White Rabbit has been granted classification to operate in significant wave heights of up to 4m at a full speed of 20 knots in 4 to 5m waves; to 6 knots in waves from 5 to 6m and in seas in excess of 6m at reduced speed and at a heading appropriate to the conditions. These Class provisions are much more liberal than for equivalent capacity catamaran or mono-hull craft owing to the reduced accelerations on trimaran designs
The hulls are framed on a longitudinal system and supported by transverse web frames and bulkheads with typical spacing between transverses of 1200mm. The, three hulls are joined together by fabricated wing structures forming the tunnel arches. The hull plating thickness varies from 25mm at the keel plate, to 10mm at the bow and generally 8mm aft. Plate thickness reduces to 4-5mm closer to main deck level. Doubler plates (12mm) are used for bow thruster and ride control strut reinforcements.
Six transverse watertight bulkheads are fitted in the centre hull to provide seven watertight compartments. The side hulls have five transverse watertight bulkheads and six watertight compartments.
The structure was designed to DNV’S Light-Craft Rules for open-ocean operation on international voyages with a Class Notation of 1A1 LC Yacht E0. The hull and superstructure were rigorously analysed to ensure the structural integrity and efficiency of the design due to the operational loads expected under this Class Society notation. The number of load cases analysed is approximately double that of a conventional catamaran, with additional cases looking at the loads transferred through the tunnel bridging structure due to various loadings applied to the vessel’s side hulls.
Designed to DNV Light-Craft Rules
Finite Element Analysis (FEA) was used to assess the structural integrity of White Rabbit in variety of load scenarios.
FEA GLOBAL TRANSVERSE BENDING ASSESSMENT
Seaway induced transverse bending of the hull structure was analysed to show which areas of the hull exhibited the highest stress loads and then to ensure that these were within allowable limits.
The seaway induced loading criteria were determined by combining the static loading resulting from a simulated heel angle of 20 degrees, with a dynamic loading factor of 2.4. The resultant nodal forces were then applied to the hull structural model to identify the resultant stress ‘hot spots’.
The high stresses located in the tunnel bridging structure in way of the transverse bulkheads necessitated the fitting of thicker plating at these locations.
FEA ANALYSIS OF FOREDECK
The combination of a large tender, higher bulwarks, and longer boom arm on the crane, necessitated a detailed assessment of the surrounding structure. In addition to ensuring the deck stresses were within acceptable limits, the aim of this analysis was to ensure the resultant deck deflection did not cause de-lamination of the teak decking.
The worst case scenario is with crane boomed out athwartship with tender attached. Careful consideration of stresses and deflections were made for this condition and found to be within tolerance.
STRUCTURE AND VIBRATIONS
A thorough analysis of engine beds and hull structure surrounding the propulsion machinery and equipments was performed using Finite Element Analysis. This ensured that excitation frequencies did not coincide with any natural frequencies of the surrounding structure.
The engine beds were considered in conjunction with the mounting arrangement of the engines and gearboxes. This is important as huge forces are transferred to the structure via the mounting arrangement.
HULL PLATING IN WAY OF PROPELLERS
Comparison of propulsion equipment excitation frequencies with the natural frequency of the designed shell structure resulted in replacement of the two longitudinals indicated in the side shell with stiffer sections.
Pressure pulses are generated as each propeller blade passes the underside of the hull. These pulses exert forces on the bottom shell plating. The magnitude of these pressure pulses is reduced in the hull design by minimizing the shaft angle and ensuring adequate clearance between the propeller tip and the under side of the hull.
Including suitable blade skew and tip loading criteria in the propeller design further reduces the magnitude if these pressure pulses.