International Shipbuilding Progress

In many existing algorithms of strip theory the relative velocity concept is combined with sophisticated methods of determining added mass and damping coefficients in order to evaluate components of the diffraction force in waves. However, the concept is purely intuitive and it is shown in the paper to be not correct as an approximating technique.

Instead the concept of equivalent motion, which is valid from that point of view, is put forward. It is found that the velocities of the apparent motion as obtained according to the two concepts can differ very considerably even for long waves. The new concept can be easily employed in the algoritms based on relative velocity approach or in calculations where tabulated values of hydrodynamic coefficients are to be used.

The paper describes the first development of a cochlea-channelled stern for a single-screw hull. The traditional reference hull is chosen among those considered by CETENA of Genoa in its research programme on ship propulsion with a large-diameter and slow-turning propeller; the hull adopted for the present project is designed for a 38.000 t bulk-carrier with a service speed of 16 kn, and referred to in CETENA’s dossier No. 36 as hull ‘B₁’ if provided with a propeller of normal size, and as hull ‘B₂’ if provided with a large propeller.

The design work is based on tests at the model basin of the Institute of Naval Architecture of the University of Triest with two models of small size having an identical displacement, length (2 m), breadth and draught – one having a traditional stern and the other an unsymmetric afterbody provided with a cochlea-channelled stern.

The designed stern shape has been gradually altered in order to optimize the combined effect of the towing resistance, the wake factor, the pre-swirl and the inflow diagram on the propulsive power, calculated in the supposition of adopting the most efficient screw allowed by both the greater hydrostatic pressure at the blade-thrust maximum and the reduced blade-thrust pulsation. The end results evaluated in this way are compared with the same evaluation, performed for the traditional reference hull provided with a propeller of normal size, on the basis of the corresponding model test results. The cumulative influence on the assessed propulsive power of the induced pre-swirl, the improved wake factor, the larger propeller, and the reduced number and area-ratio of its blades, causes a substantial improvement (up to 30.6%) of the propelling efficiency which reaches the value of 0.91. Despite a not entirely negligible increase of the effective power, deduced from towing tests performed with small models and therefore subject to partially unknown scale effects, a significant saving of propulsive power (up to 19.5%) has been assessed without, however, considering further improvements which are obtainable on the thrust deduction and the relative rotative efficiency with a cochlea-channeled stern according to the results already achieved in self-propulsion with twin-screw hulls (up to 11.5% further saving of power).

The results of the present preliminary development are such that they should rate a continuation based on self-propulsion tests with models of adequate size, in order to verify the assessed performances and the influence of unconsidered parameters as well as unknown scale effects.

The project is subsidized by CETENA of Genoa.