CAPTAIN’S LOG

21 January 2015

We are passing the southern coast of the island of Montserrat this morning. Nothing too special, except you can clearly see the lava flow, from volcano crater to the sea. Just a river of rock and gravel meandering down thevalleys. Kind of cool, but probably not if you lived there a few years ago when the mountain top blew off!

Yesterday I was commenting upon steaming rates and distance.  When I mentioned “knots” for speed I wondered if all the readers followed that. So, knowing I talk about that occasionally, I am going to “re-send” a Captain’s Log
from 17 January 2011 – four years ago. Let’s call it a “classic log”. “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 one 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!” Hope you enjoyed this detailed explanation, again!