After a quick chat with Ed, it seems clear that the best design is going to strike the right balance between speed and control. Apparently, in the past, getting a boat to go fast has not been a problem. I don't know what difference our particular fountain will make with the way the jets from above create currents. But in general, it sounded like a slow boat with quick, intuitive controls performed a lot better than the ones that went for raw speed alone.
We won't need to stay at an Atoll for too long to capture it, but we should allow time for errors and trying to position close enough. The RFID readers are reliable within only a few inches, so it might be worth investing in side propellers, especially if we have sensors on a couple sides of our boat and want to pull up to it sideways. That seems like the best plan to me because that allows us to simply drive in a ring around the whole fountain, especially if we can fit in the shallow section around the outside. And it would require less control and driving skill than having to face the front of the boat towards each RFID reader to tag an Atoll.
So, boat shapes. Higher level decisions seem to be between a single and double hull. Advantages of a double hull are stability and more flexibility with motor placement, since there would be a large space in between the two points that go in the water, possibly under our (waterproofed) electronics. Advantages of a single hull are mobility and a more compact size, since we do have to fit within the 72" perimeter requirement. One thought I had today was making a double-hulled boat, that was both wide and tall enough such that it could fit around an Atoll completely. The RFID signal would come from the inward sides of the two hulls. That would also be a way to block an opponent from capturing an Atoll we owned, if we decided not to go for more. Of course, with the Atolls being 16" wide already, it would be very difficult to fit within the perimeter, maintain stability, and leave enough of a margin to actually drive properly over an Atoll.
As for components, the things I see costing the most are good reliable motors (probably not the one that costs $100, sorry Matt) and the sensors we'll want in our remote control. Speaking of which, here are a couple accelerometers:
From Radio Shack, $35: http://www.radioshack.com/product/index.jsp?productId=2909788&CAWELAID=124299652
One of the reviewers even mentions using it on a model boat for automatic stability control (making sure the boat doesn't tilt too far to one side or the other). A cool idea, though it shouldn't be necessary if we get the mechanical design right (plus I don't think we'll be lacking in other code we have to write). Also don't think we can afford more than one of these and it'll probably go into the remote control. Anyway, this one is well-reviewed, but the technical specs are either non-existent online or hard to find.
From Adafruit, $20: http://www.adafruit.com/index.php?main_page=product_info&cPath=35&products_id=163&zenid=374729df98ba07ea843cc82c7806628b
Measures 3 axes (!) and seems pretty solid. Adafruit is also a pretty cool company.
Some other interesting sensors we can think about putting on the remote control:
Flex sensor: http://www.adafruit.com/index.php?main_page=product_info&cPath=35&products_id=182&zenid=374729df98ba07ea843cc82c7806628b
A different idea for a throttle: how much someone is bending some part of the remote determines how fast the boat goes.
Rotary encoder: http://www.sparkfun.com/products/9117
Doesn't have to be too expensive. If we can get a really solid/sturdy mechanical connection, I'm imagining a giant crank that you have to turn, or maybe a big wooden steering wheel like you find on old boats (eBay?).
There are lots more cool things out there, but that's all I have for now.
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