2 min read · Updated 19 May 2026
A square-top mainsail has a wide, squared-off head; a pin-head mainsail has a narrow, pointed one. That single difference in the shape of the top changes how the sail makes power and how it sheds it in a gust, which is why almost every modern Grand Prix boat — the Melges 40 included — carries a square top. The mainsail is the engine of the boat, and the head shape is a big part of how that engine behaves.
The square top (or fathead)
A square-top main — also called a fathead — carries a wide head, supported by a long top batten, instead of tapering to a point. Two things follow from that. First, there is more sail area high up the rig, where the wind is stronger and cleaner, so the sail makes more power. Second, and just as important, the broad, flexible head twists open in a gust, spilling wind from the top of the sail automatically before the crew touches a control.
That self-depowering behaviour is the key. It lets a boat carry a big, powerful mainsail and still stay on its feet when a gust hits, because the sail effectively reefs its own top. It pairs naturally with a fractional rig that can be bent to flatten the sail — the relationship explored in carbon masts and rigging.
The pin-head
A pin-head main tapers to a narrow point at the top, the traditional mainsail shape. It carries less area aloft and does not twist off in the same automatic way, so depowering relies more on the backstay, mainsheet and the other controls. Pin-head mains are common on masthead rigs, where the backstay directly controls forestay tension and sail shape, and they remain simple and effective in that context.
| Feature | Square-top (fathead) | Pin-head | | --- | --- | --- | | Head shape | Wide, squared-off | Narrow, pointed | | Area aloft | More | Less | | Gust relief | Twists open automatically | Via backstay and controls | | Typical rig | Fractional, modern racers | Masthead and traditional rigs |
Which a boat uses
The choice follows the rig and the era of the design. Modern fractional racers favour the square top for its power and automatic gust relief, while many masthead and older designs use a pin-head. The Melges 40 carries a square-top mainsail of roughly 72 square metres — a powerful, self-depowering sail that suits its tall fractional rig and its planing performance. For how the main works with the rest of the sails, see the sails pillar, and for the boat that flies it, the boat page.
Frequently asked questions
- What is the difference between a square-top and a pin-head mainsail?
- A square-top (or fathead) mainsail has a wide, squared-off head with a long top batten, while a pin-head mainsail has a narrow, pointed head. The square top adds sail area and power up high and twists open in gusts to depower automatically, whereas the pin-head is simpler and relies more on the backstay and standard controls.
- Why do modern racing yachts use square-top mainsails?
- Square-top mains add power where the wind is strongest, high up the rig, and the flexible head twists off in a gust to spill wind on its own. That gives a powerful sail that also helps the boat depower itself, which suits fractional rigs and fast boats. The Melges 40 uses a square-top main for exactly these reasons.
- What is the advantage of a pin-head mainsail?
- A pin-head main is simpler and is well suited to masthead rigs and boats that control mainsail twist and depowering mainly through the backstay. It has less area aloft and is easier to handle in some respects, but it lacks the high-up power and automatic gust relief of a square top.
- What is a fathead mainsail?
- Fathead is another name for a square-top mainsail — a main with a wide head rather than a point. The terms are used interchangeably. The wide head is supported by a long top batten and adds power and self-depowering twist high in the sail.