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Human Powered boat speed measurement

By Warren Beauchamp

Sure, you can put a speedometer on a bike, and know exactly how fast you are going, but how do you measure speed on a boat, short of setting up timing traps? 

Measuring Propeller RPM
If you want to find what your propeller RPM is, you can use an bike computer with the magnet mounted on the driveshaft. R.D Stockinger did the math to find that if you enter 1667 mm as the "circumference of  the tire" on a cycle computer, and set the computer to read in KPH, then the computer will read 10.0 KPH at 100 RPM, 8.7 KPH at 87 RPM, etc.

Measuring Boat Speed
George Tatum says that you can use a Cateye bike computer with with a cadence counter to measure HPB speed. They say "Water Resistant" but this is one case where the company understates its product. Hook the cadence sensor to your crank arm just like a bike. Hook the wheel sensor to your boat's shaft, just like you would to your bike wheel. Find good places to mount the pickups. Place the Cateye computer on your handlebars just like a bike. The Cateye has an instruction manual that gives you numbers to calibrate your computer for the diameter of your wheel. Use the exact pitch of your propeller, convert it to mm, then plug that number in for the wheel diameter. As George's Heron prop has close to zero slip, the computer will count its mechanical advance like it would the distance of a rolling wheel. This tends to work so well, he uses his CatEye to check behind his GPS, which is rated Waterproof but is not even water resistant. The Cateye will record fasted sprint times, distance traveled and just about anything else. If you use a prop like the APC or Bolly props, which have slip, you calibrate them for the slip by placing the Cateye into the distance mode. (It will go metric if you want it to) Ride a known distance by landmarks. George often uses, as reference, government channel markers. When you have completed a know distance, look at the Cateye. If the data figure reads over distance, then your prop is slipping. The correction is a function of the slip ratio applied back to the mechanical advance rate. If you know your cadence, your gear ratio, the pitch of your prop, and the distance and time of a run, you can calibrate for slip to the cm. With known shaft speed, prop pitch and a GPS, you can calibrate to the mm. 

Prop Pitches:
Heron - 30 inches or 762.0mm
Bolly HPB prop - 25 inches or 635.0mm
APC 16x16 - 16 inches or 406.4mm

FlareKing says:

"I have for the past year used my Cateye computer for cadence on my cat, but I did not think of using this for boat speed until I read the article on the web by Warren Beauchamp, where it is recommended that you mount the wheel sensor on the boat prop shaft. I am using a compact twisted chain drive system so I have no boat drive shaft.

To overcome this I have mounted the magnet on the inside of the crank arm as per cadence method. The detector and computer head are then mounted on the support bar so that the computer can be read and reached when peddling. The computer is then calibrated in the following method:

Calibration input number is Propeller pitch in mm x Slippage factor x Prop to pedal gear ratio.

Slippage factor should be between 0.995 and 0.94 but depends on the prop chord, diameter and RPM. Wide props have only a little slippage but slimmer props have more slippage. 

For a APC 16 x 16 use 0.95 which is correct for 5 MPH

To check the slippage factor create a measured distance on your bike using your computer alongside a straight piece of river bank. Then use the boat to check the same distance with the computer measuring distance, but do it in both directions to allow for any current. This will enable you to correct your estimated slippage factor. 

If you have a hand held GPS this will make calibration very easy.

A very small error occurs caused by the slippage increasing as the propeller RPM increases, but this can be minimized by measuring the slippage factor at your average cruising speed.

Most bike computers are suitable for a wheel circumference range of 1100 to 2999 mm which is the calibration input number, should your calibration number be larger than 2999 then divide by 2 and your speed and distances will be shown as half of the actual figure."

Orrin Christy posts:
"I started with an Avocet bike speedometer. The magnet was replace with a smaller rare earth magnet from Edmund Scientific that was epoxied to the propeller shaft. The normal sensors are simply magnetic reed switches, but since the response rate of the Avocet one was too low, I dug around my junk boxes and found one that could cycle on and off quicker than 50 milliseconds. This was all pre-tested on a test rig mounted on my drill press. 

These components were potted into a streamlined epoxy composite structure attached to a strut that would mount to the boat. The propeller was a crude structure with flat thin stainless steel blades with a diameter of about 3/4 inch and an estimated pitch of 9.3". This figure also compensated for an estimated 10% slip factor. 

The prop had four blades and maintained the same streamlines as the stationary components so in use, the unit had very low drag. The blade angle relative to the prop shaft was 14 degrees. Being realistic about our non-olympic participation, we targeted 10 mph as our maximum velocity. For this, I entered a calibration number of about 215 (2150??) into the computer - this would approximately match a 27" bike wheel. 

Theoretically at 10 mph with the prop uncertainty and slip and all, the reed switch should send 21 pulses per second to the computer and speedometer should read 100 mph - divide by 10 and you get a decimal point of accuracy thrown in. 

After in-water tests, I think the calibration value had to be changed by about 10%. We were amazed at the accuracy of the unit when we checked out the odometer feature on a surveyed 2000 meter current free course. I think the fastest anyone saw the speed up to was about 9.5 mph in a kayak. We used it on my Seacycle and saw 8.9 mph one time (as it is advertised, we could not make it up to 12 mph!!). If I can recall correctly, the minimum readable accurate speed sensed was 0.8 mph." 

Hang tough, Orrin Christy


Any other (inexpensive) methods? 

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