Up the creek without a paddle
An article by Charles Mathys
Using an Arduino Uno computer, the voltage of an 18 volt battery pack is modified to match the 12 volt trolling motors specifications. Using PWM techniques, 6 forward and 3 reverse speeds are programmed to make the trolling motor run far more efficiently.
The photo above shows the console used to propel a 10 foot kayak electrically. The console contains an Arduino Uno computer capable of generating Pulse Width Modulation (PMW) signals and a Cytron driver (also a 2 by 3 inch circuit board like the Uno) which interprets the Uno’s PWM signals and powers the trolling motor as it would a robot motor.
On the side of the console, a stock 4 Ampere Hour (AH) or 6 AH Ryobi Lithium battery pack (used to power cordless tools) provides the power for the trolling motor. The console also provides a voltmeter and an ammeter to monitor the battery charge and the power flowing to the motor. The speed of the motor is controlled by a rotary switch with 6 forward and 3 reverse speeds. The Arduino Uno’s PWM control of the trolling motor is used in two ways, first it reduces the 18 volt output of the Lithium-Ion battery pack to the 12 volt rating of the motor, second it controls the speed of the motor. This is where it excels: small trolling motors operate at less than full speed by inserting resistors in the battery circuit. These resistors transform the unused battery power into heat which is dissipated in the water and wasted. At low speed a typical trolling motor’s efficiency is about 25%. The PWM technique for controlling speed is 98% efficient at all speeds thereby preserving the full speed efficiency at any speed.
The convenience of using a battery pack to power the motor is most notable for its light weight: a 4 AH pack weighs 2 lbs. It is easy to plug into the console or into the charger which fully charges the battery in about 2 hours. Spare battery packs are easy to carry on board to extend the range of the cruise or the fishing trip. The Lithium battery is also excellent at holding its charge over long periods of time and delivering its full rated power at any speed.
The components used in this project have exceptionally good value. A 10 foot kayak costs about $200, a trolling motor less than $150, two 4 AH Lithium battery packs $100 and less than $100 for the miscellaneous electronics and parts needed for a handy do-it-yourselfer to put it all together. Building this entry level electric boat, which is perfectly suited for kayak fishing, is a great learning experience.
The Mini Transom
The equivalent of a transom is needed to mount the electric motor on the kayak. There is just enough room for a custom made mini-transom 7 to 10 inches wide (depending on the width of the kayak ) to accommodate the motor clamps which require a minimum of 6 inches in width. A good height for the transom is 3 ½ to 4 inches and a good thickness is 2 1/2 inches. A block of wood of that size can be assembled from our pressure treated 2 x 4 in the following manner. 2 layers of 2 x 4 and one layer of 1 x 4 which are glued together to obtain the necessary height. The ends of the block are then trimmed to match the lines of the side of the kayak. The thickness of 2 ½ inches is cut on the bench saw. Then, the bottom of the block is contoured to match the top of the kayak as we see in the photo below.
To attach the transom to the boat, I used ¼ inch stainless steel threaded rod with back-up plates which perform double duty by bringing the motor power from the console to the surface of the transom. On the port side we have two threaded rods (the + and – for the motor) whereas on the starboard side we only have one. Be careful to keep the rods with the power separated from each other and away from the mounting bracket of the motor as much as possible so that they don’t cause a short circuit when the battery cable connections to the motor are tightened.
Mounting the motor
The location of the mini transom to which the motor is attached is 2 inches from the end of the boat. In the past, I have built brackets for a small motor to be used on a canoe. These brackets were located near the seat so that there was no need to relocate the controls nor to add a steering device. It worked but the handling (especially the turns) felt awkward, and I did not want to repeat those disappointing results.
I have also seen large dories used by lobster fishermen fitted with a box inside the boat on which an outboard motor was mounted. The motor controls were used, and the steering was done with the outboard. All very convenient and easy to implement. Since I had already cut a hole for the centerboard in my sit-upon coral kayak, I reasoned that by enlarging it, I could mount the trolling motor there within easy reach of the paddler running the kayak. This idea did not work either, mainly because the motor was mounted too close to the middle of the boat for effective steering. But my in-water tests did reveal the following good news, although there are no easy shortcuts, when the motor is mounted on the transom, both the controls and the steering work perfectly.
The Steering Mechanism
The steering mechanism is simply an arm on a pivot near the middle of the kayak. Attached to the bottom of the arm is a rope that goes around the two sides of the boat and emerges about 2 feet from the electric motor’s control arm. Pulling on the control arm with the steering handle, in turn, pulls on the right side of the motor’s control arm. It causes the boat to turn in the starboard (right) direction. A picture of the steering mechanism is shown below.
The arm, ¼ inch thick is made with two 1/8 thick aluminum bars approximately 11/2 inches wide. The two 1/8 inch sections, 9 and 10 inches long are bolted together. The pivot hole is a 3/8 inch hole drilled 5 ½ inches from the bottom of the longest bar. It accommodates a 3/8 inch bolt that has no threads for a distance of ½ inch in order to provide a smooth rotating surface. The threaded part of the bolt will then be cut flush with the end of the lock-nut. A contoured wood spacer separates the steering mechanism from the hull of the kayak.
The only remaining part of the steering to be built is the handle which will be attached to the steering arm. Like the spacer, it is made out of a clear section of pressure treated 2 x 4. The cross-section is octagonal about 1 1/4 inch in diameter. First cut a 7 inch long piece of 2 x 4, then make it 1¼ inches square. Then, by adjusting the fence and the angle of the blade on the bench saw, a professional looking octagon is readily made. A ¼ inch slot is cut on one end of the handle into which the steering arm fits. The arm is locked in place with two 1 1/2 inch # 6 screws with lock-nuts.
The Control Box
The control box houses the Arduino Uno and the Cytron driver unit. Each one is a small electronic board approximately 2 x 3 inches and about 1 inch thick. The block diagram above shows the electronic connections to the speed control switch (10 wire cable) and the 3 wires between the two boards (PWM, Direction of the motor and Ground). The heavy lines from the Cytron driver show the two connections to the battery (be sure to observe the correct polarity!) and the two connections to the motor (the direction of rotation is determined by the polarity of the connections). Once inside the trolling motor case, these wires by-pass the control switch and connect directly to the red and black wires coming from the motor through the shaft.
The electronics inside the control box are shown in the picture below.
The two electronic boards are oriented so that the USB connector which is used to load the Uno program from a PC, lines up with a 1 inch hole in the left side of the console. The board is attached to the bottom panel of the console. The rotary switch which controls the speed, is attached to the front panel of the console. The other end connects to the prototype shield attached to the Uno computer.
Results of Water Test
The most important information to be gathered is the efficiency of the four configurations that were tested. It is determined by the amount of running time that is produced by a given amount of power in watts. Equally important is the maximum speed that is achieved at each speed setting. The results of configuration B, the sit-in kayak with the Newport Vessel motor are shown below.
The chart shows the results obtained for each of the six forward speed settings. The following data is measured: the battery voltage and current (which determines the power in watts when multiplied together) and the speed. From separate barrel water tests, the running time is determined for each speed setting. The voltage and current readings were taken from the meters on the console. The power in watts was calculated. The speed was obtained using the satellite app ”MPH” on my cell phone.
Test results for Configuration B (Red kayak with N-V Motor)
*To determine the running time, namely, the amount of time that the motor can run continuously with one fully charged 4AH or 6AH battery pack, is a time consuming task. Each motor must be run at 6 different speed settings for a total of nearly 7 hours during which weather conditions are likely to change on the water. More accurate results can be obtained by running the motors under more controlled conditions in a barrel full of water. There is more turbulence in the barrel but as long as the drain from the battery is approximately the same as when the motor powers a boat, the results are valid. The running time numbers were obtained from a curve generated from the barrel test results.
Summary and Conclusions.
Powering a 10 foot boat at a trolling speed of 1.7 MPH for two hours or at hull speed of 3.5 MPH for nearly one half hour with one small lithium-ion 4 AH battery pack is a remarkable feat. I was the boat operator and I weigh 185 pounds so, with a lighter operator the results would have been better still. An average kayak paddler’s speed is 2.8 MPH.
The main reason that excellent low speed results were obtained is the use of the Pulse Width Modulation technique. It reduces the speed efficiently compared to the resistors which waste energy by converting excess power into heat. A bonus from the PWM technique is that the software simultaneously converts the 18 volt battery pack voltage to the 12 volts required by the trolling motors.
To implement the PWM motor drive, we built a console onto which to plug the battery packs. The other two electronic components in the console are the Arduino Uno which produces the PWM signal under the control of a very simple software sketch and a 20 amp Cytron driver designed to run robot motors. The Cytron requires just 3 signal wires from the Arduino Uno. Due to their simplicity, the electronics and the software operated flawlessly during the design, construction and the testing of the project.
A more detailed story about this project can be found in a recently published 40 page article available on line, free of charge at the following address: Arduino Project Hub/electric kayak. It provides the results of 3 other configurations as well as the program code (called a sketch in Arduino talk) and detailed plans, instructions and wiring diagrams which were not shown here.
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