S) is measured in knots. One knot is equal to 1 nautical mile (nm or D) per hour (T). 1kn = 1nm/hr.

Distance travelled (D) can be found by multiplying our Speed (S) in knots by the running time (T) in hours (hrs).

Our Estimated Time of Arrival (ETA) can be found by dividing the Distance (D) in nm to our Destination by our current Speed (S) and adding that number to the current time. So if at 1300 hrs we are 5nm from Port and we are steaming at 5kn we will be running for 1 hour and arriving at 1400 hrs. (1300 + 5/5 = 1400).

If we jumped in the boat and sailed for an hour at 8 knots, how far would we have travelled (D)?

Our Distance (D) would equal our speed (8) multiplied by our time in hours (1) = 8nm.

Here are a few practice questions:

  1. How long will it take us to get to the fishing grounds 5nm east of the bar if we travel at 10 knots?
  2. What if we go a bit slower at say 6 knots? (T = 5/6)
    Well – 1/6 of an hour is 10 minutes, so 5/6 will be ? Enough time to get the gear ready anyhow.
  3. What is your average Speed when you sail a Distance of 120nm in 10 hours? (S = 120/10)
  4. Coffs Harbour is approximately 83nm from Port Macquarie. If we leave Port at 0600 and maintain 6 knots, what is our ETA for Coffs? (T=83/6)

When you are sailing single handed on a dark night and your GPS has given up, it can be very comforting to see the Tacking Point light on the horizon 10° off the port bow, just as your ‘ded reckoning predicted.

 

Navigating the NSW East Coast – Know Your Coordinates

Navigating the East Coast of New South Wales requires more than just a good eye—it demands a clear understanding of marine coordinates and local reference points. Whether you’re a recreational boater or a seasoned skipper, working confidently with GPS coordinates is essential for safe travel and effective communication with Marine Rescue.

Understanding Coordinates

Marine coordinates are expressed in degrees of latitude and longitude.

Latitude measures how far north or south you are from the equator. The equator has a Latitude of Zero degrees. The North Pole has a Lat of 90 degrees North or +90. The South Pole has a Lat of 90 degrees South or -90. The convention to label the southern hemisphere as negative degrees has arisen in the digital era through the use of GPS coordinates. Latitudes are parallel. They never converge.

Longitude measures east or west of the Prime Meridian – the zero degree of Longitude, which passes through Greenwich village in the UK. From this convention we get the term for GMT (Greenwich Mean Time). The meridians of Longitude track the revolution of the earth over 24 hours. The meridians converge at the North and South poles. Does this mean that time stands still at the Poles?

On the NSW coast, you’ll generally be navigating between 28°S to 37°S latitude and around 153°E longitude.

Key Reference Points for Coordinates

  • Point Danger (-28° 10.000′ S, 153° 33.000′ E)
  • Port Macquarie (-31° 25.85′ Long: 152° 54.53′ E)
  • Port Stephens (-32° 42.000′ S, 152° 10.000′ E)
  • Sydney Heads (-33° 50.000′ S, 151° 16.000′ E)

The coordinates above are given as Degrees and Decimal Minutes. Coordinates can also be given as Decimal Degrees or the more traditional Degrees Minutes and Seconds.

The coordinates for Port Macquarie when given as Decimal Degrees are Lat: -31.430833°,  Long: 152.908833° and Degrees Minutes and Seconds are: Latitude: 31° 25′ 51″ S Longitude: 152° 54′ 31.8″ E. Note the symbols ° for degrees, ‘ for minutes and ” for seconds.

Tips for Working with Coordinates

  • Always cross-check: Use a marine chart or digital charting app to verify your GPS location.
  • Practice giving coordinates clearly: Speak slowly and repeat if giving them over the radio.
  • Mark key points: Save waypoints on your GPS to track safe paths, anchorage zones, or hazards.

Knowing your location precisely can mean the difference between quick assistance and a prolonged emergency. Make coordinates a routine part of your checks.

References:

https://latitude.to/map/au/australia/cities/port-macquarie