Great idea by the way.
Why not make it a trimaran. With mission critical stuff in the centre hull, like comms, pi , GPS and power. Outer hulls would have aux power and sensors.
Since the boat is moving why not have a small propeller in the water which turns as the boat is moving and then use this to generate power or to charge batteries. I have used larger versions to power towed equipment at sea.
The whole system needs to be able to power itself up if for some reason there is a loss of power. Or to conserve power because the batteries are failing to keep up with demand, so it switches off then once power is at a suitable lever switches back on. another reason for aux power, as you could divert power from the sensors to power the Pi etc. Ok you loose sensors, but if it means your boat can keep going its better than nothing. So I think power should be able to keep mission critical things running for at least 3 to 4 days if the charger fails, maybe longer if you go into a low power mode.
Comms is a tough one, as anything with the word satellite in it means add a few zeros. Best would be some form of text based message service. and try and compact the message as much as possible. Plus you need to think about what is important and what is not. So to send say
24º 50' 19.2" N 48º 00" 10.2"W Time 15:00 Date 15/08/2012 Air temp 22º water temp 17º
You could send it like this
Then decode it when you get the message. You could use letters of the alphabet to send temp, a=0º j=10º etc.
Using Short wave radio may be possible but downside is atmospheric conditions can play havoc.
GSM not really possible since the time you need to know where the boat is its out of GSM range.
These links might help http://www.telemetrysolutions.com/gps-d ... -birds.php
http://www.satellitephonestore.com/cata ... -telemetry
I think you will need 2 way comms as you may need to adjust the boats course due to weather or make the odd course correction manually.
Sail needs to be hard wearing. Or you could use some form of solid aerofoil (think vertical aircraft wing). Or thinking really outside of the box what about a vertical axis sail rotor or two.
Will need a radar reflector, Navigation lights, and nav beacons.
Oh and everything needs to be more waterproof than a waterproof thing. Plus it will bounce around a fair bit. So some form of shock absorption, then there will be heat problems. Oh and what about barnacles and seaweed. Don't need too many goose neck barnacles before the boat struggles to move. These things grow at an alarming rate, about a week for them to start causing drag.
Get sponsorship, you never know you could get some really big names behind it.
Oh and lastly stick a sticker on it saying if found please phone 1234567