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CUVX Design – VT Team 2 Page 123 Concept ExplorationChapter 3 describes CUVX Concept Exploration. Trade-off studies, design space exploration and optimizationare accomplished using a Multi-Objective Genetic Optimization (MOGO).3.1 Standards and SpecificationsBased on the ADM and program manager guidance, CUVX shall be designed and constructed usingcommercial standards wherever possible, with the exception of specialized mission systems and survivabilityenhancements, or as required for LPD-17 commonality. This guidance is driven by the stringent cost threshold.Additional military standards may be incorporated into subsequent CUVX platforms once the CUVX/UCAVmission concept has been demonstrated. CUVX shall comply with American Bureau of Shipping (ABS) Rules forBuilding and Classing Steel Vessels including requirements for classification as A1, AMS, ACCU, and unrestrictedservice as applicable and where military specifications are not applied.The following standards shall be used as design “guidance”:• General Specifications for Ships of the USN (1995)• Longitudinal Strength: DDS 100-6• Stability and Buoyancy: DDS 079-1• Freeboard: DDS 079-2• Endurance Fuel: DDS 200-1• Electric Load Analysis: DDS 310-1• Aircraft Handling Deck Structure: DDS 130-1If the LPD-17 modified repeat alternative is selected, LPD standards and specifications shall be used for thoseportions of the CUVX design that are common with the LPD.3.2 Trade-Off Studies, Technologies, Concepts and Design VariablesAvailable technologies and concepts necessary to provide required functional capabilities are identified anddefined in terms of performance, cost, risk and ship impact (weight, area, volume, power). Trade-off studies areperformed using technology and concept design parameters to select trade-off options in a multi-objective geneticoptimization (MOGO) for the total ship design. Technology and concept trade spaces and parameters are describedin the following sections.3.2.1 Hull Form AlternativesSix different hull form type alternatives were considered for CUVX:• Catamaran• SWATH• Trimaran• Conventional monohull• Wave piercing tumblehome (WPTH) monohull• Modified-repeat LPD-17 monohullThe selected hull form must satisfy the following requirements: 1) displacement between 20,000 and 30,000 MT;2) low radar cross-section (RCS) where possible; 3) good low speed endurance; 4) low cost; 5) good volume forlarge object spaces (machinery spaces, hangar decks, weapons magazines); 6) a recovery deck length greater than150 meters for recovering UCAV’s; and 7) good seakeeping characteristics. Each of the hull form types wasassessed based on these requirements with the following conclusions:• CatamaranThe Catamaran or twin-hull concept has been employed in high-speed craft design for several years. Thecomponent hulls (demihulls) usually have V-type sections and a cut-off transom stern. The division ofdisplacement and waterplane area between two relatively slender hulls results in a large deck area, good stability,and smaller roll angles than monohulls of similar displacement under similar sea conditions. However, seakeepingqualities in terms of angle and rate of pitch are poor compared to a monohull. This problem can be reduced usingactive control of pitching motions.The wetted surface area ratio, slenderness ratio, and hull spacing strongly affect the resistance of a catamaran.The wetted surface area ratio is high compared with planing monohulls of the same displacement. Thus,
CUVX Design – VT Team 2 Page 13catamarans have relatively high resistance at low speeds (Fn < 0.35) where skin friction is dominant. At higherspeeds, the low wave-making resistance provides low total resistance. Beneficial wave interference can beachieved by the cancellation of part of the divergent wave systems of each demihull.Catamarans have a relatively high radar cross section, especially end-on. The displacement to length ratio ishigh and the large object volume is relatively low compared to a monohull. The cost for building a catamaran ishigher than that for a monohull of the same displacement.• SWATHThe SWATH (Small Waterplane Area Twin Hull) hull form consists of two cylindrical lower hulls that arecompletely submerged. The upper hull rides above the water and is connected to the lower hulls by vertical struts.The small waterplane area of the vertical struts greatly improves seakeeping for a given displacement, but increasesthe load sensitivity. SWATHs are considered load sensitive due to the fact that for a small load added to the ship, arelatively large change in draft occurs. Typically, SWATH ships have 50% less waterplane area than monohullships of equal displacement. Deck length available for landing and takeoff is smaller on a SWATH ship sinceSWATH ships are generally shorter than monohulls of equal displacement. This would result in greater forcesbeing exerted on aircraft to takeoff and land on the shorter deck. SWATHs have a relatively high radar crosssection, especially end-on, unless special provisions are made to reduce it, as with Sea Shadow. In order to lowerthe radar cross section by incorporating a tumblehome, the SWATH would have to be wider, increasing thetransverse bending moment and reducing structural efficiency.While the reduction in waterplane area decreases wave-making resistance, it leads to an increase in frictionalresistance. The increase in frictional resistance is due to an increase in the wetted surface area of the hull. The thinvertical struts also present structural problems. SWATH ships generally experience large transverse bendingmoments that must be countered with added structural support. SWATH ships are more expensive to build andhave less large object volume than monohulls of equal displacement.• TrimaranThe trimaran hull form consists of a very slender monohull with shorter slender hulls attached to each side.The trimaran hull form has some advantages over a conventional monohull such as decreased resistance for Froudenumbers greater than 0.3, increased stability and more deck area for flight operations. The decreased resistance ofthe trimaran hull form is important for CUVX when the ship is going faster than 10 knots. Since the endurancespeed of CUVX is greater than 10 knots, the reduced resistance is an advantage for fuel savings. The U.K. and U.S.have developed a trimaran research vessel and the concept is currently being tested.Trimarans could reduce heat signatures by ducting exhausts between the hulls. The radar cross-section of atrimaran is comparable or greater than a conventional monohull of similar displacement. Given that a trimaran hasslender hulls, the large-object arrangeable volume is relatively small and limited. The cost of a trimaran would begreater than a conventional monohull of similar displacement.• Modified-Repeat LPD-17As required by the ADM, the modified-repeat LPD-17 is an alternative for CUVX. The LPD-17 is a 25,000-ton displacement, bulbous bow, amphibious assault and transport ship that is currently being constructed atNorthrop Grumman Ship Systems, Avondale Division and Bath Iron Works. This ship is designed to reduce radarcross-section and to meet top level requirements for survivability. It has a very robust and survivable hull structure.The current design includes stern flaps, which increase fuel efficiency and speed.The greatest advantage of using a modified-repeat LPD-17 is that the design has already been completed andwould only need to be modified. This reduces design cost, improves producibility through commonality, andreduces the cost of logistics support and training. LPD-17 has more large-object arrangeable volume than a multihullhullform of the same displacement.The main disadvantage of using this hull form is that the principal dimensions of the ship can not be altered.This limits optimization of the design for the CUVX mission.• Conventional MonohullAn optimized conventional monohull hull form with bow flare is the most traditional design considered.Shipyards have more experience in building monohulls and this could improve producibility and reduceconstruction cost. Monohulls have larger large-object space than any of the other hullform alternatives for a givendisplacement. Optimizing a conventional monohull has an advantage over an LPD-17 mod-repeat in that it can be
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