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Boating Basics Dead Reckoning using Speed and Log
Using a tachometer to know where you are
Knowing how fast you are moving through the water is another key factor in the navigation equation. Steering a compass course for a known amount of time can't in itself be of much use to a navigator, but if the speed is known, the navigator can calculate how far the boat has traveled in a time period and plot it on the chart. Plotting direction and speed on a chart is called dead reckoning.
There are many mechanical and electronic devices available that determine boat speed. Many of these devices also have a log feature that automatically records the distance traveled.
Power boats with a tachometer that measures engine revolutions can also use it as a gauge to determine speed. Calibration of the tachometer can be done by motoring between two known points on the chart at a fixed engine speed and recording the time it takes. Since the time and distance between the aids to navigation or landmarks are taken from the chart, you can calculate the speed by dividing the distance by the time.
For example, in many harbors there is a measured mile painted and marked off on a bulkhead or breakwater that corresponds to one on the chart of the area. You record the time as you pass one of the marks. Let's say it takes you ten minutes to get down to the second mark. If you divide the distance by the time, you will get the speed of your boat (S = D/T). Since the formula deals in hours and we have recorded minutes, the formula is modified to S = 60D/T. So the boat is moving through the water at 6 knots per hour (S = (60 X 1)/10). If there is any current, run the same course at the same speed in the opposite direction and average the speeds.
Record the calculated speed opposite the engine RPM from the tachometer. Repeat the process at different engine speeds and you have a table showing the speed of your boat at these engine speeds. This chart will remain accurate as long as you don't drastically change the loading of your boat and keep the bottom and propeller free of marine growth.
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