How Does A Boat Speedometer Work?

The other day I was working on the boat and I noticed the speedometer on the stern of the boat was flipped up because it hit the rack I keep the boat on. I adjusted it so it would work properly the next time I am out in the boat, but this got me thinking… How does a boat speedometer work? So I did some research and thought you may find it interesting too.

So, how does a boat speedometer work? There are two popular speedometers on the market. The Pitot Tube type speedometer and the Electric Paddle Wheel type. The pitot tube design uses water to compress air inside of a tube. This pressure is measured and converted to boat speed. With the electric paddle wheel, the paddle wheel is turned by the force of passing water and the speed is registered and sent to the speedometer gauge at the helm.

Both the pitot tube style boat speedometer and the electric paddle wheel speedometer can be found on modern boats. The pitot tube type looks like a long narrow triangle. It is made out of plastic and the tip of the long end of the triangle sticks down in the water below the bottom of the hull. As the boat moves, water is forced into the tube attached to the plastic triangle. As water moved into the tube, the air inside of the tube becomes pressurized by the force of the water. The greater the pressure built up inside of the tube, the higher the boat speed and the speedometer is able to take the air pressure and turn it into boat speed for the speedometer to accurately display the speed at which your boat is traveling.

With the electric paddle wheel speedometer, speed is measured by how fast the wheel spins. As the boat moves faster in the water, the paddle wheel spins faster and faster registering your speed.

In my simple mind, the paddle wheel version is the most logical. It seems obvious to me how it works, but the pitot tube speedometer is a bit of a mystery to me.

How Long Has The Pitot Tube Speedometer Been Around?

The pitot tube type speedometer is quite an amazing way to measure boat speed. The man or woman who figured out how to use air pressure to measure speed must be really smart. I was curious who was smart enough to figure this out so I looked it up. I have to admit, I was amazed to realize how long ago this technique was discovered.

The pitot tube was invented by a French engineer named, Henri Pitot in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy. I was completely surprised by how long ago this was discovered.

How To Repair A Boat Speedometer

If your speedometer is not working, don’t worry. You can follow the instructions in this video to troubleshoot your boat speedometer and get it working right again.

Take a look…

Beyond just the history of boat speedometers, here are some frequently asked questions and answers you may find helpful:

How Is Speed Measured On A Boat?

Speed on a boat is measured in knots. A knot is one nautical mile per hour. 1 knot is equal to 1.15 miles per hour. The term knot comes from the 17th century when sailors used a device called a “common log” to measure the speed of their ships.

Why Are Knots Used On A Boat Versus Miles Per Hour (MPH)?

Knots are used to measure boat speed versus miles per hour (MPH) because ships use latitude and longitude for global navigation. On a meridian line, one nautical mile is 1 minute of arc. For this reason, ships (boats) use knots instead of using miles or kilometers.

Knot To Miles Per Hour (MPH) Conversion

Knots	Miles Per Hour (MPH)
5 Knots	5.7539 Miles Per Hour
10 Knots	11.5078 Miles Per Hour
15 Knots	17.2617 Miles Per Hour
20 Knots	23.0156 Miles Per Hour
25 Knots	28.7695 Miles Per Hour
30 Knots	34.5234 Miles Per Hour