Vinhaassa

Vinhaassa (Aibram’s ship in Birgom’s Diary) is specially designed to sail in restricted waters – narrow straits and channels, sometimes not very deep and with a risk of hazards such as sandbanks, uncharted rocks and wrecks. She beats upwind like a proa. (If you don’t know what a proa is, and what’s special about how it beats upwind, then see Beating Upwind.)

Notice the side-to-side asymmetry of Vinhaassa’s hull. This shape is somewhat like an aircraft’s wing, for the same reason: to optimize the lift:drag ratio as she glides through the water. An aircraft’s wing always flies in the same direction, though, so it doesn’t need to be symmetrical fore-and-aft like Vinhaassa. But the fore-and-aft symmetry doesn’t affect the lift:drag ratio as badly as the side-to-side symmetry of most real boats. This means that Vinhaassa can sail very close to the wind. (Yes, a boat built like her really could.)

Notice also the rudders at both ends – not vital, but much better if you’re going to be sailing in the opposite direction half the time. They, together with the short keel, also mean you can turn far more quickly than a conventional vessel.

Because Vinhaassa can be going in either direction, her rudders have to be balanced, that is the gudgeon (hinge) line has to run down the centre of the blade. A normal rudder, with the gudgeon along the leading edge of the rudder, tends to be forced hard over by the pressure of the water if you try to go fast astern.

There’s another big advantage to being laterally asymmetic. Because they’re symmetrical, conventional boats are upright in the water when there’s no wind, and heel over in a wind (R is the reaction force on the keel). If the wind is too strong, the crew have to take in sail (reduce the sail area) to avoid capsizing.

Heeling over too much also has a negative impact on the lift:drag ratio – it’s the equivalent of an aircraft’s wings angling upwards instead of sticking out horizontally.

Because the wind is always on the same side of Vinhaassa, she’s not designed to be upright in the absence of wind. In a moderate wind, she’s completely upright, heels much less in a good wind, and can handle a much stronger wind before she heels too much.

Apart from Vinhaassa and the Manafa line ships, the other ships in the book wouldn’t have looked out of place in the sixteenth or seventeenth century AD in the real world. Winghies are not really ships!

The massive painted wooden dragon on one end of Vinhaassa is designed as a battering ram for defence against pirates. Many ships in her part of the world have dragons on their prows, only a few of them designed for this purpose. Her short keel and a large balanced rudder at each end means she can turn in the water much faster than most ships her size – giving pirates who try to come alongside to board her a nasty surprise!