A spritsail is a four-sided sail laced to the mast on one edge with a single diagonal pole, the sprit, pushing its top outer corner up and out, so the rig needs only one spar beyond the mast. The sprit is about as long as the sail’s diagonal from the snotter loop on the mast to the peak (2.55 m in the 5.6 square meter example), and the sail should be sized so that its heeling moment stays inside what one crew member sitting on the windward gunwale can hold down.
Part of Small Sailboat Construction
The spritsail is a four-cornered sail held up by the mast along one edge and by a diagonal pole that props the top outer corner, used on small working boats in many regions.
One Pole Props The Whole Sail
The sail is a quadrilateral. Its front edge (the luff) is laced to the mast, its bottom edge (the foot) is tied to the tack and clew, and its upper outer corner, the peak, is held out by the sprit. The lower end of the sprit sits in a loop of rope round the mast called a snotter; the upper end fits into a cringle or pocket at the peak. Raising the sail means lifting the sprit and sail together, which is why the rig needs no separate top spar and no boom for the carrier to warn about.
Slackening the snotter lets the sprit drop and the peak sag, which spills wind. Treat that as a way to ease the sail in a gust, not as a reef: the area is still there, and a loose, drooping sail flogs. For a real reduction use reef points (see Grommets & Attachment Points).
What You Need
- A straight, light pole for the sprit, as long as the diagonal you compute below
- A snotter: a loop of rope that grips the mast and holds the sprit’s foot, adjustable so you can raise and lower it
- A peak cringle (reinforced hole or pocket) where the sprit’s tip engages
- Lacing along the luff, so the sail can slide down for storing
- A sheet run to the clew, led aft to a block or a fairlead
Method: Size The Sprit From The Sail Layout
Step 1: Draw the sail full size on a flat surface (as in Cutting & Sewing). The example here: foot 3.0 m, head 1.9 m, luff 2.3 m measured up from the tack.
Step 2: Locate the snotter on the mast. Put it about 0.6 m above the tack line for this example.
Step 3: Measure the diagonal from the snotter to the peak. Horizontal run = 1.9 m (the head width); vertical rise = 2.3 − 0.6 = 1.7 m. Length = √(1.9² + 1.7²) = √6.50 = 2.55 m.
Step 4: Cut the sprit about 5 cm longer, so the tip seats in the cringle with tension in the leech and luff taking up the slack.
Step 5: Rig and test on land. Lace the sail to the mast, raise it on the beach with the boat head to wind, and check that the sprit stays seated when you pull the sheet.
Worked Example: Is 5.6 Square Meters Too Much?
Sail area, taken as a trapezoid with foot 3.0 m, head 1.9 m and height 2.3 m: (3.0 + 1.9) ÷ 2 × 2.3 = 5.64 m², used as 5.6 m².
The centroid of that trapezoid lies (h ÷ 3) × (a + 2b) ÷ (a + b) above the foot, where a = 3.0 is the foot and b = 1.9 the head: (2.3 ÷ 3) × (3.0 + 3.8) ÷ 4.9 = 1.06 m. Allow 0.3 m from the waterline to the tack and 0.25 m from the waterline down to the centre of the board’s lateral resistance. The lever arm is about 1.06 + 0.3 + 0.25 = 1.6 m (an assumed layout; measure yours).
| Wind (Beaufort) | q = ½ρv² | Force on 5.6 m² (coefficient ≈ 1) | Heeling moment (× 1.6 m) |
|---|---|---|---|
| 19 km/h (top of force 3) | 17.1 N/m² | 96 N | 153 N·m |
| 28 km/h (top of force 4) | 37.1 N/m² | 207 N | 332 N·m |
| 38 km/h (top of force 5) | 68.2 N/m² | 382 N | 611 N·m |
What holds the boat up? A 75 kg crew member weighs 75 × 9.81 = 736 N. Sitting on a gunwale 0.6 m from the centreline of a 1.2 m wide boat, that is 736 × 0.6 = 441 N·m at zero heel, less as the boat leans. The top of force 4 therefore uses about three quarters of one crew member’s capacity, and the top of force 5 exceeds it by 611 − 441 = 170 N·m. A reef of 30% removes area but also lowers the centroid; using the same arm (a cautious simplification), 3.92 m² at 38 km/h gives 268 N and 428 N·m, just inside that single crew member’s 441 N·m. The force coefficient of 1 is an assumption, and gusts exceed the mean speed, so treat these numbers as a reason for caution rather than a safe limit.
Common Mistakes
- Using a heavy sprit — a pole that is thick at the tip raises weight aloft; taper it.
- No lacing, only a rope tied at corners — the sail then cannot be dropped fast.
- Leaving the sprit unsecured at the bottom — a sprit that slips out of the snotter in a gust drops the peak and can fall on the crew.
- Believing “no boom” means “no sweep” — see Tacking & Jibing: the foot, sheet and sprit still cross the boat when the stern passes through the wind.
Safety
A pole in compression can fail without warning
The sprit works as a column pushing the peak out. A hairline split near a knot can snap it in a gust, dropping the upper sail and a splintered pole across the cockpit. Choose straight-grained wood, avoid knots in the middle third, and carry a spare pole length.
What’s Next
- Learn next: Lug Sail Rig — a sail that reefs more easily
- Combine with: Standing Rigging — what keeps the mast upright under the loads computed above