Pages: 66 - 67
Author: David Owen
David Owen looks at the Ocean Bird, a cruising trimaran which offers speed with stability
Stability is both the strongest and the weakest point of multihull design. A normal single-hull sailing boat will heel over easily at the first puff of wind but its final stability is very great, thanks to the weight of its keel. The stronger the wind, the further it will heel - but the greater its angle of heel, the greater the righting moment of the keel, and the easier it is for the wind to spill out of the sails. A multihull, on the other hand, owes its stability to the wide spread of its hulls; most multihulls will sit upright in a moderate breeze, using more of the wind's energy than a monohull does to push the boat forward.
There are two drawbacks to this. One is that there is no 'giving' to a strong gust of wind, which means that the strain on mast, sails and rigging can be greater than on a monohull boat in the same conditions. The other - much more important - is that the bigger and more complex structure of a multihull boat can create windage problems of its own. If the wind is strong enough to bring its weather hull out of the water the boat becomes vulnerable to any increase in wind. This is where the three-hulled trimaran scores over the twin-hulled catamaran. When a strong gust of wind heels a trimaran over its leeward hull sinks deeper into the water as the windward hull lifts and its buoyancy gives the progressively righting force which a monohull gets from its keel. Catamarans on the other hand depend on the sheets being freed so that the wind can be spilled from the sails should the angle of heel reach the danger level. Safety devices like alarm bells or automatic release gear are sometimes installed to operate when the boat reaches a certain degree of tilt.
Multihulls do, of course, offer much that monohulls lack. With no heavy keel, and with bulk spread between two or three sleek hulls, friction between boat and water is minimised so sailing speeds are high and responses quick. Here again the trimaran tends to score over the catamaran, since three hulls give greater flexibility for living space and accommodation. Designs vary widely in the relationship between the three hulls in terms of size and displacement. But one of the most interesting and successful of the current crop of trimaran designs is John Westell's Ocean Bird, a Soft cruising trimaran built by Honnor Marine Ltd at Totnes and based on a unique combination of the advantages of monohull and multihull design.
Westell, like all multihull designers, is acutely conscious of the need for stability. This is why Ocean Bird is arranged as a monohull with two smaller, sleeker wing hulls mounted as outriggers - which ensures that when the boat heels it is forced to pivot about the centre hull, and the leeward hull sinks straight away to provide the self-righting action. There is no danger of the main hull lifting as well.
But Ocean Bird's most individual feature is its folding-wing capability. The wing hulls are joined to the main hull by simple tubular-steel frameworks - Westell deliberately avoided the flat bridge decks which, in really strong winds, can behave as aerofoils and cause dangerous lift. 'Once we'd decided that,' he explains, 'it seemed logical to make the wings fold inwards to save space in harbour.' So by unfastening a set of bracing wires, the wing hulls swing inwards to nestle snugly alongside the main hull. Ocean Bird can be motored and moored - though not sailed - with wings folded.
Because Ocean Bird sails at a more upright angle than a monohull yacht, the line between underwater and above-water parts of its hull is more clearly defined. This means John Westell has been able to combine a wide and roomy hull above the waterline with a sleek, shallow-draught underwater profile - thanks to the absence of a weighty keel. Ocean Bird uses a simple centreboard to give extra grip on the water and reduce leeway when sailing hard to windward.
Accommodation is certainly generous, although Westell has been careful to avoid the temptation to pack in as many berths as possible which seems to plague multihull designers. There are two berths in the forepeak, a toilet and washbasin amidships balanced by a locker for sails and oilskins on the other side. The main saloon has a settee berth on the port side, next to a permanently rigged chart table; on the other side of the cabin is a small table with a seat extending round three sides of it. The raised seats give a good view through the coachroof windows, while the table can be lowered and covered with a cushion to help make a double bunk. Next to the dining area is the galley, with gas cooker, sink and crockery stowage.
The stern cockpit is set at two levels - the helmsman's position right aft is raised up to give him a good view forward. The forward end of the cockpit is lowered to give more shelter to those not on steering duty. A wide hatch gives easy access to the big auxiliary engine: next to this is a central pillar holding the steering compass and the mainsheet mounting clear of the cockpit seats. Above decks, the width of the boat and its freedom from rolling give a safe and spacious working area. There is also a deep well in the bows for stowing anchors and mooring warps, and to give a little extra protection when changing headsails in rough weather.
A trimaran like Ocean Bird certainly seems to approach the ideal for any cruising yachtsman, giving a smooth ride in most seas and plenty of room without performance loss. But the strength-and-stability bogy still haunts many people who would otherwise be in the market, and they point to several well-publicised multihull accidents over the last few years. It should be remembered that monohulls have behind them more than a century of intensive development, while many multihull designers are still learning. John Westell, for one, is convinced of the trimaran's future. "If a trimaran is properly built to stand up to bad weather stresses, it's just about the safest thing afloat. It's light enough and buoyant enough to use as its own life raft if all else fails." Certainly on design, behaviour and appearance, Ocean Bird is a solid, well-thought-out boat which has been carefully put together. It was the only multihull to finish in the top ten in last year's Yachting World Rally for cruising yachts, and this may have done a lot towards breaking down the prejudice against multihulls once and for all.
Ocean Birds, seen racing at Cowes, far left, are swing-wing trimarans, designed by John Westell and built by Honnor Marine Ltd of Totnes. The two outrigger hulls, connected by steel beams, allow generous cabin and cockpit proportions while ensuring fast and secure sailing: outriggers can be retracted when Ocean Birds are under power or in harbour