Comparing cruising multihull design features

By Charles E. Kanter

 

Design features can be a complex subject. I'll deal with it--as the features relate to multihulls--in three separate parts.         First, let's discuss the shapes that make the boat go fast or slow, carry plenty of weight or be weight critical, and affect the boat's performance. Underbody shapes: keels and centerboards Every boat, without exception, is a series of compromises. In today's industry, we see two distinct trends in multihull cruising boat underbody design. Some of these design compromises are for simplicity and cost, others are for performance and draft.         Comparing the compromises in underbody configuration, the catamarans with fixed low-aspect ratio keels and spade rudders have the following attributes: protection from grounding damage to main hull and rudders; simplicity (no moving parts, no mechanical gear, no pivot point to leak); easier to sail (no board to trim); no trunk to clog with debris or to leak; more interior room; and less expensive construction (simpler to design with no trunk to plan into accommodation); and many vessels have easier steering and better tracking. Catamarans and sport trimarans with centerboards, daggerboards and either transom-hung or kickup rudders offer these advantages: very shallow draft; better windward performance (deeper draft board down); better overall sailing performance (less wetted surface); and ease of rudder maintenance and repair. Beam & hull-beam ratios For the individual hulls, the ratio of the length to beam (L/HB) is the clue to the performance aspect of the boat. Thinner is faster. A high performance boat will have hull-beam ratios of around 12:1 and often greater.         Dennis Conner's 62-foot Stars & Stripes of America's Cup fame has a L/HB ratio of 16 to 1. That means a 32-foot racing catamaran with a 12:1 LHB ratio will have a maximum hull-beam width of 32 inches. The higher the performance expectations, the higher the L/HB ratio. The trade-off for a high L/HB ratio is carrying capacity. As boats are designed closer to cruising specifications, they will have much lower L/HB ratios. Many cruising catamarans and trimarans are in or around the 8:1 L/HB range. While these L/HB ratios still project above average speed, it does create lower potential top speeds.         The common hull speed limitation factor (1.4 WL as expressed in monohull circles) changes as the hull-beam (HB) ratio changes. For example, a hull of 10:1 LB ratio will have a hull speed (K factor) factor of (2.8 WL) thus greatly increasing the potential top speed. This principle is the reason why catamarans are faster than comparable monohulls. However, they only are potentially faster given the right conditions. Beam overall – overall wide beam versus standard beam It must be noted that the standard length to beam overall ratio (L/BOA) for cruising catamarans during the preceding 40 years has been beam = 1/2 waterline length. (MAXB = 1/2 WL). This standard is still adhered to by many present catamaran manufacturers including Performance Cruising (Gemini) and Prout.         This beam ratio will be considered the "standard" beam for the sake of the following descriptions and has proven beyond any doubt to provide appropriate initial, athwartships, fore and aft, and transverse stability. This LB ratio is the same as used by the early Polynesian colonizers and is scientifically established as providing equal stability in all directions.         Wide beam in high-performance craft allows the carrying of significantly taller masts, allows carrying full sail area into higher wind strengths (with certain technical limitations). It also eliminates or greatly reduces the possibility of hull-wave interference and increases athwartships stability (and proportionally decreases fore and aft and transverse stability).         Wide beam in cruising boats allows for enormous amounts of interior volume, creates huge deck areas, allows creation of luxurious interior

This diagram of the waterplane (footprint) of two 32-foot vessels (a popular catamaran and a popular monohull) graphically describe why skinny hulls go faster than fat ones. The monohull has the typical K factor of 1.4 because of its 32-foot length and its 8.5 foot beam. The catamaran has a K factor of 2.8 based upon its 32-foot length and its 3.5 foot beam. and exterior layouts, and provides more headroom in central bridge deck cabins with fewer aesthetic penalties. The wider beam does require foredeck trampolines. Standard beam cruising boats in most cases are easier to handle and maneuver and allow access to marina and repair facilities through a greater length range of vessels. Standard beam also allows full decks (both fore and aft), thus eliminating the need for a trampoline. Fully decked catamarans have a drier ride, less maintenance, a simpler headstay attachment, and have better utility for anchoring and mooring. Standard beam cruising multihulls have a better ride and can be designed and built with less freeboard, thus less windage, and offer a more traditional appearance. Displacement There are several aspects to displacement, the first being designed displacement—the estimated weight of the vessel with the water and fuel tank half-full and all necessary gear installed. Installed gear would include sails and rigging.         The total displacement is the weight of the vessel as it is sailed, including the crew, provisions, dinghy, etc. Total displacement is the important figure. In any multihull, just like any other vessel or vehicle, there are total displacement figures that must not be exceeded. You do not expect your compact car to haul a pallet of bricks, and like your car, you should not expect your boat...any boat...to exceed appropriate displacement figures.         The difference between designed displacement and total displacement is the carrying capacity of the vessel. Designers and builders vary on how they arrive at the figure. Nevertheless, that is a number you need to know. You need that number on any boat—power or sail, monohull or multihull. The total displacement number dictates how much "stuff" you can put onboard.         Note the placards required on trucks: Gross–Tare–Net, with gross being the entire vehicle weight, net being the unladen or "designed" weight, and tare being the allowable load. Your boat has the same limitations and restrictions. Exceeding weight limits does the same thing to both vessels and vehicles. It makes them poor performers and potentially dangerous. It is important to obtain a realistic load capacity figure from the manufacturer of your vessel.         The following table, courtesy of designer Bill Roberts, shows graphically the effect of length/beam ratio (LB) Bill Roberts Presentation -- The "K" Factor LOA BMAX Disp Ratio Fineness Speed MAX D/L3 K Factor 27 16" 650 20 27 kts 30 5.2 27 23" 1300 14.3 20 59 3.8 27 28" 1950 11.7 17 89 3.3 27 32" 2600 10.2 15.0 119 2.8 27 72" 7500 4.0 7.3   1.4