The windlass runs off the battery, not the alternator. Any winch worth
its salt will draw far more current than the alternator can replace in
real time.

Having well (over) sized cables (up and back) to the winch will give
better results but this can be expensive. Sometimes it is cheaper and
more effective to put a dedicated battery at the winch with a light
gauge charging line to replenish the battery.

See the article at http://www.yandina.com/Bank3.htm