If you have a design, I would really like to see it.

I am trying to get a design for a 12 volt DC to 12 volt DC regulated power
supply. The design is to provide 12 volts DC + or - 5% output from an input
of between 8 volts and 16 volts DC.
The purpose of this is to run a computer in a sail boat. The battery voltage
in the boat may vary from 8 volts to 16 volts when it is charging.

I currently have an ATX computer power supply that runs off 12 volts DC, it
provides 3 and 5 volts nicely, but the 12 volt rail is actually the input
voltage. So, when the input is 12 volts all is well, if it goes high, well
that could be a real problem, and if it goes low, the computer will shut
down. If you can help it would be much appreciated