Calculating Speed Without GPS!

By Monica, Program Coordinator aboard Adventuress

We're back up sailing in the beautiful San Juan Islands! This recent group was very interested in really learning how to sail, so we accommodated. We incorporated something from the "old" days of sailing. We calculated our speed using something other than a GPS. Here's what we did: Our deck is 101 feet in length. A nautical mile is 6,076 feet. Rounding these two figures off (for ease of calculations) to 100 and 6,000 respectively allows for a very simple (and fun) way to approximate our speed. Mathmeticians will immediately see the formula which is 60 divided by the number of seconds it takes for an object in the water to travel from the bow to the stern of the ship. This simple formula results in our speed expressed in knots. For those non-mathmeticians, here is how you get there... x = the number of seconds it takes an object in the water to travel from the bow to the stern of the ship which is 1/60 of a nautical mile y = x(60) = the number of seconds it takes that object to travel one nautical mile y / 60 = the number of minutes it takes that object to travel one nautical mile z = y / 3600 = the number of hours it takes that object to travel one nautical mile To convert the number of hours to travel one nautical mile into knots (nautical miles per hour) take 1 divided by z. The mathematical laws of cancellation when multiplying and dividing result in the very simple, final formula of 60 divided by x. So any time we see an object drifting towards the ship, we can easily approximate our speed by timing the object as it passes from bow to stern and divide that time into 60. So who needs a GPS to measure speed? Students at the bow would signal students at the stern who were holding a stop watch when they saw something drifting by. As the object reached the bow, the lookouts would lower their arms signalling the ones at the stern to begin timing. When the object reached the stern, they would stop the watch and announce the length of time. We could then very quickly approximate our speed. Pretty cool. Monica