What Do You Need to See How Fast Your Boat Is Going?

Nautical Miles

• Nautical miles are the unit of measurement for distance over seas. Nautical miles are actually about 1.151 miles, or exactly 1.852 kilometers. This value comes from how maps of the world are drawn. Because the world is roughly circular, sailors set the circumference as 360 degrees, each of which were divided into 60 "minutes" (which, as you would think, are then subdivided into 60 "seconds"). Sailors found it useful to measure distances in terms of these minutes of arc. Thus the distance of a minute of arc around the world (at the equator or the meridian) became the nautical mile.

Knots: Speed Measured by Nautical Miles

• To measure their speed in nautical miles per hour, sailors contrived a rough method of calculating the distance they were moving over a given period of time that allowed them to figure their speed with a degree of error of less than 2 percent.

  By dragging a rope tied to a piece of wood, called a "chip log," behind their ships, sailors would measure the number of evenly spaced knots tied in the rope separated by distances proportional to the nautical mile. The unit of speed came to be known as 1 knot, thanks to the knotted-rope method they used to measure their speed.

The "Knots" and Bolts of Measuring Your Own Boat's Speed

• To measure your boat's speed using the same method as sailors of old, take a long piece of rope and secure it to a flat, quarter-circle-shaped piece of wood, connected at three points: the vertex and the ends of the curved part of the wood. The middle of the curved section should have a weight attached to the middle of the curve. Tie knots every 47 feet (the modern standard is actually 47.25 feet, but for many years sailors used varied lengths from 33 to 50 feet) down the rope from the wood. To measure your speed, throw the chip log overboard and begin timing.

  The weighted side of the log will cause the board to stand vertically, increasing water resistance so it will drag and pull the rope from the boat while remaining relatively stable in place. Make sure the rope runs freely into the water. Count how many knots pass into the water over 30 seconds, and that number represents a rough estimate of your speed in knots. If it takes exactly 30 seconds to reach one knot spaced at 47 feet, that means you're moving at 1 knot.

Common Speed Conversions

• 1 knot =

  1.150779 miles/hour

  1.85166 kilometers/hour

  1.687810 feet/second

Sources of Error

• Though this method is quite effective, some factors can throw off your calculations. Physical conditions, like currents in the water or a wave behind the boat going in the same direction as the boat (called a "following sea") will both complicate the measurement. Also, stretching in the material of the rope and inaccuracies in the time-keeping device will change your measurements. The simplest way to solve this problem is to conduct your measurement multiple times and average your results.