I am thinking of having the next size model be about twice this one (so about 8 times the displacement). If it is 8 feet long it could hold 2 of the 100 watt panels by the same people that make the 50 watt panel used in the first size or 4 of the 50 watt. Either way the total for solar would be about 8 lbs.
I would like to have 4 motors, one on each float, so that if one breaks we can keep going. With 200 watts it would even be able to pull a kayak. This means testing can be riding in a kayak that is pulled along by the robot. Should be fun.
In this larger model I would put the batteries down inside the floats. This lowers the center of gravity and also makes for a high rotational inertia as the weight is then widely spaced. With 8 times the weight capacity we could use more and larger batteries and so make it through the night.
The first version just uses relays so the motor is either on or off. With an electronic speed controller (ESC) we could use whatever power level we wanted. There is a board called IOIO which can talk to an Android phone and an ESC. There are other boards to go from bluetooth to ESC. There are some ESCs that can talk bluetooth.
There is an underwater T100 thruster by Blue Robotics who is also making an autonomous solar robot boat. You can get these with ESC built into the thruster. These can go up to 140 watts but at about 12 watts it seems to give about 1 lbs thrust. So 4 of these could be very nice. We would normally run all 4 thrusters at low power but if one goes out then we run the other one on that side at higher power.
The BlueESC is the T100 model with the builtin ESC. This also has the ability to be controlled with I2C, which is a protocol where one controller talks over a bus to several devices. With this a Bluetooth to I2C board could be an easy way to go. However, a bus is not so good as far as keeping independent failure modes. Saltwater getting into the wires by one thruster could disable all the thrusters.
For good independent failure modes I like having 4 different 50 watt solar panels going to the 4 different floats. Inside the foam for each float there could be a charge controller, battery, bluetooth relay, and motor. So the phone would be talking bluetooth seperately to each corner of the boat. Could have a diode protected power line from each of the 4 batteries going to the power adaptor for the phone. The diode would make sure that a short in one battery could not drain power from the others.
Could even have a backup phone that only took over if the first phone stopped working.
There are phones starting to come out with Ubuntu on them. This Ubuntu phone has compass and GPS for $192 US. This would let us write fast native code. It could be interesting for a future model.
I think having a 3D printed part to connect the drive shaft and the motor and help mount the motor would be good. With shapeways anyone could just get one printed even if they did not have their own printer.
I would like to be able to have the robot go on multi-day trips. But for this it really needs to know how much power it has. I would like a small bluetooth voltage sensor. Seeed used to make one for $39 but it seems it is discontinued. There is a $120 Mooshimeter by moosh.im that could work.