UK-based Ad Hoc Marine Designs Ltd. (AHMD) has announced a new design of Small Waterplane Area Twin Hull (SWATH) crew transfer vessels (CTVs) for Maritime Craft Services (Clyde) Ltd (MCS), also a UK company. Initially, three vessels to the new design will be built at Työvene yard in Finland.
A SWATH ship is a twin hulled vessel which concentrates a large proportion of its floatation volume into bulb-like hull sections deep beneath the waterline. The torpedo, or submarine shaped flotation bulbs support the weight of the vessel through narrow, hydrodynamically efficient hull struts and in the case of the AHMD design have additional motion stabilization with zero speed heave mode controllable fins.
The vessel’s engines, fuel and other heavy equipment are contained within the upper haunch region for easy access and maintenance from the main deck, allowing the struts to only require minimal wiring and plumbing to pass through them — thus enabling them to be made very narrow and hydrodynamically efficient.
The main advantages of a SWATH hull form over a more traditional monohull or catamaran are twofold — according to Ad Hoc officials — the efficiencies derived from a low waterplane area and decreased vertical accelerations in rough sea conditions which allow a wider window of operation with more comfort for passengers and crew.
The waterplane area is the part of a vessel’s hull that would be seen if it was sliced horizontally along the waterline. It is a major contributor to wave making drag and hence a vessel’s overall drag, as well as the vessel’s natural period of motion. A SWATH vessel design reduces the waterplane area by having only the two foil-shaped, knife-like support struts intersecting the waterline. Particularly at slow speeds, a correctly designed SWATH hull offers significant CO2 reductions over a comparable catamaran or monohull. At much higher speeds monohulls and catamarans are able to utilize their hull shape to minimize resistance, whereas the efficiency advantages of the SWATH hull form begin to reduce with increasing speed.
It is also the reduced waterplane area that brings the improved seakeeping of a SWATH hull. A vessel’s volume distribution and waterplane area are what causes it to heave and pitch as it passes through waves. Reduce the waterplane area and the designer reduces the heaving and pitching forces that waves can exert on the vessel. Adding motion control fins to the SWATH hulls as in this design reduces these vertical accelerations even further, making for a seakindly ride, at any speed and sea state.
While SWATH designs have been around for many decades, and even more on paper alone, the number of naval architects and shipyards worldwide that can routinely produce SWATHs delivering all the promised advantages can be counted on the fingers of one hand.
“For more niche designs such as SWATH hulls there is nowhere near as much prior art as there is for more conventional hull forms. Ad Hoc Marine Designs have been involved in highly successful and proven SWATH designs for more than 30 years, and these range from 13 meter in size up to current SOV designs of 71 meter,” John Kecsmar, naval architect at AHMD, said in a prepared statement. “Our back catalogue of successful SWATH designs spans from our first SWATH MV Patria in the late 1980s, still the world’s fastest SWATH, as well as the naval architecture design of the Lockheed Martin Slice SWATH, to our latest range of typhoon Class of CTV SWATHs that are extending the North Sea operational window by more than three months per year.”
An extension of the existing two Typhoon Class SWATH CTVs, these new fully IMO Tier III compliant designs offer an IMO specification ballast water management system and are hybrid ready.
The vessels will carry 30 metric tons of deadweight along with 24 technicians in comfortable business class seating with low risk of seasickness thanks to the extremely low motions of a SWATH hull form.
The new design is a larger version of the Typhoon Class of SWATHs currently operated by MCS, thereby pushing the safe operational envelope, from their existing Hs (significant wave height capability) of 2.5 meters even higher to the holy grail of Hs=3.0 meters.
In addition, the advanced hull form exhibits extremely low resistance at low/loitering speeds. This current SWATH hull form requires less than 50% of the power required by an equivalent size and displacement conventional vessel when operating at slow speed, thereby reducing fuel consumption and CO2 emissions, Ad Hoc officials said.
