Sailing Navigation Essentials – Do You Know This “Magic Triangle”?

Here’s another great navigational tip from our friend Capt. John of skippertips.com

Sailing navigation begins–as does all of navigation–with a good grip on how to use time, distance and speed. Indeed even the most advanced navigation system uses these same elements to tell you where you are, calculate the direction you need to steer, and estimate the distance and time you will arrive at a point some time in the future.

I believe all seamen need to know how to do this with just a pencil and paper. It keeps you sharp, helps you check the black box, and will make you a more well-rounded sailing navigator in all regards. Here’s my method, taught to thousands over the years. Try it out and see how it works for you!

Are We There Yet?

Sailors always want to know one of three questions: What time will you arrive somewhere, how much further do you have to sail (on one particular course or to a specific destination like an anchorage or port), or how fast are you sailing.

These are the basic “big three” in navigation. Time, Distance, and Speed. In order to answer any one of these questions, you must know the answer to the other two questions. For example:

If you want to know how much long it will take you to sail somewhere (time), you need to know how fast you expect to sail (speed) and how much further you have to sail (distance).

If you want to know how far you can sail (distance) in a specific period, you need to estimate how fast you can sail (speed), and much time you desire to use in that specific period (i.e. each day, during daylight, etc…).

If you want to know how fast you are going (speed), you need to know how far your traveled between two points (distance) and how long it took you to travel that specific distance (time.).

As you can see, each of these are used by all of us on a daily basis. For example, let’s say you are driving to work. Enroute, you decide to pop in to the local Starbucks for a cup of coffee for the road. You keep glancing at your watch as you stand in line. Why?

You are doing a time, distance, speed calculation. You have a certain “distance” remaining to get to work and you can only go so fast (speed) in street traffic. So, you might determine that you have just 15 minutes (time) until you need to be on the road again to make it to work.

Now let’s go onto the water with some realistic examples and see just how easy this can be:

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 (for distance) on the top section. Place an S (for speed) on the bottom left and T (for time) on the bottom right.
4. Cover the unknown factor with a finger.
5. Divide if you see two vertical factors after you cover the single factor. Multiply if you see just two horizontal (bottom) factors. Work through these examples:

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 are able to 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 .3h. 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!

Captain John’s Sailing Tip
Here’s a cool tip from Bill Brogdon, author of ‘Navigation for the Rest of Us‘. Want to know how long it will take to travel one nautical mile? Divide the number 60 by your sailing speed. Sailing at 4 knots? You’ll cover one mile in 15 minutes (60 / 4). Going 5 knots? It’ll take just 12 minutes to sail one mile (60 / 5).

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Now you know how to find your time to an anchorage, distance traveled, or sailing speed fast and easy with the magic triangle. Practice this often to “keep the rust off” and increase your sailing safety on the waters of the world–wherever you choose to go sailing!

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