ADVANCED HULLFORMS
In the T-Craft project the US Navy is looking for a high-speed, load carrying ‘connector’ that would carry more than existing craft such as LCACs, and be more stable and seaworthy whilst at a seabase.
• Require reliable low-risk technologies that can result in a viable near-term affordable solution.
To achieve this capability, a multi-hull was
thought to be best suited to solve the first and last of these problems, and an SES the best solution for the second, with a SWATH seemingly best suited for the third. However, no ship type is best for all objectives unless a viable hybrid of all three can be conceived. ‘Thus, our study looked at combining all
three and, at first examination, it appeared that there would be a significant opportunity for the attributes of each concept to synergistically work together to produce a vessel that is operationally transformable, to be far superior for each element of the mission than any of the three original concepts alone,’ said CDI. The concept examined by CDI is a vessel
that can easily transform itself to operate either as a catamaran, SWATH, or SES. The catamaran was chosen because of its good high-speed efficiency and seakeeping performance, low risk, low cost, payload
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flexibility, and compatibility with the SES concept. The SWATH was chosen because of its inherently good seakeeping, and the SES because of its ability to afford shallow draft with end seals deployed and to combine with the SWATH to enhance seakeeping at low speed. This latter combination is achieved by
ballasting the vessel to transform from catamaran mode to SWATH mode in combination with deploying the cushion end seals and a mid-cushion divider, and supplying air to the cushion as in an SES, with active control of cushion vent valves fore and aft of the divider. This takes advantage of the very large aerostatic
forces and moment arms available from the SES split cushion to actively control heave and pitch motion from relative-motion feedback, on a hull that already has reduced seaway excitation forces from the small waterplane area SWATH mode. This concept for the SES has already seen limited demonstration by NSWC Carderock. The combination of ballasting/de-ballasting
and end-seal deployment to control draft offers the ability to also adjust deck height and
to ‘detune’ the ship’s response as it is being loaded or unloaded. Active roll control could also be arranged, if needed, with the addition of an inflated longitudinal keel. De-ballasting can be assisted with SES cushion pressure. The approach could be used for loading at zero forward speed alongside the seabase or for low-speed ‘ship-stern-to-HSC-bow’ loading when the HSC is being towed in a partial self- propelled mode with manoeuvring control at a most favorable heading to the sea. ‘Note that this is a concept not available to non-air-cushion type vessels, which usually need significant forward speed through the water to achieve useful vertical plane motion control,’ said CDI. ‘Note also that at no time are the SES seals used for high speed, so their life expectancy should not be a major issue.’ In the process of developing the catamaran
hullform, SAIC used their potential flow code ‘Das Boot’ to optimise hull shape for minimum drag. To ensure that there were no areas of separated flow on the hull, CDIM- SDD checked pressures and velocity fields using the RANS solver CF-X as part of the process of fairing the ship’s hull lines.
WARSHIP TECHNOLOGY MAY 2007
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