So long as one can see the sun and stars reasonably frequently, I imagine a competent navigator could estimate his latitude and know which way is West accuratel

So long as one can see the sun and stars reasonably frequently, I imagine a competent navigator could estimate his latitude and know which way is West accurately enough to stay in the trade winds and make landfall along Brazil's coast, which I suppose was McVicar's only hope, given the extreme shortage of water and rations.