|Power Catamarans as pleasure boats|
Power catamarans occupy a strong niche in the leisure marine market acting as rescue craft, diving boats, water taxis, ferries and pleasure craft mainly in Australia, New Zealand, South Africa and the US, but they are gaining a stronger presence in Asia. But still, despite evident advantages in safety, performance and comfort their potential as pleasure boats is not fully understood or appreciated. The main disadvantages of catamarans are higher construction costs and docking fees due to limited availability of docking space. But still the biggest inconvenience cat manufacturers face is the stereotype labeling them "untrustworthy" in some parts of world and by certain operators; though their presence usually leads to a "chain reaction" that stimulates a stronger presence.Introduction
Architectural types and hull shape design Catamarans can have different architecture but there are some general rules. First, their layout is designed around the "tunnel" hull, having public spaces on the cabins and hulls. Catamarans are also "tall" – their higher initial stability and lack of usable volume in demihulls makes them "grow in height" making room for flybridges or upper decks on smaller boats. Catamarans are great day boats as they provide plenty of deck space. In general, catamarans provide at least 30% more usable area compared to monohulls.
Recommendations on hull shapes are based on the systematization of results of calculations, model tests and full scale trials and can be subdivided into two groups:• Factors of isolated hull shape;
• Factors of tunnel shape and hull spacing.
The catamaran's hull shape is similar to those used for monohull craft and approaches to their selection are also comparable.
The desired speed and the "slenderness" of the hull are major factors influencing the selection of the hull type. Catamarans can have a variety of hull shapes, but most can be classified into symmetrical and asymmetrical, with the few basic shapes presented in
• Round bilge displacement hull (B);
• Semi-displacement, combination of round bottom and chines at stern (C);
• Semi-displacement with so-called canoe stern (D);
• Semi-planing, chine flats and raised transom (E);
• Planing, monohedron shape (F);
• Split hull ("tunnel hull") (G);
• Split hull with stepped bottom (H).
Unlike monohull craft, catamarans possess enough stability due to the separation of the hulls and thus can feature very slender demihulls with high length to displacement ratios – with a relative length or slenderness ratio as a measure of length ratio to the cubic root of volume displacement l=LWL/V1/3. The effect of this parameter on hull shape selection is presented in fig.3. Traditional definitions of planing and displacement ranges are based on volumetric Froude numbers FnV and are not always applicable for catamarans. For a very slender catamaran, FnV>3 can still present a pure displacement mode where no significant hydrodynamic lift is developed.
Variations of the round bilge symmetrical shapes are the best choice for displacement catamarans. Properly designed planing catamarans possess sharp-chine shapes. Split hulls start to have an advantage at high speeds exceeding volumetric Froude numbers of FnV>4…5, where flat tunnel sides allows minimization of hulls interaction. Variations such as Z-bows, wave pierces are not shown, but they also offer interesting alternatives.
In design practice, hull shapes are affected by numerous factors such as materials and construction method, the preferences of the designer, builder and customer, the purposes of the boat, the desired type of propulsion, draft limitations, etc.
Factors of tunnel shape and hulls spacing are most intriguing in catamaran design. In terms of catamaran operations, seakeeping is strongly affected by vertical clearance t, and their performance is affected by horizontal clearance c. Generally, catamarans operating in the proximity of hump resistance tend to be wider; for planing craft typical values of horizontal clearance are c/L=0.1…0.2.Performance and seakeeping
Catamarans have gained a reputation as excellent performers; this is true if the craft is light and designed with speed in mind. But the advantage of high-speed catamarans is not the performance itself (sometimes their resistance is higher compared to monohulls), but the ability to maintain a higher speed at higher sea states. The key reasons why catamarans possess smoother ride on the seaway are:• Higher possible deadrise angles (as stability is provided by the separation of the hulls, not by the demihull shape itself);
• Combined chine beam of both demihulls is less than the chine beam of comparable monohull craft, especially for smaller boats.
These two main factors reduce the angle and area of slamming impact thus reducing vertical accelerations by 30-50% compared to monohull craft. This is true except for wet deck slamming; usually not something to worry about with pleasure cats. Catamarans are said to suffer from a small-roll period and "corkscrew" motions, but these are mostly features of relatively narrow catamarans and only in certain conditions.Controllability
AMD conducted a number of turning tests for a variety of catamarans and came up with an evaluation method of the turning radius and related values of the heel and side accelerations, depending on the speed, beam to length ratio and angle of steering action. Generally, wider catamarans possess a bigger turning radius R/L. The turning diameter of a typical planing catamaran at full speed and maximum steering/drive angle can comprise about DT=3…5 lengths of hull.
Most planing catamarans with symmetrical hulls heel outwards during turning though split hulls could heel inwards. Catamarans stay almost flat during circulation; their heel does not exceed 2-5 degrees at normal maneuvering speeds. This is a significant advantage for a comfortable ride on pleasure boats, ferries and water taxis. The side acceleration does not exceed 0.2g for normal maneuvers.
The manoeuvrability of cats at slow speeds and their backing performance is excellent – due to the twin-engine installation with a big turning lever between propellers, so usually no bow thruster is required unless the catamaran is in excess of 15m in length.Stability and regulatory aspects
Currently, the international ISO12217-1 standard on stability heavily penalizes power catamarans in categories A and B. This happens because wind and wave criterion in existing forms is used with a roll angle calculated by the formula for monohulls. Thus, the area under the stability curve at small angles of the heel is treated in an unfavorable way for catamarans. Usually this is large area for catamarans and at lesser angles, the stability of catamarans drops significantly.
Different methodology should be developed for those craft, which will allow a fairer application of criterion, maybe in a form used in the international HSC Code.Structural considerations
The current approach to structural design of catamarans is to consider the following loads:
• Local loads on the bottom, sides, deck, cabins - estimated for monohull boats;
• Local strength of tunnel – specific slamming loads for catamarans;
• General strength of the catamaran with special attention to transverse bending, shear and torsion.
Local loads typical for monohull craft can be defined using the ISO12215-5 standard or other appropriate structural rules of classification societies; this also include slamming loads on catamaran tunnels. Unfortunately, part of ISO12215, covering multihulls, has not yet been published.
The general strength of the catamaran is due to its hull, side, tunnel and deck plating and transverse bulkheads. The values for transverse bending moment Mtb, transverse torsional connecting moment Mtt, and transverse connecting shear force Qt were all considered. General strength conditions,were by default satisfied for small craft (LH < 24m) of normal proportions. This practical conclusion was confirmed by calculations (including FEA methods) performed by AMD and their colleagues.
Materials used for the design of composite catamarans are different. We widely use multiaxial fabrics and infusion processes in our designs, though simple solutions with hand lay-ups are still required by many builders. The application of sandwich structures is the key element in making catamaran design successful. Along with foam cores, we increasingly use polypropylene honeycombs as sandwich core material for some craft; unfortunately, not all builders are aware of their advantages. Honeycomb's shear strength is 20-30% less compared with PCV foam core of similar density. This restricts use of honeycombs to areas without significant impact loads; strength and resistance to buckling loads for honeycomb are also much higher when compared to foam. Certain types of honeycomb possess certification from German Lloyd and Det Norske Veritas for use in marine structural applications. For smaller boats and non-racing craft honeycombs do not make a significant difference in structural performance but do increase cost savings.New developments
Even with over 30 catamaran designs launched, AMD continues to develop new concepts, as a number of their catamarans are currently under construction in Thailand, China, the UAE and Russia.
Given this experience, AMD has found the catamaran platform perfect for developing new types of pleasure craft with interesting features. Recent developments include the Gerridae 26' – 7.9m innovative folding trailerable cat concept with electrical propulsion. The idea for this craft is to re-think pleasure boating towards fuel economy and green technology, with the ability to operate where boats with internal combustion engines are not allowed, and with wheelchair access. In Europe, 20% of the households have people with restricted mobility; an aging population should have access to leisure marine craft.About the author:
Albert Nazarov is naval architect (1996) having a Ph.D. (2004) in the field of sailing craft hydro aerodynamics. He is currently the managing director of the Thailand-based design office Albatross Marine Design, the biggest boat design company in South-East Asia. Together with his team Albert is developing pleasure, passenger and special high-speed craft (55% of which are catamarans), for builders in 14 countries.