SWATH HULL

SWATH hulls, or Small Waterplane Area Twin Hulls, are a type of catamaran design that aims to reduce the hull cross section area at the sea surface, where most of the wave energy is located. By minimizing the ship's volume near the waterline, SWATH hulls can achieve lower wave resistance and higher stability than conventional monohulls or catamarans, especially in high seas and at high speeds. The main features of SWATH hulls are two submerged hulls connected by thin struts to a platform above the water. The submerged hulls have a large displacement and a small waterplane area, which means they have a low vertical center of gravity and a high metacentric height. The platform provides a large deck area and a high freeboard, which means it has a high vertical center of buoyancy and a low metacentric radius. The combination of these factors results in a low natural frequency of heave and pitch motions, which reduces the ship's response to waves and improves the comfort and safety of passengers and crew.

However, SWATH hulls also have some disadvantages and challenges compared to other hull forms. First, they have higher surface drag than conventional catamarans due to the increased wetted surface area of the submerged hulls. This means they require more power to achieve the same speed and have lower fuel efficiency. Second, they have a deeper draft than conventional catamarans due to the length of the struts. This limits their operability in shallow waters and increases their vulnerability to underwater hazards. Third, they have a complex structural design due to the bending and torsional stresses on the struts and the platform. This increases the construction and maintenance costs and requires sophisticated control systems to ensure stability and maneuverability. Fourth, they are sensitive to changes in payload and trim, which affect their draught and waterplane area. This means they need to adjust their ballast or speed to maintain optimal performance. One of the main challenges of controlling a SWATH ship is to avoid excessive pitch motions that can affect its performance and safety. To achieve this, SWATH ships are often equipped with control fins that are mounted on the inner sides of both hulls near the bow. These fins are small underwater wings that can generate lift forces by changing their angle of attack according to the oncoming waves. By adjusting the fins in a coordinated way, the ship can counteract the wave-induced pitch moments and maintain a level trim. The control of the fins can be done by using feedback sensors that measure the ship's motion and wave characteristics, and applying a suitable control algorithm that determines the optimal fin angles. There are two types of control fins: active and passive. Active fins are powered by hydraulic or electric actuators that can move them in response to a control signal from a computer or a human operator. Passive fins are not powered but rely on hydrodynamic forces or springs to move them in response to the ship's motion or wave pressure. The difference between active and passive control fins is that active fins can provide more precise and effective control of the ship's pitch, but they also require more energy, maintenance, and complexity than passive fins. Passive fins can provide some degree of pitch stabilization without any external power source, but they also have limited control authority, performance, and reliability than active fins. The choice between active and passive control fins depends on factors such as the ship's size, speed, mission, cost, and availability. SWATH hulls are an innovative hull form that offers superior seakeeping and deck space for certain applications, such as oceanographic research vessels, pilot tenders, yachts, and cruise ships. However, they also have some drawbacks and limitations that need to be considered and addressed in their design and operation. SWATH hulls are not suitable for all types of ships or sea conditions, but they are an interesting alternative for those who seek a smooth ride on rough waters.