A lug sail is a four-sided sail hoisted on a yard, a horizontal or slightly raked spar, with one edge on the mast side; the standing lug hangs the yard to one side of the mast and is simple and efficient downwind, while the balance lug carries part of the sail forward of the mast for better performance upwind. Its main advantage on a small boat is reefing: with reef points at roughly one third of the sail height, tying the lower part down removes about 30% of the area in a few minutes.
Part of Small Sailboat Construction
The lug sail is a four-cornered sail hung from a yard rather than propped by a diagonal sprit, widely used as a working-boat rig because it is simple, reefs easily and stows compactly.
A Yard And A Boom Make The Area Adjustable
The sail’s top edge is laced to the yard, the spar the halyard lifts. The bottom edge is often laced to a boom. Because both edges are on spars, the cloth keeps its shape when hoisted, and it can be shortened by lowering the yard part way and tying the surplus cloth down along the reef points. Lowering the yard and boom together lets the whole rig lie flat along the mast, which is why the carrier says it stows alongside the mast for rowing or portage.
Standing lug: the yard hangs to one side of the mast and the sail’s tack is fixed. On one tack the cloth presses against the mast and loses its shape; sailors accept this because the rig is easy to set up.
Balance lug: a short portion of the sail extends forward of the mast, so the wind pressure on that part helps swing the yard and keeps the cloth away from the mast. It sails upwind better, at the price of needing the yard’s attachment point balanced by trial.
What You Need
- A yard about as long as the sail’s head, and a boom about as long as the foot
- A halyard and a traveller or rope strop that holds the yard to the mast
- Reef points: short lines sewn through the sail in rows (see Grommets & Attachment Points)
- A downhaul at the tack, so the luff can be pulled tight
Method: Hoist, Set And Reef
Step 1: Lay out the spars on the sail. Lace the head to the yard and the foot to the boom, leaving the luff free to sit beside the mast.
Step 2: Fit the halyard bridle to the yard. Start about a third of the yard length from the forward end and slide the point back and forth during trials until the yard balances horizontally when hoisted; this is a trial value, not a fixed number.
Step 3: Hoist on land with the bow into the wind. The cloth should fill the shape of the spars with a few soft wrinkles along the luff, not hard vertical creases.
Step 4: Reef before you need to. If the sail begins to heel the boat past comfortable, lower the yard until the first row of reef points reaches the boom, gather the surplus cloth along the boom, and tie the reef points in a flat knot. Do this in sheltered water; see Essential Knots.
Step 5: Check the leech and the sheet. After reefing the sail should show smooth cloth between boom and yard, with no loose bag at the bottom.
Worked Example: What A Reef Buys
The carrier gives 5-8 m² for a 4-5 m boat, puts reef points at roughly one third of the sail height, and counts about 30% of the area removed by that first reef. Starting at 6 m²: 6 × 0.30 = 1.8 m² removed, leaving 4.2 m². At the top of force 5 (38 km/h = 10.56 m/s), dynamic pressure q = ½ × 1.225 × 10.56² = 68.2 N/m². Force on the full sail ≈ 68.2 × 6 = 409 N; on the reefed sail ≈ 68.2 × 4.2 = 286 N, a reduction of 123 N. A force coefficient of about 1 is assumed, as in Spritsail Rig. The moment falls by slightly more than the force because the centre of the cloth moves down with the reef.
Common Mistakes
- Reefing late — a reef tied while the boat is heeled and the sail is full is difficult and dangerous; reef before the first white crests are frequent.
- Letting the yard hang unattached — a yard that slips off the mast in a gust swings and can hit a crew member.
- Sewing reef points too weakly — they take the whole load of the reefed cloth; see Cutting & Sewing.
Safety
The yard is a heavy spar overhead
A softwood yard 3 m long and 5 cm across weighs about π × 0.025² × 3 × 450 = 2.6 kg (450 kg/m³ is the midpoint of the 400-500 kg/m³ seasoned-softwood range in Logistics & Load Planning) and hangs above the crew. If the halyard parts or the sail is dropped in a gust, the yard and boom swing and fall. Wear a hat, keep heads clear of the line between the mast and the yard, and rig the halyard with a knot that cannot slip.
What’s Next
- Learn next: Lateen Sail — the rig that points highest, with a different set of drawbacks
- Combine with: Running Rigging — halyard, sheet and downhaul detail