650 amps at 12 volts is 6500+1300=7800 watts of power, DC.
The rectifier (converting AC to DC) as well as a transformer (step
down from AC power voltage to 12 volts DC) will have some losses as
well. I believe that you will find such a rectifier to be impossibly
expensive for such an application. So costly that several alternatives
become affordable:
- Another battery, kept charged
- A small winch driven by a suitably geared gasoline engine
- A suitable tow line capable of attachment to a vehicle

There is no information regarding the anticipated current drawn by the
winch at stall speed, nor is the effective load of the boat (including
both friction drag and angle of incline on the railway).
Surely the winch does not demand maximum battery current along the
entire hauling path.

On 13 Nov 2006 17:50:52 -0800, wrote:

Hello, I use a 12 volt winch to pull my boat up on a marine railway at
my seasonal cottage in Canada. I do not have permanent AC power to the
location, but can get a big enough extension cord to it if need be. I
want to buy a more powerful winch which requires a battery rated at "12
VDC - 650 Cold Cranking Amps" which is not a problem. I would like to
buy a back-up 12 volt AC to DC converter to power the winch if the
battery fails. Please advise on how to rate the converter based on the
battery requirement. Much thanks! D. MacQueen.