One leg we are in a hurry, the next one we're dogging it. We knew the trip over to Veracruz would be a lazy one, where we set the speed at 11 knots. So, as it has turned out we carried a little more. So this morning we slowed down a bit more, 40 revolutions per minute of the propeller, which should give us about 9 knots.
Speed of ship is much like speed in an aircraft - where the external forces affect it very much. We've all heard the pilot of the Boeing say 'We are experiencing headwinds, so our arrival will be delayed' and the same is true for ships. Cars, trucks and trains have what is called positive friction with the land, whereas ships and aircraft are operating in a fluid medium 'air and water actually act similarly from a dynamics perspective.
How we determine speed starts with the propeller - on the Kennedy our propeller had a 22.946 foot pitch - or in other words, in one revolution it will drive the ship exactly 22.946 feet ' multiply that by the revolutions per minute and you can compute the ship's distance traveled over time ' or what is known as speed. As we look at our effective speed, we then can compute the 'engine distance' compared to 'observed distance' to figure out the percent efficiency, and a thing called 'slip', where head currents or following currents can change the propeller efficiency.
Yes, we confuse it one more step by using nautical miles (so do airplanes!). Our miles out here are not based upon a British Standard, but by the Earth itself. One mile equals on minute of arc on average of the two great circles on Earth ' the Equator and all the Meridians. So, it happens to be just about 6076.1 feet in length. When the sailor or the aviator travels one nautical mile per hour, we differentiate from land by saying we are moving in 'knots' - or a nautical miles per hour. Wow, stop me now as I start thinking about the age old argument ' is it a knot because of the old chip logs, or simply because to is slang for 'naut' - I simply don't know!