Lab.y.Schepbcwwkun Tecimsche hoy';scíioo Recent develoPments in marine propeller hydrodynamics DdfL ú/: # - /2.B40 /r. /:_I 7f4/ 3e.- / I97 Dr Jr \V. C. Oosierveld and Ir P. van Oossanen ¡ Netherlands Ship Model Basin, \Vageningen ¿4 1q73. Introduction complete integration of the design willi the wake at the propeller position is obtained. A short exposition of the On looking back on a 40 year period of work in the principles of this method is given. field of ship hydrodynamics at the Netherlands Ship odcl Basin (NSMB) a number of memorable highlichts Experimental projlIer characteristics such as open- çn he discerned. Anumber of these highflghls are noted water test results are increasingly used in preliminary c!cwhere in this book. Most of these memorable propeller design and parameter studies by means of jtivities vere the result of seif-sponsoced research with high speed computers. For this purpose it is veryad- the aim to promote the efficient designing of ships. vantageous to have these experimental propeller charac- :riicuiarly intensive research has constantly been devot- teristics in the forni of polynomials or formulas. Among cd to the various aspects of ship prcpulsion, the results the well-known screw series developed by Schaifran, of which were nearly always published in well-known Gawn artd ailiers, the Wageningen B-screw series of the maeaziiies and periodicals. Netherlands Ship Model Basin are perhaps the most used. This screw series has through the years been ex- li, not the intention of this paper to review past tended and now comprises some 120 models with blade activities in the field of ship propulsion, but rather to numbers ranging from 2 to 7 and binde ai-ca ratios from rc.nsider more recent work in this field, carried out or 0.3 to 1.05. The cross-fairing of the open-water test data Fing carried out, in particular at the NSMB. by means of the computer for a Reynolds number of 2 x lO& has now been completed, and it was considered ' his paper is divided into the following four main topics: appropriate that these results should be pubhshed for the I. Theoretical propeiJer theory. first time in this paper. The thrust and torque polyrio- 2. Experimental (conventioria i) p opcller characteristics. mials given incorporate the results of some 40 years of .3. Characteristics of non-conventional propulsion open-water testing at the NSMB. ices. 4. Propdlcr testing techniques and facilities. In the chapter on non-conventional propulsion devices, particular attention is given to the characteristics of the i he state-of-the-art of sub-cavitating optimum propeller various propulsion devices. The application of non-con- theory has progressed to a stage ensuring the highest ventional propulsion devices is increasing due to the fact possible efficiency in both the free-running and wake- that modern ships sometimes demand specific propulsion ndaptcd cases. A short review of past developments in requirements which can not be obtained with the con- theoretical propeller theory particularly concerned with ventional screw propeller. The propulsion devices con- ub-cavitating optimum propellers is included in this sidered are the following: p.per.With the unmistakable trend towards higher ship - ducted propellers, sreeds and larger displacements, howcver. non-optimum - contra-rotating propellers, propl1er design in connection with securing the best - overlapping propellers, psib1e cavitation properties is becoming relativelymore - controllable pitch propellers, important. As a consequence, many research institutes - vertical axis propellers. have recently intensified workon this aspect of propeller c-:the above mentioned. ducted propellers are being theory. At the Netherlands ShipModel Basin a method increasingly applied behind large tankers. Ducted h.tbeen developed to determine overall and local propellers are therefore considered in more detail. PrOpeller geometry in accordance with obtaining maxi- Accelerating, decelerating and non-axisymetrieal nozzles mum latitude to the angle of attack. In this method are regarded. 51 Lastly, this paper deals with some recent developments Propeller Theory in the cavitation testing of propellers, both from the viewpoints of techniques and facilities. Rev/en' of past developments Model tests are often employed to determine the best Before considering some recent progress made in propeller-afterbody configuration. The main reason for propeller theory, a survey of past developments is this is the difficulty involved in the theoretical determina- appropriate. In 1865, Rankine [3] developed the fore- tion of unsteady propeller action, cavitation, and propel- runner of momentum theory as it is known today. This ler-afterbody interaction effects when the propeller opera- theory is based on the change of momentum and the tes in a wake. related axial motion of the fluid passing through an With modern propeller theory the determination of actuator or propeller disk. [n 1889, R.E. Froude [1] unsteady propeller forces in the non-uniform velocity considerably extended this theory and it has subsequent- field can, to a certain extent, be realized. Theoretical ly become known as the Rankine - Fronde axialmornen- assessment of the cavitation properties and, in turn, the turn theory. The effects of the rotational motion of the influence of propeller cavitation on propeller action and slipstream were included by Betz [5] in 1920. This interaction effects, has not progressed that far. In conse- theory is today used in various propeller problems. The quence, model testing is particularly employed to deter- fact that it gives no indications of the geometry of the mine propeller cavitation properties, induced vibrations propeller causing the changes in the flow is a large and other adverse effects of cavitating ship screws. The drawback, and in fact the reason for it not being used in tests to determine propeller-induced vibratory forces general design problems. acting on the afterbody of a ship vere up till now performed in conventional towing tanks. In this ship The first to attempt to formulate the relation between model testing facility the effect of propeller cavitation is propeller geometry and the associated propeller thrust not taken into account. It has recently been established, and torque was W. Froude [6] in 1878. His crude blade that the effect of cavitation on the vibratory forces on element theory was the forerunner of all theories relating the ship's afterbody and appendages and on the the lift and drag of an clement of a blade to its geome- propeller itself is considerable [1, 2]. Complementary try. Later, Drzewiecki [7] considerably extended this tests with model propellers in cavitation tunnels in theory and suggested performing tests to determine the wake-simulated flows are therefore often necessary to lift and drag forces experienced by blade section forms obtain an impression of the cavitation properties of the at various angles of attack when he found that he could propeller. Actual interaction effects between propeller not calculate them. The uncertainty as to the character- and afterbody are, however, not taken into account in istics which must be assumed for such sections was, and this way, while it is found extremely difficult to simulate to a certain extent still is, an unsatisfactory feature of the required distribution of the axial and tangential such theories. Furthermore, early workers in this field wake components. These and other difficulties have led failed to recognise finite aspect ratio effects, causing the the Netherlands Ship Model Basin to construct a depres- arithmetical results obtained with this theory to be surizcd towing tank, in which the air pressure can be far from satisfactory. lowered to such an extent that ship model testing can be In consequence of the large discrepancies between the performed a the correct cavitation index. The climens- momentum theory and the blade element theory, at- ions of this towing basin are such that ship and propel- tempts were made to combine the two, and to use ler models are of a size with which it is possible to the induced velocities as determined by the momentum avoid unpredictable scale effects. theory to reduce the angle of attack in the blade element theory. In this way large differences between experiment Besides a short description of this facility, this part of and theory were avoided but duc to the still unaccount- the paper includes a review of the many problems as- ed finite aspect ratio and chordwise effects, and the still sociated with ship model testing including such subjects unknown variation of blade section characteristics with as scale effects and cavitation scaling. Reynolds number, these differences remained unaccep- table, in particular for broad bladed marine propellers. In 1907, Lanchester [8] put forward a new theory which accounted the lift of an aeroplane wing duc to the 52 development of circulation around each section in the difficulty inherent to the finite blade nunìbcrcase lies span direction. He postulated that the vortex movement in the complexity of calculating the induced velocities around such an aeroloil is continued in the fluid in the caused by the system of trailing vortices constituting a forni of vortices trailing from the ends, and in the case finite number of vortex sheets. Particular credit must be of propeller blades, passing downstrean in approxinia- paid to J3etz's paper, not only for determining the op- tely helical paths from the tips. This concept of the timum radial