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1. 2N3055 on a suitable sized heatsink. Collector=12V, Emitter=6V out Base=centre-tap of two 'turn signal' light bulbs connected in series across 12V input. When it's night time a bonus is you have a light to see by when the pump is running.
I tried the light bulb but it used up to much juice. For right now i am just using a separate 6v lantern batt. I was also thinking of using a nicad pak and just using the boat batt to charge it. Thanks for the help.
Can you tell us more about it "used up too much juice"?
If you mean it drained the boat battery, I guess the series light bulb trick had saved you the time and trouble of building a 6.000V high precision, fan-cooled, lightweight voltage regulator that would of drained the battery just about as fast!
The Black regulator is the right approach(ie SMPS) , but not the right circuit. Your 12A current requirement is way beyond Black's circuit.
Even if you have a 100% efficient SMPS, it will still draw 6A from the 12V input. If you are using a 6V battery, the amount of energy lost during charging the battery will far exceed what can be done with a 6V 12A SMPS running on 12V input.
I looked into the Black regulator like Mike said not beefy enough. What i was trying to say about the bulb using to much juice is that when i hooked the 6v blower motor to a the 12v lead acid batt. with a 12v auto bulb in series the blower did not turn fast enough to do the task and about 25% as fast as when using 4 d cell batts.
The "right approach" I think is just to PWM the pump's DC motor with (about) a 50:50 duty cycle. The huge inductance of the pump motor should do the trick, all he needs is a 555 timer, a mosfet, and a freewheeling diode across the motor terminals.
The solar magazines sell cheap kits and devices that do this, to power 12v water pumps from 12v system with energy savings because they PWM the voltage to the motor as many water pumps dont need a full 12v to operate. The same cheap kits would be ideal for this application.
There is also another consideration: What else is the boat battery running while the pump is running. If there are any audio devices (Stereo/CD/XM/IPOD) involved, then the PWM or SMPS current pulses can get into the audio and drive you knuts. Ask me how I know. If this is a consideration, then go with a linear regulator and waste some heat.