A halyard pulls a sail up the mast, and a sheet controls the angle of the sail to the wind once it is up. Pulling a sheet in is called sheeting in or trimming, and letting it out is easing.
Both are ropes, but they do different jobs.
- A halyard hoists a sail. It runs from the top of the sail (the head) up to a sheave near the top of the mast, and back down to the deck or cockpit. You pull it to raise the sail and let it go to lower it. On a typical yacht there is a main halyard for the mainsail, and there may be others for the jib and the spinnaker.
- A sheet controls the angle of the sail to the wind. It is attached to the clew, the bottom back corner of the sail. The mainsheet pulls the boom in or lets it out, and there are two jib sheets, one on each side of the boat, so the jib can be trimmed on either tack. The one in use is the working sheet; the other is the lazy sheet.
So a halyard is mostly set once, when the sail goes up, and then adjusted occasionally to change the tension along the luff of the sail. Sheets are adjusted all the time as the boat changes course and the wind changes. Pulling a sheet in is sheeting in or trimming, and letting it out is easing.
On a yacht, sheets and halyards carry large loads and are usually led round a winch and held in a cleat or a jammer. Take care when letting one go under load: leave the turns on the winch and ease it out round the drum under control, keep fingers clear of the drum, and never wrap a loaded rope round your hand. Never open a jammer on a loaded line until the line is on a winch and the winch has taken the load.
On a boat a sheet is always a rope, never a sail.
More in the book: Chapter 1, The sailing boat