The performance of a sailing boat is governed by the power-to-weight ratio and stability. A sailing dinghy can have an enormous sail plan, but it will be unsailable if the first gust knocks the boat on its side. To effectively use its sails, a sailing boat requires stability to hold the rig upright and convert the wind's energy into power. To achieve more stability, design the boat wider, but this becomes inefficient so the next scheme is to split the boat in two.
The Pacific islanders knew of this concept for centuries with their outrigger canoes and twin hulled boats astonishing the first Europeans visiting the region. It was not until after the Second World War that yachtsmen began experimenting with multihulls.
An advantage of a multihull is that it achieves stability from its width, and the individual hulls by being attached to each other stabilize each other and then can be designed slim with a semicircular section akin to a rowing eight.
Slim hulls are easily driven and able to reach a high speed without planing. As soon as the first catamarans like the 16-foot
Shearwater made their appearance, it was apparent they had a speed advantage over the typical monohull designs.
Early sailing days, multihull sailors were regarded as radicals and their boats were a nuisance, taking up so much room in the launching area and difficult to classify in normal events because of their speed. Because of this it was years before multihulls attained any popularity and acceptance into the sailing scene. This happening was due in a large part to boats such as the Hobie cat and Dart catamaran, which are easy-to-use production models.
Though they vary in shape, the Hobie Cat and Dart Catamaran share a common characteristic: neither has a centreboard. Californian Hobie Alter design was a deeply rockered 'banana' shape with crew on an aluminium trampoline frame above the hulls which could be run up the beach without damage.
Rodney March, already having designed the Olympic class Tornado took a different approach, designing the Dart catamaran with a streamlined skeg incorporated into the hull moulding but effective in stopping sideways movement.
For high performance there is no substitute for an aerofoil-section daggerboard, fitted to the Tornado and other racing catamarans. There is the expense and difficulty of constructing them strong enough to resist the exceptional side force that a catamaran develops at full speed.
As catamarans are much faster than normal dinghies, it was found that the usual rig of cloth sail set on a round mast was hindering the boat's potential. Aerodynamic solutions were required which lead to streamlined masts with rotation to be aligned with the airflow, and fully battened sails of stiff materials that emulate an aerofoil shape.
In major international events, including the International Catamaran Challenge Trophy, solid wing sails have been used but are impractical for normal racing because of expense and the need to be de-rigged every time the boat is ashore to prevent it sailing on its own.
Racing a catamaran is different from racing a dinghy as tacking is a slow manoeuvre because of the width and kept to a minimum, whereas small variations in tuning results in large differences in speed. Therefore Catamaran techniques dictate sailing to windward in a series of long, fast tacks instead of tacking dogfights like small racing dinghies.
The apparent wind becomes all important downwind as the catamaran’s speed pulls the apparent wind ahead and increases it. The most efficient way of keeping this effect going is by reaching downwind in a series of gybes as sailing straight downwind is slower.
Catamarans sail ably with one hull immersed in the water and the other gliding over it reducing the 'wetted surface’ of the hull. This may involve the crew deliberately sitting on the lee side of the boat to cause heeling, which is bizarre for a boat whose advantage is its stability.