Via one of the sailmakers websites...these guys say it a lot better than I can.

Sail Shapes

It's rare in sail design when an innovation can add power in light air AND reduce power in heavy air.

Physically, these Mainsails feature an additional batten at the head placed at 45 degrees to the luff. This batten supports the 'Square Top' leech profile. Rather than the standard aluminum headboard, the square top is over 30% wider with a conventional stainless ring which replaces the headboard.

The additional sail area creates more power and efficiency. Some power is added simply due to the increased size. Efficiency is greatly enhanced as the 'Square Top' profile increases the effective aspect ratio which raises the lift coefficient for better upwind performance. Additionally, the added sail area is behind the mast turbulence zone which lengthens the effective luff for enhanced tell tale flow in the upper 30% of the Mainsail. The net result is more speed on all points of sail in light to medium air.

Depowering in heavy air is the second half of the 'Square Top's' advantage. All of the additional sail area is added behind the load path from head to clew. When excessive wind pressure is added, this portion of the sail behind the load path line 'hinges' open automatically which depowers the upper portion of the Mainsail. That's right, automatically. Even if the mainsheet isn't eased, the top of the sail depowers. In many 'marginal' conditions, reefing the main can be delayed as the sail conforms to the puffs automatically.

When reaching downwind with the Asymmetric Spinnaker, the 'Square Top' Mainsail is again a major improvement. By easing the mainsheet, the 'Square Top' automatically 'twists' open at the top 1/3 of the leech. This aligns the Mainsail with the apparent wind angle which is progressively further aft in the upper portions. Your first observation will be the steady leeward tell tale flow, even the ones at the top. In this condition, a standard 'pin head' main twists open in the middle where it has the most roach, leaving the top stalled and under-powered.


Squaretop / Pinhead

Why the new 'Square Top' Mainsail profile? Why do classes with a choice go 'Square Topped? It seems that everyone including A Class cats, to the latest rage in Europe, the Formula 18's, the Nacra Express, Super Cat 22's, and Farrier Trimarans are all winning with Square Top Mains.

Let's start with the obvious. Square Top Mains look bigger than Pin Heads. Sail area can generally be expanded by 7% with a Square Top profile. Just remember that, "Sail Area is King".

But wait, there must be something more than just area. The A Class and the Formula 18 Classes chose the square top without any additional sail area bonus. Yes, the advantages are in fact numerous and spread around the whole race course.

Upwind the Square Top essentially "stretches your mast length" by increasing you effective aspect ratio. This translates into enhanced lift to drag ratios which means more forward power, less hull flying, and higher pointing. In windy conditions a more dynamic advantage for the Square Top is it's ability to lower the center of effort. At first this sounds ludicrous. Is appears that all the added sail area is up high and should raise the center of effort. Actually excessive wind loads push on the square top aligning it with the wind which effectively makes it invisible to the wind. It's as if the Square Top automatically reefs the to portion when overpowered.

A Pin Head unfortunately depowers where the roach is largest; in the middle rather than up high. the result in high winds is the Pin Head must ease the traveler to reduce excessive hull flying which immediately hurts pointing ability.

In summary Upwind: The Square Top can point higher in light air due to the increased Aspect Ratio. In heavy air pointing is enhanced due to the lower center of effort which enables the traveler to stay centered. In all winds you can sail with more sail area while pointing higher.

Off the wind the Jib (and Asymmetric spinnaker) backwinds the lower portion of any Mainsail. This makes the bottom the Main 'see' apparent winds from well forward while the top of any main 'sees' the apparent wind much further aft. Due to the fact that the wind speed is greater at the top of your mast than the bottom the apparent wind is further skewed vertically along the Mainsail. It's no wonder that it's so hard to get the leeward tell tales to flow on the top of Pin Head Mainsails.

Square Top mains solve this dilemma by twisting at the top thanks to the excessive sail area in the head. By adjusting the Mainsheet, twist is easily manipulated. When combined with proper vertical camber distribution (your sail makers job), this makes the entire luff match the 'twisted' wind directions vertically. The bottom line? All your leeward Mainsail tell tales along the luff flow continuously.

For those of you that consume numbers, a sail with attached flow (leeward tell tales are flowing) has 180% the power of a stalled sail (leeward tell tales drooping or spinning). So, the top portion of a Square Top Main that enjoys attached flow is producing 80% more power than an equal sized Pin Head with stalled flow. So a Square Top is really bigger than life.

It's rare in sail design where an innovation can enhance performance in all wind speeds and wind directions. Why didn't we discover the Square Top long ago??