Over the past ten years significant advances have been made in identifying the causes of excessive vibration observed in the afterbodies of ships. Ship trials, model experiments and analytical studies have shown that unacceptable levels of aft-end and superstructure vibration can be attributed both to structural causes, such as inadequate stiffness or resonant excitation of aft-end configurations and shafting systems, and also to hydrodynamic causes such as pulsating propeller cavitation. When cavitation grows and collapses in the upper-half of the propeller disc, large excitation forces can be produced at blade frequency and multiples of blade frequency.
This report summarises the results of BSRA work on the hydrodynamic aspects of propeller-excited vibration (PEV) carried out as part of a three year project, which was jointly funded by the General Council of British Shipping and the Ship and Marine Technology Requirements Board, as well as the UK shipbuilding industry.
The hydrodynamics work has followed three main streams :
(i) development of analytical prediction methods for non-cavitating and cavitating propellers;
(ii) correlation of model/full-scale and analytical model results, and
(iii) preparation of design guidelines and criteria for acceptability.
Within the limitations of the state-of-knowledge, these studies have led to the adoption of a design strategy which is described in Section 1.
A number of computer programs have been produced during the development of analytical prediction methods. These are capable of calculating the following :
Open-water propeller characteristicsBlade loading fluctuations in non-uniform flowBearing forces and momentsFluctuating hull pressures due to a non-cavitating propellerDetermination of cavitation inception and cavitation geometryCalculation of fluctuating hull pressures due to a cavitating propeller.Scale model/ship correlation is discussed using the examples of a 6 200-tonne d.w.pallet carrier and a 30 000-tonne d.w. container ship. Recommendations are given relating to model testing procedures together with the criteria with which to assess measured wake flow and hull pressures from model tests. Guidelines for the design of aft-end forms are also included.
It is expected that use of the design guidelines and application of these acceptability criteria within the framework of the design philosophy presented here will reduce the likelihood that a given propeller and hull form will give rise to large excitation forces.
