'So he shortened the main mast, then designed that flexible curved boom you see that sits on top. The boom strengthens and supports the leading edge of the sail so that we can face it into the wind. A traditional square-rigged sail would simply flutter and vibrate and lose its shape. But with the boom, the sail forms a smooth curve so that we can redirect the driving force of the wind much more efficiently. The result is, the ship can move at an angle to the direction the wind is blowing from. In effect, we can sail against the wind.'

He paused, seeing a few questioning faces, then amended his statement. 'All right. Across the wind. But it's a huge improvement on the old square sail. That's unusable once the wind is any farther forward than dead abeam.'

'But you've duplicated that thin top boom and the sail,' Evanlyn said. And she was right. On the deck, lying fore and aft, was another boom, with its sail furled around it. It lay on the opposite side of the mast to the boom that was currently in place.

Gundar favoured her with a smile. 'That's the beauty of this design,' he told her. 'As you can see, the sail is currently on the starboard side of the mast, with the wind coming from the port side, so it's blown away from the mast into a perfect curve. When we tack…' He glanced quickly at Alyss but she kept her expression blank. 'The wind will be on the starboard side, forcing the sail against the mast, so that the perfect wing shape would be spoiled. So we rig another boom and sail on the port side. Then, when we tack, we lower the starboard sail and raise the port sail. The two are linked by rope through a pulley at the masthead, so that the weight of one coming down actually helps us raise the other one.'