PBS KidsZOOM Home Back to Activity
Science Rocks!

Measuring Boat Speed


your results

Sent in by:
The ZOOMers

A cool, calculating, speed-measuring trick.

Materials Needed

  • tape measure
  • life preserver
  • rope
  • stopwatch
  • calculator
  • pencil and paper



  1. When the ZOOMers went on a boat trip, they used a life preserver, a rope, and a stopwatch to figure out how fast their boat was going. You can do this, too.
  2. First, you need to know how long your boat is. If you don't already know, measure it with a tape measure to find out how many feet long it is.
  3. Next, you need to time how long it takes your boat to pass something that's staying still in the water - like a life preserver. You can do this by tying a life preserver to a rope, and throwing the life preserver overboard from the front of the boat.
  4. As soon as the front of the boat passes the life preserver, start your stopwatch. Stop your stopwatch when the end of the boat passes the life preserver. Then use the rope to haul the life preserver back into the boat.
  5. Now you know how long it takes your boat to travel one boat length. But you need to do some math to figure out how many miles per hour your boat travels.
  6. Start by dividing the length of your boat in feet by the number of seconds it took to pass the life preserver. This will give you the number of feet your boat travels in one second.
  7. Multiply this number by 60 (since there are 60 seconds in one minute) to get the number of feet your boat travels in one minute.
  8. Multiply by 60 again (since there are 60 minutes in one hour) to get the number of feet your boat travels in one hour.
  9. Now divide this number by 5,280 (since there are 5,280 feet in one mile) to get the number of miles your boat travels in one hour. Is it faster or slower than you thought it would be?
  10. You just found out how many miles per hour your boat travels. But most of the time, boat speed is actually measured in nautical miles per hour. Nautical miles are based on the distance around the earth at the equator. There are 5,280 feet in a mile, but there are 6,076 feet in a nautical mile. So to calculate your speed in nautical miles per hour, go back to step 9 and instead of dividing by 5,280, divide by 6,076.
  11. Nautical miles per hour are also called knots. That's because sailors used to measure boat speed by tossing the end of a knotted rope overboard. As the water pulled more rope into the water, sailors would count how many knots went by during a given time period. The knots were spaced out so that each knot represented one nautical mile per hour.
  12. If you don't have access to a boat, you can use a similar method to find out how fast a river is flowing, or to calculate how fast you can ride your bike. Test it out, and be sure to send your results to ZOOM!

Some of your Results

Kolton, age 9 of Penticton, BC wrote:
I tried it with my bike and it went 7 miles per hour.

Lucas, age 11 of Ashland, MA wrote:
I have a friend, and he lives in Martha's vinneyard and he tried that Measuring Boat Speed AND THAT REALLY WORKS!!!

not yet implemented