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Equations in Cat Design: Develop a critical eye for proportions
Many factors determine how a cat functions, that is, how well it delivers its promise as a sailboat and as a living space. Designers go to great lengths to create the desired level of appointments while balancing hull hydrodynamics and weight distribution to provide the requisite combination of performance, stability, and ride characteristics.
As in monohull design, one goal is to keep weight close to the center of the vessel to reduce pitching. But if the weight is too concentrated, the pitch frequency may in fact become uncomfortably short. This won't bother adrenaline junkies but might put the less sanguine on edge. Moving weight away from the center of gravity will lengthen the pitch frequency but also increase its amplitude. Too far, and the motion both tires the crew and slows the boat.
Hull design, particularly how finely the bows and sterns are drawn, also contributes to pitching characteristics. Full ends with generous reserve buoyancy will act quickly to damp the motion, perhaps uncomfortably so. Fine ends may not damp the pitching enough, resulting in hobbyhorsing and, perhaps, the potentially dangerous immersion of the bows. Fine ends work best on boats groomed for speed with centralized weight, whereas full ends better absorb the long- period pitching of a heavier boat with a more widely distributed load.
It follows that boats designed for the performance end of the spectrum have short bridgedecks that keep the weight centered; pure cruising boats spread the accommodations, and the load, more longitudinally. The bridgedeck of the built-for-speed Gunboat 48 is 53 percent of the boat's overall length. On the Switch 51 and the Catana 47, it's 55 percent. At the opposite extreme are the Gemini 105Mc (89 percent), some Lagoon models, and the Royal Cape Catamarans 50 (82 percent). Most cats fall between the Nautitech 40 (63 percent) and the Lagoon 420 (77 percent).
Another key factor in the agility-vs.-amenity equation is the length- to-beam ratio of the hulls at the waterline. Narrower hulls are faster but don't have the payload capacity of beamier hulls. Most cruising-catamaran hulls have length-to-beam ratios of about 8:1, but at this ratio, even a 40-foot hull offers tight quarters, taxing the designer to create accommodations that aren't claustrophobic. Ways of creating additional living space include flaring the hulls above the waterline or stepping them inward in way of the wingdeck. This sometimes results in increased drag when sailing in anything other than flat water.
Builders run the gamut from mass producers to one-at-a-time custom yards, and each weighs the features that affect performance, comfort, and cost, hoping to strike the balance that will attract the right customers in the right numbers. Clues as to the builder's thinking are visible on the boat's exterior.
If peppy performance is on your wish list, look for a bowsprit, used to project the tack of a cruising chute of some kind. Look also for daggerboards, which particularly improve windward performance and maneuverability. Windage is a big drag on cats, so boats designed for speed and agility tend to have a more streamlined appearance: rounded deck edges; smooth, lozenge-shaped deck saloons; lower total profile. Faster cats also have shorter bridgedecks, so eyeball the trampoline area forward and the hull extensions aft.