They didn't have a GPS navigation system, so they had to use planets and stars to get an idea of where they were and what direction to go. If there was cloud coverage for an extended period, where they couldn't reference the stars, they could just as likely end up in Charleston, SC than their destination port of Boston.
Could you imagine trying to figure out the necessary food and water for the crew considering the tremendous variance in how long it took to cross the Atlantic? It's not like you'd want to be better safe than sorry to a large degree because any space taken up to provide for the crew, meant less merchandise/supplies to sell the colonies and vice-versa as far as our exports to Europe.
I can’t imagine how challenging it was to live during that period of our history. I admit I’m a spoiled and grateful 20th Century kid.