by Steve Callahan from Cruising World, April 1997

Almost no heeling, scalding speed, unsinkability and accommodations to rival your home -- sounds like an ideal boat by any measure.

A catamaran's light, narrow hulls slice the sea with little resistance while the twin hulls' wide stance provides enormous stability at low heel angles. The easily driven and enormously stiff platform offers the potential for higher speeds under sail or power and increased range under power.

However, the twin light hulls float high on the water to provide interior headroom. These, combined with voluminous connective bridge decks, create more windage than low-slung monohulls. As wave-making and frictional drag decreases, windage becomes a greater percentage of total drag. Also, designers and crews are enticed to fill the cat's enormous lateral space with accommodations and gear. Although cruising cats can be designed with a reasonable anticipated payload, each pound composes a higher percentage of a light vessel's weight, so performance is affected more. Weight and windage can eventually destroy any performance advantage that the multihull offers. As bulk increases, the higher center of gravity and sail plan detract from ultimate stability, which can be critical in a storm offshore.

In this special section, we'll review the latest multihulls to hit the cruising market, and outline the range of choices that designers make to create catamarans that lie along a wide spectrum, from flat-out racing machines to full-powered auxiliaries.

At a cost to stability but to promote maneuverability and docking, the designer may choose a narrower beam, which requires less strength, so it can be built lighter. The designer can redeploy this weight by making the hulls more voluminous and/or giving the boat more payload capacity, increasing drag but also regaining some stability. Of course, a narrower, heavier, more commodious boat would be even more stable if widened again, allowing a larger rig for more speed.

The designer seeks the compromise that will fulfill its prime directive while he shuffles numerous other attributes as well, such as choosing boards for better upwind performance or keels for simplicity and freeing the accommodation. For pure speed, he will likely choose boards, very narrow hulls, very wide beam, low freeboard, high sail area and no bridge deck, but the boat will likely be wet, spartan and difficult to maneuver in tight quarters. For living in harbor with palatial accommodations, he designs wide hulls, extreme freeboard, very high cabin, and full bridge deck. But don't count on this boat giving significantly faster passages, or even greater speed over short courses, than many performance-oriented monohulls, although it may give brilliant bursts when reaching in moderate-to-stiff airs. The safest and fastest cats for ocean passages will feature moderate freeboards, high underwing clearance, low bridge-deck cabin height and moderately broad beam.

Many designs are available only on a custom or semi-custom basis, but most of today's production cats combine satisfying, often exhilarating sailing with spacious and comfortable living aboard.

Cat Facts On Beam

Wide Beam Benefits: Greater righting arm and wider "footprint" on large-wave contours to optimize stability for greater sail-carrying power, speed and ultimate storm stability; minimal rolling; more space for bridge-deck accommodation; wider sheeting angles to optimize off-wind sailing with jibs and spinnakers.

Narrow Beam Benefits: Quicker turning; easier docking and fitting slips; reduced connective structure weight; less stress on boat structure; more rigid structure for rig stability.

Although the boat's maneuverability and stability are ruled by the spacing of the hulls' centerlines, overall beam measurements are more widely available and can be used for comparisons. Overall beams in excess of 50 percent of boat length are normal, and some trimarans are even "square," or as wide as they are long. However, there is a point when the boat becomes less stable longitudinally or diagonally than athwartships, so there is a balance. For ultimate performance, multihull designer Peter Wormwood once remarked, "When you've made the boat wide enough that you're not sure whether it will first capsize to the side or pitchpole, then you have the dimensions about right," but for maneuvering in tight quarters and finding docking space, narrower beams still make more sense and will result in a boat unlikely to pitchpole.

Cat Facts On Bridge (Wing) Deck Configurations, Underwing Clearances And Freeboard

Short Bridge Deck With Low Cabin Or No Bridge Deck Benefits: Reduces weight and windage for better speed and pointing ability; minimizes weight in ends to reduce pitching; sail plan can be carried lower to increase stability for safety and sail-carrying power; large passing waves can vent through open areas of connective, reducing pounding and overturning moment from storm seas; a low-profile bridge with underwing high off the water for less (or no) pounding from normal offshore waves can still contain berths with sitting headroom.

Longer-To-Full Bridge Deck With Full Cabin Benefits: Greater interior space; well-lit saloon with good view unites hulls' interior layout; more rigid support for rig (especially if extended to jibstay attachment) which reduces shock loads and fatigue of fittings; longer bridge deck spreads stress between hulls and connectives, keeps deck drier and eliminates area of inter-hull trampoline, which must be replaced periodically.

Benefits Of Heights Off The Water: Freeboard provides a drier ride and more interior volume for headroom and accommodations in hulls; bridge-deck cabin height provides headroom; raised bridge-deck underwing reduces slamming by waves for comfort, speed and less structural stress.

Benefits Of Low Heights Off The Water: Lowers the center of gravity, increasing the range of stability (important offshore in storm conditions); lowers rig height, giving more sail-carrying power and dampened pitching; minimizes windage for better pointing ability; more pleasing aesthetics. Low freeboard gives less wind drift and wave impact in heavy seas; easier boarding and disembarking from the side to promote docking.

Bridge-deck underwing clearance forward is vital, but it is important throughout the length of the bridge deck. Clearance is less of an issue in flat water, but offshore it is imperative for comfort and safety. Waves slamming underwings have damaged many boats. When the going gets tough, slamming of the wing can really rattle things around in the boat -- it has even bounced people out of their berths -- adding structural and psychological stress and slowing the boat, perhaps even preventing progress to weather. Wider-spaced and longer hulls span more ocean, allowing swell and chop to affect the underwing more and demanding more wing clearance. Many boats use complex curves under their wings, which helps to dissipate any slamming and excess loads.

Multihulls appear as ugly boxes to some sailors while others think they are as elegant as seabirds. One catamaran aficionado wisely directed his designer to draw up a boat with a typical 50-footer's accommodations, then make the boat 65 feet long. The result was elegant and not much more costly. Freeboard for a given length is one key to catamaran aesthetics. A heavy monohull sits deep in the water, so freeboard to provide headroom need not be excessive. A light catamaran's hulls cannot be very deep, so freeboard tends to be high. Even a trimaran's freeboard is lower because the boat's weight is supported by a single deeper hull, and its freeboard is masked somewhat by sleeker outer hulls. Many multihull designers note that it becomes exceedingly difficult to create an elegant cat with a luxurious accommodation and standing-headroom across the bridge deck in a vessel less than 40 feet long.

Cat Facts On Displacement/ Length Ratios

Light Displacement Benefits: Less weight and narrower hulls have to move less water so are faster; quicker acceleration and deceleration provides livelier ride; shallower draft; less energy to power up or stop, so can use smaller, lighter and less expensive sails, engine and other gear to achieve a given performance; less expensive to build unless using exotic construction methods/materials.

Heavier Displacement Benefits: More forgiving sailing -- less sensitive to perfect helming and sail trim; carries way farther; easier tacking; greater stability; greater volume for accommodation and payload; slower, more comfortable motion though usually greater pitching.

Catamaran weight is a critical issue and published catamaran displacements have proven highly suspect. Due to designer optimism and construction quality control, errors of 10 or even 20 percent or greater have been common. Also, sailors often overload their boats. Rarely are actual boat weights verified by weighing or measuring them when floating and recalculating their displacements. Added weight increases stability but also increases structural loads and dramatically affects comparative ratios such as sail area/displacement and the reserve buoyancy available to carry payload safely and efficiently.

For fair comparisons and if available, use the boat's true half-load displacement (with gear ready to sail minus crew and cruising payload). Don't be confused with empty boat weight or total cruising weight, which may differ by as much as 50 to 100 percent. Also, beware of claimed displacements significantly less than other boats with comparable bulk and accommodations unless the lighter boat is built with sophisticated techniques and materials. Otherwise, the figures are faulty or the construction must be correspondingly lighter.

Cat Facts On Hull Shapes In Section And Hull L/B Ratios

Beamy Hull Benefits: More payload capacity; shallower draft and increased maneuverability for a given displacement.

Narrow Hull Benefits: As hulls narrow from 8:1 to 12:1 or greater, wave making resistance decreases dramatically, yielding speeds in excess of traditional concepts of Òhull speedÓ as well as less pounding and better tracking.

Semicircular Sections Benefits: Minimizes wetted surface for better light-air performance; balances soft motion in choppy waves, load-carrying and shallow draft; relatively easy to build. Drawbacks include limited interior volume and tunnel-like feel -- flared topsides help mediate this. Quasi-semicircular -- wider than deep underwater -- increases load carrying while increasing resistance from wetted surface and lost hull fineness.

Flared Bell Benefits: Greatly increased interior volume, especially at and above seat/berth heights while retaining ideal underwater shape. Drawbacks: Added windage and structural weight; more wave resistance in a chop; more difficult to build.

V'd And Rounded V Sections Benefits: Good wave penetration for soft motion; simpler to build from sheet materials; deeper for same displacement so lower freeboard; good tracking. Drawbacks: Provide little pitch dampening; very narrow sole in the hull; slower to turn.

Hull shapes tend to be complex, often mixed. Obviously, most are sharper and more V'd forward, but the current trend is toward beamier hulls aft to dampen pitching and increase payload. The waterline beam and payload buoyancy of more V'd hulls increases quickly as they depress under load, but so does drag from increasing hull beam and wetted surface. Semicircular hulls depress less quickly initially, but then more evenly under increasing loads, which increases wetted surface but retains hull beam. However, if boats with transoms begin to drag their sterns from overloading, heeling, or pitching, performance suffers. Waves may create resistance when transoms dip or if the knuckle of bell-shaped hulls is lowered too much by overloading. Asymmetrical hulls and box sections appear on some designs, but are rare on production cruisers.

Cruising payload -- the weight of crews and stores -- varies, but 2,000 to 4,000 pounds seems reasonable for most sailors. You can compare different boats' load-carrying capacity by using pounds per inch immersion (PPI), the amount of weight it takes to sink the boat one inch deeper on its lines. To approximate PPI, multiply the hull waterline beam times its waterline length times .6 times two (for both hulls) and divide by 64.

Cat Facts On Sail Areas

Large Sail Area (higher numbers) Benefits: Speed; added sail power momentum helps resist pitching.

Small Sail Area Benefits: Easier handling; reduces costs of rig and sails; less likely to accidentally sail into a capsize; shorter, lighter rig produces less pitching moment. Developed by Edmond Bruce, father of the Bruce anchor, the Bruce Number is the square root of the sail area in square feet divided by the cube root of the displacement in pounds. This is similar to the performance indicator Sail Area/Displacement ratio. Both are power-to-weight comparisons. Bruce theorized that the Bruce Number should approximate the speed that a vessel can sail compared to wind speed. If greater than one, the boat can exceed wind speed on some points in some conditions. When comparing boats, it is important to use working sail with 100 percent foretriangle (not the genoa area that is sometimes used in brochures) and half-load displacement.

As noted, light rigs produce less pitching moment, but often in the real world, boats that are undercanvased will pitch more than boats harder pressed, because sail power contributes to momentum and the dynamic mass of the sailing machine, resisting knocks from waves. Wave conditions and course will dictate the most efficient amount of sail to fly.

Cat Facts About Safety

After some capsizes and notable structural failures in the 1960s and 1970s, it was only natural for people to doubt modern, western multihulls, even though well proven at sea for millennia. Today, several experimental righting systems have been tested, but few multihulls are fit with them and almost none are self-righting. However, it is extremely rare for a cruising boat to capsize, and even rarer for it to be sailed into a capsize. In addition, most are unsinkable, and most provide multiple collision-protection bulkheads. There is also evidence that sailing with minimal heel provides some added safety to the crew.

Although statistics are sparse, a study of 35 publicized multihull capsizes between 1975 and 1985 contained only three cruisers, one anchored in a 170-knot hurricane. Ninety-one percent were racers, designed and sailed to the edge, and 60 percent occurred during racing or record attempts. A full 54 percent of the boats were eventually salvaged, some floating for months before retrieval. Ninety percent of the crews survived, and half of those lost were on a single boat shadowing the infamous 1979 Fastnet Race that claimed so many monohullers. What percentage of sunken or even rolled monohulls and their crews survive? We just don't know. Designer Chris White also has studied statistics and can only

conclude that, in recent decades, multihulls have proven to be up to 23 percent safer than monohulls, but again admits that the samples available are too small to make definitive conclusions. At worst, it appears that multihull capsizes are marginally more common than monohull sinkings, and in the cruising world are exceedingly less common.

Like all vessels, the multihull must be prepared for the worst with a plan for making the hulls habitable upside down, including handholds under the wing, access hatches to allow entry and exit from the hulls, places for the crew to perch and sleep inside, and secured stores, especially emergency equipment. So prepared, multihulls have proven to be exceedingly safe, making superb survival craft.

Due to their wide beam, light multihulls actually provide a lot more righting moment, measured in foot pounds, than monohulls of similar length, but they reach maximum stability at low angles of heel (as soon as a cat's weather hull lifts from the water). Cruising cats should never be pushed to "fly" a hull, but storm forces may lift one. Maximum stability can be approximated by multiplying 1/2 of the centerline-to-centerline beam (or 75 percent of overall beam for narrow cruisers; up to 90 percent for wide racers) times the catamaran's displacement. Dynamic stability is more complex and is affected by factors such as the slope of large waves, but especially by the height of the boat's center of gravity. To maximize the angle at which the boat will reright, weight must be carried as low as possible.