John Shuttleworh Yacht Design.
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This 35ft catamaran exemplifies the new generation of performance cruising multihulls. Designed and engineered with the latest thinking on structures and computer designed hull forms, the boat is nevertheless well suited for skilled amateur construction. The first boat is currently under construction in Australia from wood although the design lends itself equally well to foam sandwich construction, and plans are available for either type of construction.

The Cruising 35 has been developed from the highly successful Spectrum 42, which itself represented a major advance in cruising catamaran design. The 42,s now have several ocean passages behind them, and the features of exceptional pitch damping and bow slightly raised on a reach have been well tried and tested, and have proven to be outstanding features in improving general cruising comfort at sea, along with greater safety and fine seakeeping qualities. ( Ref 1) (Ref 2)

One of the main aims of this new design was to produce a boat of outstanding comfort and living accommodation space, with as little sacrifice as possible to the excellent sailing performance, windward ability, and seakeeping qualities of this well proven type of hull form.

Apart from the great benefits that can be derived from flaring the hull above the waterline, in terms of keeping the bow up on a reach, the flare gives a massive increase in interior volume without increasing the windage of the hulls at all. For instance, above the waterline the hull flares outwards to give a maximum beam at eye level of 1.95m, the impression of space is tremendous and cannot possibly be matched by any similar length catamaran whose topsides rise almost parallel from the waterline.

Construction and long term fatigue.

By using integrated structural techniques the hull weight can be carefully controlled and the net effect is a strong, light displacement hull with a high long term fatigue life.

Integrated Structure is a computer aided design method originally developed by the designer for the highly stressed ocean racers of the North Atlantic circuit . One of the main principles of which is to dissipate all highly loaded areas into the body of the hull in a similar manner as a reefing patch in a sail. (ref. 3). On a wood epoxy design this is achieved by laying unidirectional fibres along the line of greatest stress in highly loaded areas, and spreading the fibres into the body of the hull or deck. Fibre quantities are carefully calculated so that the stress remains constant, thereby avoiding any stress build up in the structure at vulnerable points, and hence increasing the fatigue life of the boat dranatically.

Windward performance.

We have just completed an extensive study to quantify the factors which cause drag in multihulls and in particular the exact effect of all the drag factors, including aerodynamic hull drag, on windward ability.

In the past it was not unusual to find a bridgedeck saloon type Cruising Catamaran which would tack through 100 degrees or more. This poor pointing ability is caused by the combined effect of heavy displacement, inefficient under water foils, small sail area, and unstreamlined above water hull form.

By keeping the boat light (through Integrated Structure), by using efficiently shaped daggerboards and rudders, by minimising the bridgedeck height (we moulded the saloon around a model of a human figure on the computer!) and by using computer aided design to streamline the bridgedeck for minimum drag, we believe this boat will have excellent windward performance. The open bridgedeck boats both the 35 and 42 are already tacking through 78 to 80 degrees and this bridgedeck saloon version should lose only 2 degrees per tack.

Because the boat is light and the hulls are easily driven, and because hydrodynamic and aerodynamic drag has been minimised a large sail plan is not required for good performance. The modest sail plan drawn will provide a more than adequate power to weight ratio and the smaller sail area will mean that the boat is very easy to sail short-handed. Also smaller winches etc. are required leading to a further reduction in weight and cost.

References

  • Ref 1. Spectrum 42 - New Production Cat. John Shuttleworth, Multihulls , March/April 1985.
  • Ref 2. 35ft Catamaran, John Shuttleworth, Multihulls Magazine, Jan/Feb 1985.
  • Ref 3. Cruiser/ Racer Multihull Design John Shuttleworth, Multihull International November 1985, and 2nd Multihull Symposium talk, Annapolis 1985