These triangles from Captain John’s Skippers Tips may serve as an easy-to-remember primer, or refresher, on how to determine the basic three factors in coastal navigation: Time, Distance, Speed. Even if your electronics work perfectly all the time (and they won’t), you’ll still want to know how to reach your destination safely without them. Take a look:
These are the basic ‘big three’ in all navigation, whether visual, electronic, or celestial. Time, Distance, and Speed. In order to answer any of these questions, you must know the answer to the other two questions. If you want to find time, you need to know speed and distance. If you want to find distance, you need to know time and speed. If you want to find speed, you need to know time and distance.
Five Easy Steps to Compute Time, Distance, or Speed
1. Draw a large triangle.
2. Divide into three sections, as shown in the illustration.
3. Place a D in the top section, an S to the lower left, and a T in the lower right.
4. Cover the unknown factor with a finger.
5. Divide if you see two vertical factors. Multiply if you see two horizontal factors.
How to Find TIME
You are sailing at 5 knots and you have 13 miles to go to your destination. It’s 1030. What time can you expect to arrive?
Solution:
Known factors: Distance; Speed. Cover up: Time. Note that you have two vertical factors. That means you must divide. Divide Speed into Distance and convert to Time. 13 nautical miles / 5 knots = 2.6 hours = 2h 36m. 1030 + 2h 36m = 1306 ETA (Estimated Time of Arrival).
How to Find DISTANCE
You are sailing at 5.5 knots. How far will you travel over the next 24 hours if you can maintain this sailing speed?
Solution:
Known factors: Speed; Time. Cover up: Distance. Note that you have two horizontal factors. That means you must multiply. Multiply Speed X Time to Find Distance.
5.5 knots X 24 hours = 132 nautical miles.
How to Find SPEED
You are sailing to an anchorage 27 miles away and want to make it there before sunset, which will occur at 1915 this evening. It’s now 1400. What speed do you need to make to get to the anchorage before sunset?
Solution:
Known factors: Distance; Time. Cover up: Speed. Note that you have two vertical factors. That means you must divide. Divide Time into Distance to find Speed.
You must always convert odd times into hours and tenths of an hour. In this problem, we start at the destination time and compare it to current time:
1915 – 1400 = 5h 15m to go to sunset. To convert 15m to hours, we divide by 6 and round off. 15 / 6 = .25 or 0.3 hours. So, the total time to go is 5.3 hours. Now you are ready to complete the calculation.
27 nautical miles / 5.3 hours = 5.1 knots. You need to average at least 5.1 knots to make it to your anchorage by sunset this evening. Easy!
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