Cruise Training / May 6th

Yesterday, we discussed the origin of Latitude and Longitude.  Today, we want to talk about distance at sea.  If you have not noticed, we use the nautical mile for our distance and the knot for our speed.  A knot is one nautical mile per hour. 

The Nautical mile is equal to about 6076 feet, while the statute mile that we use on land is about 5280 feet.  The nautical mile is very useful at sea as it is derived from latitude.  The earth is about 21600 nautical miles in diameter, or somewhat over 24,000 statute miles.  That means that if we have 360 degrees of latitude to go around the world, that each degree is about 60 miles, (21600 divided by 360.)  There are 60 minutes of latitude in one degree and thus, one minute of latitude equals one nautical mile.  With this in mind, it is easy for the navigator to use nautical miles and minutes of latitude interchangeably. 

The knot is also interesting.  Knot is not short for nautical mile.  It is the measure of the number of knots that pass a point in a set amount of time.  By throwing a log over the side with a knotted rope tied to it and letting the rope pay out for a set amount of time, and then counting the knots, one could tell speed if the knots were spaced correctly in relation to the set time.  Interesting enough, the rope was tied to a log, which is the name used for speed devices today. 

Today we have the Doppler log.  This tells us speed by change in frequency of the echo from the water.  Older speed logs used water pressure or impellors to turn gears like your car speedometer. 

Slowly, each part comes together to develop the complete picture of navigation.  Tomorrow, we will see how time plays into this puzzle.