Slot effect

The ‘slot’ is the gap formed between Headsail and Mainsail. The best way to sum it up is to say

that if you have a sail behind a mast, you can improve the airflow over the lee

side by careful adjustment of an overlapping headsail. The slot so formed will

speed up the air flow over the lee side of the main and reduce the turbulence

caused by the mast. But the slot is not a propulsive device of itself: if you

were to make a single sail equal to the combined area of main and jib, and if

you were to hank it on a forestay so that it was free of the mast’s turbulence, its

propulsive effect would be greater than that of the two sails and their slot.