When rigging the boat, the main hull is pre-assembled with keels and rudder attached and is introduced to the water and oriented with at least a component of wind from the bow. The outriggers and aero/mass balanced wing-sail planks are attached, one at a time.

When towing the boat, the two horizontal wing-sail planks are locked relative to the outrigger and control locks are installed. This enables the boat to be safely towed with the true wind from any direction.

Why the ability to tack?

• Practical reasons; with a tacking boat we will get far more than twice as many runs for a given window of conditions

• Towing a single tack boat back to the start point for each run would increase the risk of accidents

• A tacking boat adds little complexity. Being able to change the effective twist of each wing-sail gives good efficiency and balance across the speed range. It is not just there to give tacking ability

• The drag of the redundant outrigger is a small contribution to overall drag, and does not effect the maximum speed, which is set by the keel cavitation speed

• Last but not least, breaking the outright speed record will be a very effective way of introducing a new class of sailing boat. A degree of automation will make it possible to use this configuration to sail longer missions and to break other records. Not possible with a single tack boat

What defines and limits the maximum speed?

The top speed of the boat is the incipient cavitation speed of the keel and rudder (the speed at which cavitation first occurs). This is maximized by using a cambered keel section and transposing between two keels as the boat tacks. To increase the cavitation speed still further, the keel and rudder sections become thinner and the wetted areas become larger with increased viscous drag.

Setting the drag

The aim of the skipper is to control the speed of the boat to hold it just under the incipient cavitation speed of the keel. He does this by actively controlling the height of the boat above the surface, thereby setting the amount of keel/rudder penetration into the water. The keel and rudder are the largest contributors to overall drag.

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