More Sailing Navigation Terms

Elements of Navigation

Determining Your Speed

Knowing your speed is critical for navigating. If you get into the habit of writing the time on the chart every time you bass a buoy, lighthouse or any easily known location, you can calculate your speed.

The basic equation for calculating speed is Speed x Time = Distance and therefore: Speed = Distance/Time

If you passed two buoys two nautical miles apart in thirty minutes, you can conclude your speed to be four knots. The trouble with deducing your speed this way is that you only know your speed for a given section of your voyage after you have completed that section. If your speed is changing frequently because of the weather or course changes, this won't be very helpful. If the fog closes in and you want to know how long before you should start listening for those breaking swells on those rocks on the chart, you will definitely want to be able to determine your speed at any time. In fact, good navigation requires that speed checks be done at least every thirty minutes and more often if your course (and point of sail) are changing rapidly.

The Methods

There's more than one way to discover how fast a pulling boat is moving, but they all have on thing in common: they all measure how many seconds it takes to go one boatlength (approx. thirty feet).

The Compost Technique

Have someone stand by with a stopwatch. Take a small piece of

compost and have the bow watch toss it ahead of the boat and off to

the side about three feet. As the bow passes it, the bow watch yells,

"Mark!" and the timer starts the watch. As the rudderpost passes it,

the helmsman yells, "Mark!" and the timer stops the watch. The

watch now reads the number of seconds it took the boat to go one

boatlength.

Advantages: No equipment needed; can be done anywhere

Disadvantages: Not Leave No Trace; gives speed through the

water (affected by current), not speed over

ground

The Chip Log Technique

You need a floating, non-hydrodynamic object on thirty feet of

cord. The old-time sailors used a pie-slice shaped piece of wood

(hence "chip log"), but a rubber ducky or full water bottle works well.

Have someone stand by with the stopwatch. Flake the thirty feet of

cord on the deck. When the chip log (or whatever) is dropped in the

water, call, "Mark!" and the watch starts. When all the line has run

out and comes taut, call "Mark!" and the watch stops. The watch

now reads the number of seconds it took the boat to go one boat

length (approx. thirty feet).

Advantages: Leave No Trace; can be done anywhere

Disadvantages: Requires a little equipment, and care that the

line runs out freely; gives speed through the

water not speed over the ground.

The Lobster Buoy Technique

As you maneuver through the buoys, look for one that will be right

alongside your course. Proceed as with the Compost Technique,

taking care not to change your course to bring a buoy closer.

Advantages: Leave No Trace; gives speed over the ground

(not affected by current).

Disadvantages: Can only be done where there are lobster

buoys; any significant distance off the boats

course makes readings from the bow and stern

unreliable

Converting to Nautical Miles Per Hour (Knots)

This is easy. All you have to do is divide 18 by the number of seconds on the stopwatch. For example, your speed check gives you one boat length in six seconds.

Here is an example:

18/time on stopwatch=speed in knots

18/6 seconds = 3 in knots

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