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Thrust is provided by a Minn Kota Riptide series trolling motor rated for 50 lbs of thrust at 12 volts. Several smaller thrusters were tried and did not provide sufficient thrust when towing a modem. Traditionally the thruster is mounted in the stern of the vehicle, but for several reasons we opted to mount the thruster near the bow of the vehicle and mount a skeg astern of it. The flat bottomed hull means that the kayak does not track straight on its own, so a rear mounted thruster would probably make the vehicle very difficult to control and drive in a straight line. The kayaks are also expected to station keep frequently and a bow mounted thruster should provide greater maneuverability and accuracy when doing so. Finally, a bow mounted thruster means the modem can be hung astern of the thruster and the vehicle driven forwards without risk of the propeller hitting the cable.
Tow 80/20 mounting rails are bolted to the bottom of the hull about eight inches apart from one another. Rather than run the whole length of the kayak, these rails run from the front of the flat-bottomed section to around the halfway point in the cockpit. This allows the thruster and several other pieces of hardware to be mounted to the rails in the front, while also freeing up more space in the stern for electronics and batteries.
add overhead picture of kayak here
The mounting and sealing of the thruster shaft and its azimuth servo were the most challenging mechanical aspects of the design and construction. Currently we use the MS530-1 Mega Servo from ServoCity to turn the thruster shaft. The output of the servo is clamped directly to the 1 1/8 inch shaft used by the thruster. Previous designs used a smaller diameter shaft to couple the servo to the thruster shaft, but the smaller circumference made it difficult to prevent slipping. Where the thruster shaft enters the hull a plastic cylinder housing an o-ring and sleeve bearing is bolted directly to the hull. The servo assembly attached to the top of the shaft is mounting to a horizontal plate, which is in turn supported by two more plates connected to the main mounting rails. Instead of holes, slots are used for many of the mounting bolts to allow this upper plate to be adjusted in any direction. This allows us to better align the servo with the shaft seal after the shaft is already installed. Only when the servo is properly aligned and the shaft rotates smoothly do we fully tighten all the securing bolts.
detailed picture of thrsuter mounting, perhaps more description of the shaft seal
Power
The original kayak design used a number of Lithium Polymer batteries connected in parallel inside the main CPU box. This arrangement provided good energy to weight ratio and simplified wiring, but batteries could not be swapped. LiPo batteries are also notorious for their tendency toward fiery accidents and charging had to be carefully supervised.
Lithium Iron Phosphate (LiFePO4) batteries are now used instead. While not as high as LiPo's, this chemistry still provides an energy density about twice that of a conventional lead acid battery. They are also much more stable and there is almost no risk of explosion or fire, even when overcharged. We use 12.8V 100Ah battery packs purchased from www.batteryspace.com. Tests with a DC load have shown the 100Ah capacity rating to be accurate to within a few amp-hours.