Modern conventional fast ships rarely go over speeds of 30 knots, simply because conventional hull forms do not allow higher speeds without unacceptably powerful and complex powerplant (a la LCS), which then means less volume and displacement avaliable for other necessities. High speeds are also very rarely necessary, which means that most ships can sacrifice high speed in exchange for greater firepower, survivability, or endurance. However, combat and peacetime operations have always required ships capable of achieving high speeds, for purposes such as reconnaissance, surprise attacks and combat in general, especially in littoral waters where small hulls dominate and enemy can appear from basically anywhere. High speed is also desireable for deployment of rapid-response forces as an answer to an unexpected crisis, and as airlift is of limited capacity, high-speed sealift is desireable. This means balancing out requirements for high speed, range and high payload.
In conventional ships, developing ships with higher length-to-beam ratio allows installation of more powerful powerplant for a miniscule increase in drag, thus allowing both longer range and higher top speed. Conventional hull form also has major advantages in range, and in that they can be of basically any size. However, they cannot economically achieve very high speeds, which is why specialized – unconventional – hull forms are required for ships that have such requirement. In all these, the goal is to minimize contact with surface of the water. Hull forms can be divided into:
- hull forms that sail on the surface of the sea like conventional ships, supported by the static bouyancy. Examples are catamarans, trimarans and SWATH ships
- hull forms that sail on the surface, supported partly by the static bouyancy and partly by the hydrodynamic lift. Examples are speedboats.
- hull forms which sail above the surface of the water, supported by the hydrodynamic lift. Examples are hydrofoil and various hybrid form ships.
- hull forms which float / fly above the surface of the water, supported by the static or dynamic air lift. Examples are hovercrafts (ACV – air cushion vehicle), side-walled hovercrafts (SES – surface effect ship) and ekranoplans (WIG – wing in ground effect).
Reason for development of unconventional hull forms is both the inefficiency of conventional hull forms at very high speeds, as well as the inability of conventional displacement hull forms to achieve speeds above 40 knots or any high speeds in heavy sea states. Reason for this is water resistance, which can be divided into three types. First is wave resistance, which depends on the hull form and energy used to produce waves. Once speed of a ship goes above a certain value (proportional to a square root of ship’s length), wave resistance starts rapidly increasing with further increases in speed, and eventually becomes dominant form of water resistance. Second is water friction, which depends on surface area in contact with water, water density and ship’s speed. Third and minor portion of resistance appears due to differences in water pressure between ship’s bow and stern, as well as due to water vortices created by ship’s movement.
At low speeds, friction resistance is a dominant form of water resistance. First ships shaped to purposefully reduce friction resistance are thought to be galleons of 16th and 17th century, which had length-width ratio of 4. Only in 19th century, with the appearance of steam engine, does the wave resistance become dominant. After a certain speed, wave resistance massively increases and any increases in speed past that point make no sense, as major portion of any energy added on top of the energy already being used will be used to produce waves, and only a minor portion will actually contribute to ship’s speed. This was somewhat countered by designing very slim ships – some World War II destroyers had length-width ratio of 10, with light hull, limited superstructure and armament, and could achieve speeds up to 36 knots. However, such overly slim hulls have stability issues, especially if heavy equipment is present above the waterline. Thus unconventional hull forms appeared.
Catamarans and trimarans
Catamarans are ships with two hulls connected by the above-water platform. They achieve excellent stability due to very wide hull, but are more susceptible to pitch and heave responses. It is used in small ships such as patrol ships and minesweepers. It has good fuel efficiency at high speeds, and excess stability allows equipment to be mounted high above the water line. Distribution of displacement between two hulls as well as minimum contact with surface of the sea allows them to minimize wave resistance. However, they are sensitive to changes in weight and position of the cargo, and do not allow access to shallow waters. Catamarans routinely achieve speeds of 35-40 knots, but have limited range and seakeeping quality.
Trimaran has a central main hull and two side hulls. Main hull has length:width ratio of 14 or so, which reduces water resistance despite the existence of side hulls. This leads to reduction in required power output, fuel consumption and acoustic signature. It has higher stability, lesser displacement for the same cargo capacity, and large deck area. Proper placement of side hulls can also be used to considerably reduce wave resistance. However, side hulls can reduce its maneuverability compared to conventional hull forms, and increase in wetted area reduces fuel efficiency at lower speeds.
SWATH hull is similar to catamaran, but displacement is produced by two fully submerged cyllindrical hulls. These are connected with main hull by two slim nacelles, allowing low crossection at the water line (small waterplane area). Main advantage of SWATH hull form is its ability to tolerate rough sea states, especially its ability to utilize full range of speeds it is capable of irrespective of the sea state. Experimental Sea Shadow ship, for example, could maintain speeds of 30 to 40 knots even at sea state 6. In fact, SWATH ship has the same amplitude of movement as a conventional ship of twenty times its displacement. However, due to small water line crossection, it is sensitive to balance and trim changes. Compared to conventional ships, both SWATH and catamaran form ships are far more sensitive to underwater damage, as assymetric flooding resuls in far greater tilt. It also suffers from greater water resistance for the same displacement than monohulled ships due to far greater wet area (17% water increase at the same displacement). This leads to increased fuel consumption.
SLICE is a SWATH variant with four short hulls (pods) instead of SWATH’s two long hulls. This design significantly reduces the wave-making resistance, leading to power efficiencies 20-35% greater than those with conventional SWATH designs at speeds in excess of 18 knots. Lockheed Martin’s SLICE prototype could maintain 30 knots with waves up to 12 ft (4 meters, or sea state 5) in height. Compared to the conventional monohull, it should have higher speed for same power, lower fuel consumption and lower wake signature at high speed.
Air Cushion Vehicles
ACV rides on a cushion of a low-pressure air, held in place by a flexible skirt along the periphery of the vehicle. Fans or blowers are used to maintain supply of air and thus maintain the pressure which keeps the craft above ground as air escapes from the bottom of the skirt. This allows the hard body to ride well above the sea or ground, and flexible skirt offers little resistance to forward motion of the vehicle. ACVs have demonstrated calm water speeds well in excess of 80 knots since early 1960s, making it useful for fast attack missions. Its amphibious nature also gives it beach-assault capability. Since its hull is not in water, it is less susceptible to damage from mine explosion, making it ideal for mine hunting. In moderate sea states, it performs about as well as monohulls.
Surface Effect Ships
Surface effect ships also use air cushion, but unlike ACVs they utilize rigid catamaran-style sidehulls. When air cushion pressure raises the craft, its side hulls remains slightly immersed to contain the air cushion. Flexible skirts fore and aft allow waves to pass thorough the cushion area. Presence of the sidehulls enhances underway stability, maneuverability and seakeeping. This makes it a good candidate for fast assault missions. Disadvantage is that the air cushion causes a destabilizing effect on the roll restoring moment due to the water level inside the air cushion being lower than the waterline. Compared to catamaran, SES uses less power and maintains higher speeds, but also suffers greater speed losses in waves.
Hydrofoils are monohulls with structural attachments that behave like aircraft wings to lift the main hull clear of the water. Foils can either be surface-piercing in V or U configuration, or fully submerged. At foil borne speeds hull of the craft can be completely lifted out of the water. Higher speeds result in greater lift, but lift can be controlled by changing foil’s angle of attack. Once hydrofoil slows below the take-off speed, foils can no longer produce sufficient lift and craft sinks onto the sea surface. Size of the hydrofoil craft is limited by the practical size of the foils required to lift hydrofoil’s hull out of the water. Due to the square-cube law, where hydrofoil lift grows by the square but mass grows by the cube of the dimension increase, the scale size and weight of the foils grows disproportionately with increases in hydrofoil vessel’s displacement. As a consequence, hydrofoil vessels are limited to about 500 tons of displacement. High speed plus the ability to operate in rough water make the hydrofoil ideal for the fast-attack role in restricted waters.
There are multiple hybrid hull forms, such as SES which uses powered aerostatic lift to supplement hydrostatic lift from buoyoancy. HYSWAS, hydrofoil small waterplane area ship, uses dynamic lift generated by hydrofoils to supplement buoyancy support. Tests on a 27 foot HYSWAS research vessel have revealed several advantages: 1) much less roll, pitch and heave compared to monohull, 2) better hydrodynamic efficiency than monohull at speeds above 20 knots, 3) reduced drag and power hump compared with pure hydrofoil vessels, 4) very little wake and 5) increased range and reduced fuel consumption due to hydrodynamic and propulsive efficiencies. The only limitation is vessel’s large draft.
Trimaran combines possibly the best of the both worlds between monohulls and catamarans. Main hull is kept very slender which means that the increase in wave resistance at high speeds is kept within reasonable limits. Required stability, typically the limiting factor in building slender monohull ships, can be obtained from side hulls, which can be kept small and slender as well, producing little resistance. While increase in wetted area reduces fuel economy at lower speeds, at higher speeds considerable gains are possible.
While trimaran’s high surface area places it at disadvantage at lower speeds where frictional resistance dominates, at high speeds dominant is residuary resistance, primarily composed of wave making resistance. Both wave making and form resistance is reduced as vessel becomes more slender. Further, by properly placing side hulls a considerable wave reduction is possible, resulting in lower wave resistance. Thanks to stability from side hulls, trimaran can also have very slender hulls and reduce residuary resistance, which at higher speeds will outweight frictional resistance increase due to larger wetted area. Overall, trimaran is a good choice only if the vessel is expected to spend significant portion of its operating time at high speeds.
Trimaran’s hulls can accomodate many different machinery arrangements. The center hull, with its greater continuous width in way of the machinery spaces, allows the use of larger, more efficient engines than a comparable catamaran. Other options might include the installation of waterjet thrusters or propulsors in the side hulls. This means that trimaran can have more powerful propulsion group than a catamaran, or have two different propulsion groups optimized for different regimes.
Additional advantage of a trimaran, shared with a catamaran, is the additional upper deck and upper ship space created. For the same displacement / volume as the monohull, trimaran creates a ship with greater length as well as greater useful upper deck beam. This can be used in various ways, such as better spacing and thus improved performance of sensors, or carriage of auxilliary aircraft (helicopters, UAVs). Greater stability provided by the side hulls also facilitates the ability to mount sensors and other equipment higher in the hull, improving early warning capability as well as reducing the effect of shock levels, thus improving equipment reliability in action.
Trimaran has advantage in seakeeping over both catamaran and monohull configurations due to long, slender central hull. Compared to monohull, it improves symmetric seakeeping by reducing heave and pitch motion due to the increase in length to displacement ratio of the main hull. It also has significantly improved damaged stability over monohull and catamaran designs. Side hulls also offer protection to critical central hull machinery spaces from both collision and weapons fire.
Trimaran can be used to significantly reduce ship’s IR signature by venting exhaust gasses down into the space between the main and side hulls. Radar cross-section can be reduced by incorporating an inward sloping, or tumblehome shape to the deckhouse. Another advantage is reduced wake wash.
Hrvatski Vojnik, Broj 14., Godina VI., Kolovoz 1996. (Croatian Soldier, No.14, Year VI., August 1996.)