limiting servo current consumption
Before you do anything else, wire up a 555 and a one-transistor inverter and PLAY with your servo.  Take some measurements.  See what you're really up against.

An RC servo control circuit knows where the limits of motion on the servo are, and it stops the motor, and stops drawing current, when it hits the limits.  You're going to be at one stop or the other when you're tacking, and you're going to be rudder amidships, or almost amidships, when you're holding a constant point of sail (beating upwind, beam reach, broad reach, running straight downwind).  In this situation, the rudder is just a trim tab, to compensate for the boat's pointing moment.  (Beating upwind, especially, the forces on the sails produce a net torque that tries to turn the boat into the wind.  The rudder acts against the moving water to produce a countering torque.)

What will hurt you is servo "buzz", which is caused by instability in your control system.  The easy cure for this is a big electrolytic capacitor across the servo power supply.  For a Tower Hobbies Royal Titan Standard servo, 250 uF worked great for me, and I would NEVER attempt to operate ANY RC servo without such a capacitor in place.