FLY BY NIGHT
Most people think that the term ‘fly by night’ means someone who will turn up to do something only to have disappeared by nightfall, usually after being paid for a job half finished. May be so but a fly-by-night was also a large sail used only in specific wind conditions. The fly-by-night was employed instead of the usual collection of smaller sails, making it easier to manage. It could also only be used when sailing downwind and, more or less, the only time it would be used was during the night when most of the crew were asleep below.