Ad Hoc Marine Designs (AHMD) of the UK has announced a new design of Small Waterplane Area Twin Hull (Swath) crew transfer vessels (CTVs) for Scottish operator Maritime Craft Services (MCS).
Initially, three vessels to the new design will be built at Työvene yard in Finland. With the Swath hull form, a large proportion of its flotation volume is concentrated deep beneath the waterline, to support the weight of the vessel through narrow, hydrodynamically efficient hull struts. In the case of the AHMD design there is additional motion stabilisation 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 – 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 considered a major contributor to drag, so the Swath design minimises drag, particularly at slower speeds, when 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 utilise their hull shape to minimise resistance, whereas the efficiency advantages of the Swath hull form begin to reduce with increasing speed.
The reduced waterplane area brings improved seakeeping, reducing 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.
While Swath designs have been around for many decades, the number of naval architects and shipyards worldwide that can routinely produce Swaths delivering all the promised advantages are considered few.
John Kecsmar, Naval Architect AHMD said: “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 13m in size up to current SOV designs of 71m. 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 world-class typhoon class of CTV Swaths that are extending the North Sea operational window by more than three months per year.”
These new designs are fully IMO Tier III compliant, offer an IMO specification ballast water management system, and are hybrid ready. The vessels will carry 30t of deadweight along with 24 technicians. The design is a larger version of the successful Typhoon Class of Swaths currently operated by MCS, thereby pushing the safe operational envelope up to a significant wave height capability of 3.0m.
Menno Kuyt, Commercial Director MCS said: “The major advantage of the Swath design is its superior response to high seas allowing operation up to an extra 100 days a year over a conventional catamaran.”
In addition, this 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.
Image: AHMD Swath CTVs for MCS (credit: AHMD)
