18ft Skiff - Design Considerations

Design Considerations

The first flying 18 footers were either carvel or clinker built clinker planked with multiple steam bent frames. Cotton sails were used and spars were solid wood. The crew number varied according to the wind strength, often with a boy carried to bail out water. Initial designs were conventional displacement shapes with emphasis on narrow waterlines. In the early 1950s The Sydney boats put emphasis on carrying large extra sails down wind called ringtails. These were set outside the main with light spars top and bottom.They were carried in addition to spinnakers. In very light conditions watersails were carried under the main boom.The most revolutuonary boat of this period was the lightweight New Zealand Result which was cold moulded with 2 skins of Kahikatea glued together and nailed to lightweight Mangeao bent steamed frames every 21 1/2 inches. At 6 ft 3inches Result was narrower than the conventional boats but still had the same full bow sections typical of the displacement style boats. Result had spaceframes of wood to support the mast and centreboard. In any breeeze over 10knots it was faster than any previous 18s because of its planing ability. From this period increased knowledge and understanding of hydrodynamics and aerodynamics, coupled to the availability of plywood and reliable waterproof glues saw dramatic changes. Clinker and carvel construction was dropped and glued up hollow pear shaped masts became standard. By the late 1960s a greater understanding of the science behind planning saw hulls made with increasingly less rocker, very fine forward with very flat aft sections. Once trapezes were introduced the number of crew dropped to 4 by the early 70s and then to 3. Designs became wider, especially aft. As speeds increased so did capsizes, so an effort was made to install built in buoyancy and some self draining capabilities with transom flaps, venturi floor drains built in tanks, especially forward and false floors.

Foils became more hydrodynamic, especially after the availability of tank test models. Wood was dropped as a foil material in the 1980s and replaced by stiffer synthetic materials such as fibreglass then kevlar and carbon fibre. By 1970 hollow wooden spars were replaced by tapered aluminium of much smaller section and lighter weight. These in turn were replaced by much stiffer and lighter carbon fibre masts in the 1990s. Sail area got progressively larger especially when wings were added. This enabled small crews to sail with extremely large sails in stronger wind strengths. As most boats were sponsored they could have multiple rigs to match the wind strength. Other modern features are carbon fibre prods (bowsprits) supported by bob stays, dolphin strikers and bow struts. This enabled quicker setting and dropping of the large downwind sails. Huge spinnakers could be set quickly by having halyards geared by miniblocks running internally in the mast. Dacron sails were replaced by much lighter stiffer less porous material such as mylar plastic. Weights of the bare hulls dropped quickly when New Zealand designer Bruce Farr, using his experience in Moth and Cherub designs used thin 3mm ply supported by multiple lightweight stringers and stiffened with tissue fibreglass. These were quickly followed by foam and fibreglass hulls first designed by Russell Bowler. Bare hulls weigh 120 lbs but are very strong with the use of carbon and kevlar to locally reinforce mast steps, centreboard cases, chain plates and wing attachment points. Rudders were placed 500mm aft of the hull to give more control in extreme planing conditions when only the last few feet of the hull was in contact with the water. These innovations have made the skiffs very fast but very expensive to build. In New Zealand this has seen the class decline dramatically as top sailors were attracted into the more conventional one design Olympic classes. By 2005-8 experiments were carried out largely in Switzerland using hydrofoils based on successful experiments with the Moth class. The experiments showed foiling was possible in flat water or small waves after the distance between the 2 foils was improved. Experiments showed that a conventional spinakker slowed the boat down compared to sailing with main, genoa and or genakker. With very wide tube and net decks up to 29feet wide modern boats can carry very large powerful squareheaded mainsails that have the controls to twist the head of the main in gusts to depower. Flying 18 footers can now regularly exceed the actual windspeed when planing on all points of sailing.