The clew (aft lower corner) of the mainsail is secured to the aft end of the boom, usually using a line called the outhaul that can be adjusted to tension the foot of the sail.
The sail’s foot (the bottom edge) itself may or may not be secured directly to the boom. On some boats, a rope sewn into the foot (called the boltrope) slides into a groove in the boom. The clew enters the groove first, forward by the mast, and is pulled back in the groove until the whole sail’s foot is held to the boom in this groove.
The boat shown here uses a “loose-footed” mainsail. This means the sail is not inserted into the boom groove. But the clew is held at the end of the boom in the same way by the outhaul. Thus both ends of the sail’s foot are firmly attached to the sail and drawn tight – making the sail work the same as if the whole foot was also in the groove.
A loose-footed mainsail allows for more sail shaping, but the sail cannot be flattened quite as much. (Sail shape is discussed in Part 3 of this course.)
With the clew secured and outhaul tightened, the mainsail luff can now be secured to the mast and the sail raised to go sailing.