Speed, Time, & Distance

 

In navigation we need to be able to easily solve simple math problems involving speed, time, and distance. I always used to instruct my students: "Just memorize Distance = Speed X Time." Many books talk about using 60D =  S x T and call it 60 D Street as a memory aid. Then, I had a sailing student who taught middle school math and he showed me triangle math. I thought this was pretty cool at the time and I still use it today.

Triangle math showing relationship between speed, time, and distance.

Here is the math triangle. To use it simply put your finger over the letter you are solving for. If you want to know S for Speed, then putting your finger over S gives D/T (distance divided by time). If you want to solve for D then covering it up you have S x T or speed x time.

Let's talk about the units of measurement we'll be using in nautical navigation. Distance is always going to be in nautical miles and tenths of a nautical mile. Time will be in its usual hours and minutes. Speed will always be in knots and tenths of a knot. Remember, a speed of one nautical mile per hour = one knot. So, going at one knot for one hour we would cover a distance of one nautical mile.

D (distance in nautical miles) = Speed (knots) x Time (hours)

But, what if we are traveling at 5.5 knots for 1/2 hour?

S = 5.5 knots (nautical miles per hour) x .5 hours = 2.75 nautical miles. We usually round to the nearest tenth in nautical navigation so we could either drop the trailing 5 or round up to 2.8

The only tricky thing about our equation is that time comes in two varieties; hours and minutes. It's not a simple decimal system like speed and distance. There will be many times when you need to convert minutes to hours and vice versa. Converting hours to minutes; multiply by 60. Converting minutes to hours divide by 60.

Example. What's our distance run if we are going 6.2 knots for 23 minutes? We know from our triangle that since we are looking for D we are going to simply multiply speed times time. But, the formula calls for time in hours so we first convert the 23 minutes to decimal hours:

23 minutes/60 minutes per hour = .38 hours. Now we can plug this into our equation to get distance:

D = 6.2 knots x .38 hours = 2.4 nautical miles.

Just make sure a couple of hand held calculators are a regular part of your boat's equipment!

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