Square Top Sails

[Bill Roberts]

Part 1: Technical Discussion

Square top mainsails have been around for about 30 years, first showing up on beach cats in the mid 1980’s. Prior to this most mainsails came to a point at the head of the sail called a pin head. Now square tops are showing up on high performance monohulls. This major change in sail planform is the biggest change in sail design in many years.

Why the square top sail? What's going on? Is there something special or aerodynamically superior about the square top sail? The answer is YES. The square top sail planform is an aerodynamic improvement to the airflow field in the top region of the sail. The square top planform corrects an airflow anomaly that occurs naturally at the top of a pin head mainsail.

We all know that as the airflow field approaches the leading edge of the sail at the mast, it divides. Some air flows to the windward side of the sail and some flows around the leeward side of the sail. The airflow on the leeward side travels a longer path around the curved sail surface and this causes the leeward side airflow to accelerate and move faster. The faster moving leeward side airflow brings with it a lower static pressure, this is known as Bernoulli’s Principle*. The windward side of the sail airflow did not have to accelerate along its straight line path and therefore is at a higher static pressure. It is this static pressure difference across the sail, the delta P, which becomes sail thrust. This static pressure difference across the sail is large in the front half of the sail and reduces to a small difference at the leech.

The airflow anomaly on a pin head sail occurs in the top region of the sail. It occurs here because the leech is not vertical (side view), but is bending/curving forward to the head of the sail at the mast top. At the leech the static pressure difference (the delta P) across the sail causes the air from the windward side of the sail (high static pressure) to flow over the leech to the low static pressure side of the sail. When the leech is vertical this flow across the leech happens just downstream of the sail leech and the mixed flows blow off in the distance downwind. Where the leech of the mainsail is not vertical and is bending forward toward the head of the sail at the top of the mast (pin head) “there is sail cloth and sail area below the leech”. The delta P at the leech causes windward side airflow (higher static pressure) to flow over the leech and down the leeward side of the sail to regions of lower static pressure. This is called “Spanwise Flow”. These airflows mix on the leeward side of the sail but the resulting static pressure is higher than the non mixed static pressure level would normally be. This anomaly reduces the delta P across the sail and reduces sail thrust in the non-vertical leech region of a pin head mainsail.

This Spanwise Flow effect on airfoil performance has been evaluated in wind tunnel testing on airplane wings. The wing planform that comes to a point at the tip has the lowest Span Effectiveness or lift per square foot. A wing planform where the tip chord is 40% of the root chord has the highest performance. A wing with a tip chord that is 25% of the root chord has much improved performance over a pointed wing tip.


*Bernoulli’s Principle: Pstatic = Ptotal – airdensity x Vel**2/2g