How to Tell How Fast Your Boat Is Going

The term knots came from the rope used by mariners on sail boats.

The majority of modern boats have a built-in system to tell the driver how fast the boat is traveling, similar to a speedometer in a car. In the past, boat speed, or knots, was measured in a different way using a knotted rope. If the speedometer on the boat is broken, or if the boat does not have one, it is relatively easy to work out how fast the vessel is traveling using a few basic items, some of which should be on board anyway.

Things You'll Need

• Life preserver
• Rope
• Tape measure
• Stop watch
• Calculator

Instructions

1. Work out how long the boat is in feet. Most boat owners will know this, but if not use the tape measure to find the distance from prow (front) to stern (back.)

2. Attach a rope to a life preserver, or any other floating object. Make sure the knot is tight if you do not want the object to float away.

3. Throw the life preserver overboard from the very front of the boat and hold onto the rope.

4. Start the stopwatch as soon as the life preserver passes the front of the boat and stop it when it passes the stern. Pull the life preserver back in.

5. Divide the length of the boat by the time it took the life preserver to do a whole length. This gives you a calculation for feet per second.

6. Multiply this number by 60 to get feet per minute and by 60 again to get feet per hour.

7. Divide this number by 6,076 to get nautical miles per hour, or knots. Alternatively, divide it by 5,280 to get miles per hour.

Tips & Warnings

• This reading is not going to be 100 percent accurate, but will give a pretty good estimation of speed.

• Wear a life jacket at all times when holding the life preserver over the edge in case of accidents.

References

• PBS Kids: Measuring Boat Speed