56 seamanship secrets
me an ETA, why go through this calculation?” First, it’s always good to
check the result of one navigation method against the result of another.
And second, a manual speed-time-distance calculation will give you an
ETA even when you have no working GPS receiver. All you have to do is
estimate your boatspeed some other way—for example, using a speed/
RPM table.)- To fi nd the distance traveled from speed and time en route
Set the speed factor fi rst. Hold it lightly in place with your nondominant
thumb to allow the large wheel to turn underneath.
Turn the large wheel. Watch the time window as you turn it, and stop
when you reach your time en route.
Before reading the distance, check the speed window to make sure it
hasn’t shifted. Adjust as needed. Then read the distance.
For example: In this example we do not know our distance traveled.
It’s 1600 in the afternoon. We’ve averaged 4.5 knots under sail since 0930
this morning. How far have we traveled?
Because you do not know distance, set the speed window first.
Hold it lightly in place and turn the large wheel until the time window
reads 6.5 hours. Check both windows for alignment and then read
your distance: 29.25 nautical miles. Round this off to 29, since the
precision of the method is not high enough to carry to two decimal
points.
Harness the Power of the 3-Minute Rule
When you need quick solutions and your nautical slide rule isn’t handy, try
using the 3-minute rule. Find the distance (in hundreds or thousands of yards)
your vessel travels in 3 minutes and move the decimal two places to the left. For
instance, if you travel 2,000 yards in 3 minutes, your speed is 20 knots. Travel
620 yards in 3 minutes and your speed is 6.2 knots.
To calculate distance traveled, simply reverse the rule by moving the decimal
two places to the right, adding zeroes as necessary. If your speed is 4 knots,
you’ll travel 400 yards in 3 minutes. If you are traveling at 18.5 knots, you’ll
travel 1,850 yards in 3 minutes.
Use the same method with one extra step for fi nding 1-minute increments
of travel. For instance, if your speed over ground is 4 knots, in 3 minutes you’ll
travel 400 yards. Divide 400 by 3 and you’ll cover 133 yards every minute.
Now try a few examples.
If your speed is 4.6 knots, how far do you travel in 3 minutes? By moving the
decimal point two places to the right, we get an answer of 460 yards.
If your speed is 22.7 knots, how far do you travel in 3 minutes? 2,270 yards.