DC motor and battery selection for saltwater environment?

I’m working on an autonomous sailboat for an ocean crossing next year. I’d like advice on which battery technology (NiMH, LiPo, etc.) and what class of motors (stepper, brushless, simple DC) to use. The boat is just under 1m in length, with a total weight of < 10 kg and a single sail of 0.15 square meters.

It needs to survive more than 90 days in a marine environment. I’m taking great pains to keep saltwater away from the electronics and motors, but small boats in big oceans tend to get thoroughly wet. (The motors and electronics will be under the deck, above the waterline, inside a sealed otterbox filled with mineral oil.) All power will come from solar panels, and the batteries will have to power the Arduino, a handful of sensors, and the two motors.

The motors will have a very low duty cycle, generally 10 seconds of operation per hour at most. One will simply turn a weather vane in the mechanical self-steering gear, the other will wind in the mainsheet about 30 cm with a reasonably high gear ratio. Power consumption and torque are not likely to be big issues, but survival with some exposure to saltwater will be essential. Over 99% of the time, the idling arduino will be the only power drain, as the motors, sensors, GPS and modem will be powered down while the purely mechanical autopilot holds a course.

I was looking at a LiPo battery / 6V solar panel / charger combo, but I’m wondering if the 3.7v battery may not be the best choice for powering an Arduino and electric motors as most of them require 5v or 6v inputs. Is LiPo still a decent choice, with a small voltage regulator to bump up the voltage? Battery weight and size aren’t big issues…within reason.

The LiPo battery has a small self-discharge, and contains a lot of energy. A LiPo battery would be my first choice. Discharging them too much could be a safety risk, so if you don’t trust the electronics and the environment you could use a few closed/sealed lead gel batteries. Submarines used to have lead batteries.

A LiPo battery is between 3.0V (or a little higher) and 4.2V. The 3.0V is not enough to run a normal Arduino. So you could use a boost converter.

But you could also run the ATmega32U4 (the microcontroller in the Arduino Leonardo and Arduino Micro) from a LiPo battery. It runs even at 2.7V at 8Mhz. For example the LilyPadUSB board is an ATmega32U4 at 8MHz. So you could use that board or make a compatible board yourself.
LilyPad USB: https://www.sparkfun.com/products/11190

Take a look at this, that seems just what you need, http://www.adafruit.com/products/390

A stepper motor and a brushless motor don’t have the contacts inside that could corrode.
Perhaps you need a brushless motor with a gear.

I found this movie:

Thank you, Erdin, for a very complete answer. The charger controller you linked to was the one I plan on using. I guess I'll just use a voltage regulator to provide 5V or 6V to the Arduino and motors. With a 6V solar panel, the panel will effectively drive the system during daylight, and the voltage regulator will draw from the batteries and step up the voltage at night.

Does anyone know if there's an issue with submerging a LiPo battery pack in a sealed otterbox filled with mineral oil? Do they vent gases or require exposure to air for any reason?