Last updated Thursday, 04-Dec-2014 20:59:22 AEDT
Status: In-Use almost every day.
This project started because I wanted to improve my times at the local club's weekly time trials. Without some means of continuous feedback, you can't be sure how efficiently you are paddling, or how much effort is being maintained. Just measuring the time to complete a fixed course is a very rough indicator and doesn't help greatly with improving technique or speed. All that you know is the time was slow so you must have been tired or you slacked off at some point or your paddling technique was off somewhere.
The primary goal was to develop a cheap and reliable speedo that did't burn through batteries continuously.
The image below shows the finished speedo which sits between the paddlers legs. You simply read of the speed from the water level in the vertical tube - what could be simpler! The black bracing/steering pedals can be seen with adjustment straps. You can click the image to see a much higher resolution image (599kB). Note that the top of the yellow tape marks the water level at rest. The readings on the scale are calibrated in km per hr.
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This design started with a lot of Googling for mechanical and electronic options.
+ Design Options
There are may options for speed measurement on water.
- GPS Unit - This high-tech option relies on signals from multiple satellites. It provides lots of information including distance, speed, bearing etc. The main problems are regular battery replacement, slow response to speed variations and risk of damage or loss whilst paddling.
- Mechanical Paddle Wheel - This type of speedo is used on some power boats and uses a small paddle wheel that is in contact with the water on one side such that its speed is proportional to the water speed. In order to use this as a speedo, some electronics are required in order to convert the rotational speed into pulses and then to some readable format like an analogue or digital display.
- Ultrasonic Doppler - This is a complex, expensive and bulky way to measure water speed. Ultrasonic sound waves are transmitted through the water stream to 2 detectors. The difference in received frequency indicates water speed.
- Magneto-dynamic Voltage - This is a variation on magneto-hydro-dynamic propulsion like that used in "Hunt for Red October". In this case we apply a strong magnetic field across a water stream and measure the voltage at 90 deg to the water flow and magnetic field directions. The voltage is proportional to the water speed. Aside from the complexity and cost, this one has the problem that the water stream must be conductive. Salt water (ocean) works, but the calibration will vary with salinity.
- Pitot Tube - This method measures water speed based on a balance between static and dynamic water pressures. A tube projects into the water stream and is balanced by a column of water. No batteries and simple. This is the design chosen for this project.
The theory behind the balance of static and dynamic pressure in the Pitot Tube is well explained on Wikipedia.
The relevant formula for this design is the speed vs water height which is:
km/hr = 3.6 x sqrt(2 x 9.81 x h)
where h is in metres. It is interesting to note that the height of the water column is not altered by the diameter of the tubing.
You can use the following to make your own scale. Note that you may need to stretch the scale depending on the size and length of your tubing. If you plot the shape on an X/Y axis, it has a parabolic (quadratic) shape.
- 0 mm = 0 km/hr (note that the zero point will be about 40 mm above the floor of the kayak).
- 2 mm = 1 km/hr
- 8 mm = 2 km/hr
- 17 mm = 3 km/hr
- 30 mm = 4 km/hr
- 46 mm = 5 km/hr
- 66 mm = 6 km/hr
- 90 mm = 7 km/hr
- 120 mm = 8 km/hr
- 150 mm = 9 km/hr
- 185 mm = 10 km/hr
- 224 mm = 11 km/hr
+ Building a Pitot Tube based Speedometer
Of all the above options, the Pitot Tube was the simplest, required no batteries and gave a direct speed reading. Follow the steps below to build your own kayak speedo.
- Get the parts - You will need a 5 mm right angle barbed bend (used for drip irrigation with a 2.5 mm internal diameter for water flow), a 300 mm length of 5 mm (inside diameter) clear plastic tube, epoxy resin, a 5 mm drill bit and some duct tape. The parts including the speed scale are shown below.
- Drill a hole - Drill a 5 mm hole. This should be in the bottom of the kayak, directly under the leg opening at the point closest to the front of the vessel. Place some tape over the other side of the hole before drilling to limit any damage from drilling. Drilling a hole in the bottom of my shiny fibreglass kayak was the most painful decision of this project, but I figured it is a small hole that can be patched later if necessary. Initially, I thought that it may be better to drill a hole at the front of the kayak below water level, but soon realised that this would be impractical as I would need to drill through about 50 mm of fibreglass, through a bulkhead and run a couple of metres of tubing. Access was difficult and it would have been difficult to ensure a good water seal. The opening at the front would likely be out of the water in waves and surges in the long length of tubing would affect the accuracy and response of the speedo.
- Glue in the angle - Insert the right-angle plastic component so that an open end is facing forward and the other end in facing upward into the kayak. Glue it into position using 5 minute araldite, being careful not to block the holes and making it as streamlined as possible. In the image below you can see the small right angle epoxied into the bottom of the fibreglass hull. In over 12 months of use, this epoxy has provided a water tight seal that is still as good as the day it was installed.
- Attach the tubing - Once the glue has set (24 hrs?), plug one end of the tubing into the inside end of the right-angle. Use some tape to attach the other end of the tubing the the front of the leg opening in the kayak so that the tubing is straight and vertical. In the following image, you can see how the tube attaches to the right angle inside the kayak.
- Attach the speed scale - Plot and laminate the speed scale from the data in the theory section of this page. Use clear tape to attach it behind the plastic tube. After some use, I found that it was better to attach the tubing to the kayak via using a spring to allow for flexing of the kayak during paddling, loading and unloading.
- Test and Calibrate - When you get into your kayak after installing this speedometer, the water will rise up the tube about 40 mm. This is your zero speed setting so move the scale zero mark up to match the water level. If you have a GPS, paddle at about 8 km/hr and check that the water level corresponds with the marking for 8 on the scale. Check at other speeds.
- Done! - Trim the plastic tubing above your maximum speed and you are done.
+ The Results
The results... - The water level in the tube fluctuates by about 0.3 km/hr on each stroke and could be damped if necessary with a larger diameter section in the bottom of the vertical tube if needed. I have been using the speedo for about 12 months now and found that the constant feedback has enabled me to develop a more efficient paddling technique and keep more pressure on myself during training. The overall result has been a steady reduction in paddling times over the same course. I have managed to raise my average speed from 8.5 to 9.5 km/hr and may be able to achieve 10 km/hr over the 10 km distance soon. Not bad for a 4.5 m 20 kg sea kayak! I have to concede that some of the improvement is probably also due to better fitness.
I have found that the positioning of the pitot tube is not ideal. I tend to keep an eye on the speed, and being low, I tend to slouch whilst paddling. A heads-up display would be great, but probably not practical for this simple design. Using a spring to attach the top of the tube to the kayak is better because it keeps tension on the tubing and scale, and can compensate for flexing of the kayak.
Probably the one thing that would be nice to change to to move the speedo out of the way. Being positioned between the paddlers legs makes it a little difficult getting into and out of the kayak.
I would be interested in any other ideas on nautical speedometers or variations on this project.
- TBD - .
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