We started by coming up with a cone to guide water-flow from the rear edge of the submersible to the propeller. The first idea was to have the end near the submersible be square (to make it look like a continuous piece) and have only the end near the propeller be a circle.
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| Square to Circle Thruster Cone |
This shape had two advantages over a circle to circle cone. First, it is more streamline with the rest of the submersible - this helps some with the water flow but is mostly for visual appeal. Second, it would allow us to use the flat part of the surface to bolt fins onto for the thruster nozzle. We would probably have used this design if it did not require remaking the nozzle.
Instead we switched to a circle to circle cone so that we could use the same nozzle as last year.
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| Circle to Circle Thruster Cone |
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| Nozzle |
By using a circular cone and last years nozzle we are able to cut down on the number of pieces we need. The fins that connect the cone to the nozzle are 1/8" plastic and will be epoxied into the grooves that can be found in each piece. While the cone itself is circular we did leave a square part on the larger end allowing for an easy connection with the square submersible.
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| Cone and Nozzle |
Once we had the external portion completed we moved on to the inner parts. First cutting a hole for the shaft and motor so that they can be set into the rear cone and not take any space from the submersible. We then created a mounting bracket for the large thruster motor which would suspend it inside of the thruster cone.
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| Thruster Cutout |
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| Motor and Bracket |
When all of the pieces are put together the result is a pretty weird looking, but highly functional piece of our submersible. For information about the coloring see the general CAD post.
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UPDATE (7/13/2015): A New Cone Design
In our first attempt at creating the cone piece we tried to cut a large block of plastic on a 3-axis router – it did not work well for a couple reasons. Thus we decided to 3D print the piece instead. Although we were skeptical at first, this method turned out to work well. The design however, was changed in a few ways to make printing easier.
1) The first change we made actually didn't have anything to do with printing: we decided to return the the square to circle lofted cone. This is because it gives us a much better form factor and looks better when attached to the body of the sub. The only down side is that there are three fins going into a four sided object, which just means that we have to make a custom fin for each slot as they are not the same due to a lack of symmetry.
2) Another important change we made is the use of threaded inserts (discussed in this post). Without these inserts a printed cone would not work with out current mounting design because threads do not hold well in printed plastics.
3) A final change came in the form of moving the mounting bolts. While this may not seem related to printing the cone it is a change that has only come about because we can make multiple versions easier and cheaper using this rapid prototyping method.
We decided that we want to make as many of the external bolts in one line so that we have the smallest impact on our coefficient of drag. This type of modularity also allows us to mix and match the different sections of the submersible. We decided to move moved the bolts so that all four are on the sides, rather than having one on each of the four sides.
Below is a picture of the final design:
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