Propellers are the most commonly used propulsion systems on unmanned surface vehicles. This is due to the simplicity of their design and implementation, however, they offer no protection to their surroundings. A simple duct can be added to protect the blade tips while offering some protection to users, but it doesn't reduce the chances of entanglement. Therefore it was decided that waterjets would be the ideal solution as they are inherently safe, allow for operation in shallow waters, reduce the risk of entanglement with ocean algae, and provide a range of design optimization options. The design is based on an axial flow pump and it follows a standard pump curve. It is made up of a number of features, these being the inlet passage, impeller, nozzle, and thrust vectoring components. Additional features are added such as an o-ring to reduce pressure loss and bearings to reduce friction are also used. Within these there are more features that can be modified to vary pump performance to suit the user or task requirements.
Jet design began with different inlets as this is the major difference between propeller and jet propulsion. A range of designs are shown below with it progressing from left to right. The initial design had a very small opening and sharp edges which resulted in turbulence due to the flow separation. The later designs improved the transition between the water and the inlet which allowed the flow to stay attached.
The inlet length and inlet angle have been optimised to reduce the overall inlet swirl and wall cavitation. This was done by having an inlet angle of 30-35 degrees which still leaves room for mounting a range of motor sizes. After the inlet angle, enough length is needed to house an impeller while still ensuring a very close fit to improve pressure differences.
A range of impeller designs were tested, initially they started off unrealistic but became closer to that of full size commercial impeller designs which resulted in faster priming and greater thrust.
The idea of the nozzle is to redirect the rotating flow to axial to increase the jet exit velocity. Progression can be seen from left to right, with the initial being 4 simple guide vanes. The designed one has 7 guide vanes with slight curvature to them and a thrust vector mounts internally. A reversing bucket was also added to allow the platform to reverse.
This was the initial test set up, the only shaft we had at the time was not rigid so it led to a number of issues as RPM increased. However it served as a solid starting point.
The design above is one of the later designs, probably around 5/6th iteration and was made to a diameter of 30mm rather than the previous 40mm. It used a fixed shaft and provided water cooling to the motor via the jet inlet.
A load cell was used for testing as this would measure the thrust output of the jet. Current clamps were used on the battery side to measure the current draw.
This testing was rather rudimentary but provided a key difference between the potential output of the 40mm jet vs the 30mm jet. The majority of the difference is due to the fixed hub size due to the size of the bearing in the nozzle and shaft diameter. Therefore continuing on we stuck with the 40mm jet as we saw there were great optimizations to be found.