60 D Street, the quintessential navigation equation
There are many equations to learn to master coastal piloting,
The equation for finding time, speed or distance may be the most handy and important of them all. We call this equation 60 D Street, as a memory aid. Knowing this equation, and some simple algebra will allow you to know how far you’ve gone, how long it will take to get somewhere, or what speed you actually made getting there. When on watch, you will use this equation all the time.
So, what is it, and how do we work it?
60 x Distance = Speed x Time or 60D = ST
The 60 D Street should be pretty clear at this point. 60D St, all you need to do is add the equal sign and you’ve got the basic equation. 60 times the distance in nautical miles, will equal the speed made good in knots times the time in minutes. So, using some basic algebra we can re-arrange this to find any of the variables we need to know.
D = (ST)÷60
If we want to know how far we have gone in a certain period of time, and we know how fast we have been travelling and for how long, we can use 60D St to solve. Simply multiply the speed made good, in knots, by the time travelled in minutes, then divide the product by 60 to get the distance made good.
S = (60D)÷T
To find out how fast we were actually going over the ground during a known period of time and known distance, we again rearrange the equation to solve. Our Speed made good over the ground, will equal 60 times the distance in nautical miles divided by the time it took in minutes.
T = (60D)÷S
To find out how long it will take to get somewhere, knowing how far away it is and estimating how fast we will go we can again rearrange the equation to solve. First, multiply the distance in nautical miles by 60, then divide the product by the speed we estimate we will make good along the route.
There is a bit of art involved in this when used in practical application. When we estimate how long it will take to get somewhere for instance. We could look at the GPS, plug in our destination and it will calculate our speed over ground at the moment and plug that in to 60D St to get a time in transit, add that to our present time and get an ETA for us. This would most likely be accurate for short distances. However, in many areas of the world, especially the inside passage and puget sound where I work, it lacks a crucial factor. We must take into account the set and drift, or change in our speed and direction made good created by the wind and current. We will cover that, but not here. For now, just keep in mind that we know how to figure what the machine tells us, but we need to add our experience and knowledge to the numbers before we use the ETA or speed our GPS gives us in a log entry or VTS check in.
Fair winds and following seas to you always!