Trans RINA, Vol 152, Part A4, Intl J Maritime Eng, Oct-Dec 2010 A SOURCE OF PROPELLER EXCITED BROADBAND VIBRATION ON A HIGH-SPEED
TWIN SCREW SHIP (DOI No: 10.3940/rina.2010.a4.191) J W English, Retired Marine Propulsion Consultant, UK SUMMARY
The cause of the severe propeller excited broadband vibration on a twin screw ship’s stern was investigated in a cavitation tunnel using conventional modelling methods. At first sight the results did not indicate anything untoward about the stern and propeller designs tested, apart from some unusual cavitation patterns in the propellers’ slipstreams. The fluctuating pressure levels on the model hull varied considerably depending on the propeller designs and loading conditions used in the tests, but these did not provide an explanation for the vibration on the ship. Some unusual patterns in the relative levels of the harmonic pressures on the stern were noticed in addition to the presence of a large cavitation disturbance in a propeller trailing vortex that was captured in a single frame of a video recording. This latter observation led to a plausible explanation for the broadband vibration on the ship’s stern.
NOMENCLATURE n
pn δτ ω
Harmonic number (n =1 to 5) Pressure of nth harmonic (Pa) Time duration of pulse (s)
Propeller speed of rotation (rad/s)
ABBREVIATIONS bpf
rms tdc
bdc
QPC 1.
Blade Passage Frquency (n=1)
Root Mean Square. See Appendix for definition
Top dead centre. Reference blade in the vertically-up position
Bottom dead centre. Reference blade in the vertically-down position Quasi Propulsive Coefficient
INTRODUCTION
The source of the vibration described in this paper occurred on a high-speed twin screw vessel typical of Container and Cruise Ships. It was fitted with open propeller shafts supported by brackets on either side of a centre-line skeg and twin rudders mounted abaft the propellers. Four bladed outward turning propellers were installed with tip skew angles in excess of 30° in what may be described as a compact after end design.
In service the vibration occurred in the after region and was notable because it was not directly related to the shaft or blade frequencies and their harmonics. In fact the vibration response of the ship’s after structure was high level and non-periodic over a frequency range covering several harmonics of blade passage frequency (bpf). Furthermore there was a feature of randomness in the ship’s vibration.
2. MODEL TESTS
In the course of producing the underwater ship design, conventional model tests were conducted in a towing tank to obtain the resistance and propulsion
©2010: The Royal Institution of Naval Architects
measurements and the nominal wake at the propeller position. On the basis of the nominal wake measurements outward turning propellers were selected. At that time nothing unusual was detected in the underwater hull design and the aft end in particular. The longitudinal clearances of the propellers in their apertures conformed to acceptable practice and it was only after the ship was completed that the vibration problem emerged. This led to further model tests being conducted in an attempt to discover the hydrodynamic cause of the problem after it was concluded it could not be overcome with local structural modifications.
Initially suspicion centred on the design of the propellers and their cavitation performance, but after modifications to the designs and repeating model cavitation tests it was realised that the problem was more
obscure than
anticipated, and cavitation tests in a large tunnel in which a complete model hull with its necessary appendages should be used. The primary advantages of a cavitation tunnel facility being that
relatively high Reynold’s
numbers can be achieved and some control over the susceptibility of water to cavitate is available by adjusting its free gas content.
3.
CONVENTIONAL FLUCTUATING PRESSURE MEASURMENTS
The fluctuating pressures on the model hull surface were recorded and analysed using established methods for dealing with periodic
signals
multiples of the shaft frequency. These methods with minor variations were, and probably still are, in common use at most of
having frequencies at the main propeller cavitation testing
laboratories. They presume that the recorded pressure fluctuations on the hull surface are periodic and that representative rms pressure levels and phases together with their dispersion statistics can be produced for harmonic frequencies up to, say, five times bpf. The cut off frequency in the measuring system can be quite low, depending on the frequency of the highest harmonic pressure of interest. This procedure is probably suitable for the vast majority of cases where the propeller(s) is the
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