SWATH- A new concept for the Safety and Security at Sea

SWATH significa Small Waterplane Area Twin Hull ( Cascos Dobles con Pequeña Área de Flotación ) embarcación que garantiza un excelente comportamiento marinero. El fundamento de este tipo de buque nació en los años 1960 y 70, cuando se introdujeron en el mercado offshore las plataformas semisumergibles. Desde entonces los arquitectos navales de Abeking & Rasmusen han desarrollado más y nuevos diseños que han sido utilizados como embarcaciones de prueba para la marina armada, transporte de pilotos, yates, barcos de pasajeros, etc.

Th e seakeeping capability of conventional ships depends mostly on their size and displacement. Naval architects usually optimize seakeeping behavior by well chosen main dimensions and refi ned hull lines.
Depending on sea state, a vessel is exposed to alternating accelerations, rolling and pitching motions. Th e diffi culty is not to design a vessel strong enough to withstand those environmental forces, but exposing sailors operating the vessel to these adverse conditions over a longer period of time.
Various stabilization systems have been developed to reduce rolling and pitching, some with remarkable results, but all of these systems have their limits as they cannot push beyond the limit set by physics.
By mastering a unique design, which focuses on minimizing the waterplane area of the vessel, A&R succeeded in reducing roll, pitch and accelerations in bad weather conditions to a comfortable level, even for non-sailors.
Since 1999 SWATH@A&R®-vessels are in daily service in the German Bight providing pilots a safe and stable platform with exceptional seakeeping capabilities. Over the years other applications were explored, designed and built and by today more than 20 SWATH vessels have been delivered.
Th is paper presents an overview of semi-submersible technology at the state-of-the-art for twin hull vessels and the special features developed by A&R.
Th e buoyancy of a SWATH is provided by fully submerged, torpedo-like bodies, which are connected by single or twin struts to the upper platform. Th e cross-section at waterline level is minimized and thus lowering the hydrostatic restoration forces at design draft. Th e resulting wave induced motions in the vertical plane are reduced about one quarter of those of comparable monohulls. Fig. 1 shows a comparison of waterline areas for diff erent hull confi gurations. Th e buoyancy of a SWATH is provided by fully submerged, torpedo-like bodies, which are connected by single or twin struts to the upper platform. Th e cross-section at waterline level is minimized and thus lowering the hydrostatic restoration forces at design draft. Th e resulting wave induced motions in the vertical plane are reduced about one quarter of those of comparable monohulls. Fig. 1  Using the precise wording of the German War Weapon Control Law, a SWATH is a craft with reduced waterplane area (A WL ) relative to its displaced volume (∇), specifically: Two design justifications for reducing the waterplane area are: • to reduce the wave-making resistance • to reduce sea-induced ship motions The original intention for the design of SWATH vessels was probably to develop unconventional fast crafts by reduction of wave-making resistance. A&R concentrated on the reduction of sea-induced ship motions to improve seaworthiness, crew effectiveness and vessel availability in bad weather conditions.
A SWATH@A&R® is by definition a fast displacement vessel. It is designed for operation at speeds up to a Froude number of 0.75, without slamming. These are speeds higher than those of frigates, refrigerated carrierers or corvettes, when related to ship length.
Semi-submersibles can be classified by the number of displacement bodies in each configuration. Table 2 divides SWATH configurations into design configurations.
The evolution of SWATH design from first generation to third increases the complexity of the development process because of the increasingly complicated interactions of the displaced bodies. The analysis of hydrodynamic and structural factors, including the optimization of resistance, becomes more difficult, increasing the required qualifications for naval architects during design development.
The greater the number of displacement bodies in a SWATH design, the greater the potential for meeting specific design requirements. At the same time, increased complexity of the design procedure adds to the sensitivity of the design to construction inaccuracies, expressed as excess weight or inadequate hull structure.

Generation I SWATH: single strut
Advantages: • good accessibility to spaces in lower hulls • good transverse strength • simple construction • significantly reduced ship motions compared to monohulls and catamarans Disadvantages: • dissimilar transverse and longitudinal stability • hydrodynamic efficiency (resistance and seakeeping) limited by layout • sensitive to speed dependent trim moments (Munk moments) (1)

Generation II SWATH: twin strut
Advantages: • more options for hull form shape to optimize resistance • longitudinal stability can be matched to transverse for optimal seakeeping compared to Generation I SWATH (single strut) • higher speed with stability in smooth water or a seaway • compared with Generation I SWATH (single strut) further reduction of ship motion • more sensitive reaction of ride control action, even stability longitudinally is increased Disadvantages: • limited accessibility to lower hull spaces ("bottle necks" in the struts for maintenance, ventilation, manholes, emergency exits) • increased requirements to an optimized hull structure Human beings which are permanently exposed to sea response motions (i.e. alternating accelerations) suffer from lack of concentration, exhaustion and seasickness. Even if experienced sailors get accustomed to motions of a vessel, it only prolongs the period of time when exhaustion and fatigue occurs.
In order to keep crew and passengers fairly comfortable and enable them to perform their main duties, limits have been defined, e.g. by the U.S. Navy or by ISO for the permanent allowable accelerations on board of seagoing vessels.
Following limits are widely accepted: • Crew and passenger comfort: 0,2 g • Operating limit experienced Crew: 0,4 g To stay within the "Crew Limit" of 4 m/s², the maximum speed of the DeepVee vessel needs to be reduced to 16 and 20 knots depending on sea state.
In comparison, the 25m SWATH@A&R® Pilot Tender maintains its top speed of 20 knots even without exceeding the "Crew Comfort" limit of 2 m/s².

General Design Concepts
A&R has until today developed and built four different groups of SWATH vessels defined mainly by length and deadweight capacity. Each group is characterized by the same underwater   body hull form, strut configuration, platform design and hull structure. Given the constraints for weight and centers of gravities, almost any other change to the designs are practicable which has been successfully proven by A&R in several cases (Table 2).

Propulsion Concepts
Both diesel-electric and diesel-mechanic propulsion concepts are available for all SWATH@ AR® vessels. Depending on the main task of the vessel, the most suitable concept with regards to operational and economical requirements is installed.
The contrast between both propulsion concepts is noticeable, especially for the smaller 25 m SWATH. Two diesel-generators are installed within the platform above water level providing power for electric motors which are installed within the displacement bodies below water level. Each of the electric motors drive fixed pitch propellers through reduction gearboxes enabling speeds up to 20 knots. A&R succeeded in implementing a diesel-mechanic propulsion where propulsion diesels are installed within the displacement bodies below water level driving controllable pitch propellers through reduction gearboxes enabling even higher speeds up to 21.4 knots.
Advantages of the diesel-mechanic propulsion are: • added space within the platform which can be utilized for extra cabins or other purposes • low noise and vibration levels since exciters are installed below water level remote from personnel onboard

General Design Concepts
A multi-purpose tool contains a variety of elements in one basis. The different tasks can be fulfilled by "switching" from one to the other element. By the nature of such a tool, the integration of the different elements requires compromises with regard to handling the tool and the integration of other elements. Some bulked variants of the well known Swiss Army Knife represent best whereto multi-purpose might lead. A modular tool consists of one common basis which can be supplemented by modules for a dedicated task. By exchanging the modules the tool can be configured for various different tasks and at the same time each configuration represents a dedicated tool. A socket wrench set is the best example for such a modular tool. Dedicated, single tasked vessels are hardly affordable in an era of shrinking budgets while the mission variety is increasing, which is the case especially for navies and authorities. With decreasing number of hulls within most fleets, modular or multi-purpose "tools" are considered to be the solution for executing all required naval tasks.
A&R responded to these requirements by implementing a containerized solution to its 25 m SWATH. One module with the size of a standard ISO 20ft. container can be stored in front of the superstructure in-between the stems (Figs. 5 and 6).
The gun module fitted with a medium calibre gun contains all subsystems necessary for operation of the gun, including ammunition. Only electric voltage (440V) will be supplied, target data will be transmitted by a data bus. Due to this limited interface requirement, the gun module can also be used on other units and even onshore.
Under condition of a compatible target assignment format, modules can be used by army units as well, e.g. for camp protection tasks. This brings the advantages of modular ship design into a "joint" account.
Instead of the gun module, other modules are possible for secondary tasks. The interfaces remain the same, i.e. only electric voltage, connection to the ship's data bus plus seawater and possibly freshwater. These interfaces will be available on almost every other craft of opportunity thus broadening the basis of using A module equipped with sonar ROV will enable MCM operations, such as e.g. route survey. Even a hyper baric diving chamber and breathing air compressor might be fitted for diving operations. The stable SWATH makes a perfect platform for diving operations. Access to the water is conveniently given by a hydraulic lift on the starboard side which reaches from main deck level well below the water surface.
Other modules might contain equipment for oil pollution control or hydrographic research equipment. Further applications can be developed as long as it is within the geometrical dimensions and the maximum weight. This allows cost effective future upgrades and enhancements of secondary missions.

Redundancy
Depending on the requirements and operational profile of a vessel, different redundancy levels for propulsion and ship's service system can be achieved with a SWATH. In any case two independent compartments comprising both lower hulls including two identical propulsion systems (one in each hull) are available. Given this compartmentation, gensets, auxiliary systems and electric panels need to be located in separate and independent engine rooms within the superstructure to achieve extra safety and redundancy effects. This can be achieved with small structural changes within the platform and of course with duplication of systems as required by redundancy class. An additional benefit is the large spatial distance between both propulsion trains providing additional safety.        In 2008 the Latvian Navy placed an order for five 25m SWATH@A&R® Patrol Boats based on the design of the 25m Pilot Tender. The decision was explicitly made to benefit from the well proven seaworthiness of the compact vessels. A crew of eight can remain at sea for one week to fulfill patrol and surveillance tasks, SAR missions and to participate in international assignments.

SWATH@A&R® in Service
The new vessels features a modular mission bay at the fore ship. By fitting appropriate mission modules, such as a medium caliber gun or mine hunting equipment, the capabilities of the vessels can be flexibly enhanced to suit the actually assigned task.