The influence of hull form on the slamming behaviour of large high-speed catamarans
thesisposted on 2023-05-27, 07:23 authored by Rafie Shahraki, J
Wave-piercing catamarans are used extensively for both defence and commercial sea transportation because of their many advantages. Issues such as the impact of the bow into the water when operating in large waves, better known as wetdeck slamming, can affect their mission capability and cause structural damage. However, the design criteria for this type of craft are not yet sufficiently accurate. Based on an exploratory study, wave-piercing catamarans such as the vessels built by INCAT Tasmania often have a centrebow that provides reserve buoyancy under the wetdeck to reduce heave and pitch motions and avoid deck diving in following seas. Over time, for INCAT large catamarans, hull parameters such as tunnel clearance, centrebow length and centrebow volume have decreased as the vessels have grown in length. To evaluate the effect of various bow forms on motions and slamming loads, an adjustable hydroelastic segmented model was designed and constructed as part of the collaboration between AMC, UTAS and INCAT. This segmented model is a scaled model of a 112m INCAT wave-piercing catamaran and has two transverse cuts and a separate centrebow. The centrebow was equipped with two six degree of freedom force/torque transducer to measure slam forces both in vertical and horizontal directions. The model shell was built from a sandwich panel of carbon-fibre layers and Divinycell foam core. Lessons learnt from an existing hydroelastic catamaran model were taken into consideration to design and construct this model. Three centrebow volumes (lengths) were designed and tested in head seas in the AMC towing tank for 2.76 m regular wave heights and 20 knots speed. The results showed a significant variation in slam loads when comparing the three centrebow lengths, with the highest loads found on the longest centrebow, caused by larger water volume constrained between the centrebow and demihulls. The slams in shorter centrebows occurred further forward and at more inclined angles. Results also showed that the longer centrebows have higher pitch motions in slamming conditions due to higher vertical forces on the centrebow. It could be concluded that in the tested condition, the shorter centrebow performed better overall in waves. Also a study with Smoothed Particle Hydrodynamics (SPH) was conducted investigating slamming especially for enclosed sections. Various vessel bow sections were dropped into water to replicate slamming. The simulations included single-phase, two-dimensional and three-dimensional modelling of two simple wedges, a wedge with two side-plates and a fully INCAT catamaran bow section comparing with experimental drop tests results. It is concluded that SPH is capable of simulating monohull sections slamming successfully. However, simulation results of enclosed sections did not match well with the experiments, most likely due to inclusion of air under the section which will be the subject for future research.
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