International Shipbuilding Progress

The process of estimation of manoeuvring derivatives from oscillatory captive model tests is analyzed and it is demonstrated that, contrary to common belief, all important information concerning quasi-steady force components can be extracted by means of only two terms of the Fourier expansions of the output time histories of force measurements: the mean values and the first-order out-of-phase coefficient values. Additional involvement of the second-order coefficients does not alter substantially the estimates of the regression coefficients and in most cases does not make the resulting regression model more certain and accurate, as shown with an example.

Tunnels or ‘propeller pockets’ are often a necessity in planing crafts, in order to accommodate propellers and minimize the shaft angle. Computational Fluid Dynamics (CFD) is increasingly being used as a design tool for the purpose of modelling ship flows. This is due to advances in computational methods together with improvement in performance and affordability of computers. Qualitative information to decide the relative merits of aspects, such as flow alteration in and around the ship hull, can be usefully deduced from careful CFD based analysis. In this paper, work has been undertaken to assess the pressures and resistance characteristics of a single chine high speed planing hull. A relatively full sized tunnel has been introduced in the numerical model. Using k−ε turbulence model in FLUENT, combining the predicted trim angle from equilibrium considerations and an iterative process, the stable equilibrium flow conditions have been modelled. The dynamic pressures have been evaluated and by integration, they have been matched with the total weight of the vessel. Single phase flow has been used to obtain the dynamic pressures in the underwater hull region. The numerical model predicts more favourable trim and qualitatively reduced resistance. Experiments conducted in a towing tank, using a model with and without the tunnel, confirm that by providing the tunnel there is improvement in the resistance by appreciable reduction. Pressure measurements confirm the validity of the numerical predictions obtained from CFD. It is quantitatively established that tunnels may be designed with beneficial effects for resistance.

Free trim hydrostatic ship characteristics are calculated applying the genetic algorithm (GA). According to the equilibrium condition of ship free floating, the calculation of free floatation comes down to a multi-objective constrained optimization problem, solved by a GA with real number coding. Necessary improvement of the GA is made in practice in order to speed up the evolution. As the foundation, the method of the hull surface representation, plane–surface intersection, tanks division and capacity calculation are described briefly. The hull surface and deck are represented with a single NURBS surface interpolating the waterlines, and deck side line(s) and the camber arcs, respectively. On the basis of an efficient intersection algorithm and the geometric properties calculation method, the virtual subdivision is realized and the capacity at any inclination surface calculated. The floatation calculation results and their comparison with the conventional results of full-scale ships verify the feasibility of the solution with a GA. The above 3D hull surface and tanks representation and the related calculation method determine the increased precision and convenience of the floatation calculation than that based on 2D ones. Compared with the other iteration algorithm, it is unnecessary to specify the initial iteration point and nothing is needed other than the ship hull offset, the total weight of ship and the center of it.

Buckling of laminated sandwich plates is studied using an efficient finite element plate model recently developed by the authors. The plate model is based on a refined higher order shear deformation theory, where the transverse shear stresses are continuous at the layer interfaces with stress free conditions at plate top and bottom. It is interesting to note that the plate model having all these refined features requires unknowns at the reference plane only. However, this theory requires C¹ continuity of the transverse displacement, which is difficult to satisfy arbitrarily in any existing finite element. To deal with this, a new triangular element developed by the authors is used in the present paper.