Devices to Measure Ship Speed
As ship speed developed by leaps and bounds since men learned to sail across oceans, so as the devices used to measure it. Early sailors like Columbus might have never imagined that time will come that measuring ship speed is as easy as pressing a button. The devices and the methods of measuring ship speed have come a long way since the log and reel method to today’s global positioning and other ship speed measuring devices, indeed.
The log and reel method
The earliest standard of measuring ship speed is through a log or a piece of wood tied to a long take up reel with a rope that contains equally spaced knots on it. The knots on the rope are spaced exactly 47 feet and 3 inches from one another, and together with a 30 seconds hourglass, early sailors now have a pretty accurate device to measure their speed. Measuring ship speed using this device is as easy (or not) as tossing the log or wood off the stern of the ship while turning the hourglass at the same time and holding the rope to count the number of knots that pass within 30 seconds. One knot measures one nautical mile per hour today or equivalent to 1.151 mile per hour on your car’s speedometer. That’s how the knot came to be the unit of measure for ship speed until today.
Pitometer log to measure ship speed
Perhaps the next devices that came after the log and reel were the Pitometer log devices some of which are still used today. Pitometer log devices measure ship speed relative to the water or with the aid of the water that the ship passes by. Among the Pitometer devices (some ships especially submarines still use today) are the Impeller Log and the Propeller RPM devices. The Impeller Log works by lowering or extending a small propeller into the water and measuring the rate of rotation of the propeller. And the Propeller RPM measure ship speed proportional to the ship’s propeller rotation.
The GPS ship speed measuring device
But most ships today even tug boats are fitted with a GPS device that measures and points every mariner’s needed navigational data including ship speed. The device uses satellite and radio receiver and transmitter systems to measure ship speed in real time as it glides on the water. A GPS device is the most accurate device ever to measure ship speed.
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