Gavan has got just under 1000 Nautical Miles to go. Here Henry Lupton explains why they use Nautical Miles and why it is not metric?


While lots of boaty terminology harp back to the days of the British Admiralty who charted much of the world, the infamous Cap’n Bligh having measured Galway bay, the nautical mile is a bit older and is based on something useful rather than some arbitrary distance between two points. One nautical mile equals one minute of latitude. So for every degree (60 minutes) you move north or south you have travelled 60 nautical miles. 

The world that we want to navigate is almost round and round things are easier to measure in angles of degrees and minutes. Its hard to drop measuring posts all over the ocean so figuring out where you are used to involve measuring the angles to various planets and stars at precise times using a sextant and referral to books of data. Its not much different from how your car GPS measures via satellites at various points above us. 

When the 3 dimensional Globe is transferred onto 2D paper it is done by various projections. The projection most used at sea for nautical navigation is the Mercator projection. Imagine wrapping a sheet of paper around the equator and having a light inside the globe. The parallel lines of latitude like the equator and the tropic of cancer and capricorn for instance and the meridians of longitude, with zero degrees passing through Greenwich in London, are projected as straight lines onto the sheet of paper. This projection preserves the angles of the solo rower as they row along their rhumbline from start to finish. The parallel lines of latitude spread further apart as you move from the equator to the poles which is why the land masses look distorted on some maps and charts. In a boat that is pitching and rolling around it is easier if you simply measure distance directly from that side of the chart rather than doing measurements and conversions.

The electronic charts that Gavan uses have been modernised by the likes of our own Marine Institute using very precise instruments (check out  http://www.marine.ie/Home/site-area/data-services/interactive-maps/interactive-maps ) and make navigation easier. However were Doireann to be struck by lightning he could lose his electronics and would resort to the paper charts and compass that he carries to find his way from mid ocean to a safe part of land.

The speed of the boat is referred to in knots or nautical miles per hour. They used to gauge it by throwing a weighted rope with spaced knots over board. An hour glass is spun and the line is thrown overboard and if 4 knots run in the time for the sand to run out means you are doing 4kts.  Modern boats use a spinning paddle wheel or GPS and the nautical mile is now an SI unit based on metres. Gavan relies entirely on GPS so If the Donald decides to play games with the GPS satellites we may have to go look for him! Roll on the EU Galileo satellite system.

1nm = 1.1 statute miles = 1.85 km

Anyone interested in watching the wind where Gavan is right now check it out here... https://www.windytv.com/?26.426,-44.723,