Great increases in the cost of fuel and the advent of very large tankers and bulk carriers have focused the attention, during the last decades, on means to enhance the efficiency of ship propulsion. An obvious way of obtaining an efficiency increase is to use propellers of large diameter driven by engines at low revolutions, as can be deduced from the developments in Chapter 9. Such a solution is, however, in many cases not practically possible. This has then given impetus to the study and adoption of unconventional propulsion arrangements, consisting, in general, of static or moving surfaces in the vicinity of propellers.

A distinct indication of the serious and extensive activity in the development and use of unconventional propulsors may be seen in the report of the Propulsor Committee of the 19th ITTC (1990b) which lists seven devices including large diameter, slower turning propellers. Here we summarize the hydrodynamic characteristics of six of these devices omitting larger-diameter propellers. The six devices are: Coaxial contrarotating propellers, propeller with vane wheel, with pre-swirl stators, with postswirl stators, ducted propellers and propellers operating behind flowsmoothing devices.

Emphasis is given in the following to a variational procedure which, in a unified fashion enables nearly optimum design of several of these configurations.

Propulsive Efficiency

Propulsive efficiency is conventionally thought of as the product of the open-water efficiency of the propulsor, the hull efficiency and a factor termed the relative-rotative efficiency.

The flows about wings of finite span are sufficiently analogous to those about propeller blades to warrant a brief examination before embarking on the construction of a mathematical model of propellers. A more detailed account of wing theory is given for example by von K ármán & Burgers (1935).

A basic feature of the flow about a straight upward-lifting wing of finite span and starboard-port symmetry in a uniform axial stream is that the velocity vectors on both the lower and upper surfaces are not parallel to the longitudinal plane of symmetry. On the lower side the vectors are inclined outboard and on the upper side they are inclined inwardly in any vertical plane or section parallel to the vertical centerplane. This is a consequence of the pressure relief at the wing tips and the largest increase in pressure being in the centerplane on the lower side and the greatest decrease in pressure being in the centerplane on the upper or suction side. Thus there are positive spanwise pressure gradients on the lower side and negative spanwise gradients on the upper side which give rise to spanwise flow components which are obviously not present in two-dimensional flows.

In many treatments of wing theory one finds figures purporting to show the flow about a wing of finite aspect ratio in which there is a continuous line of stagnation points near the leading edge, extending from one wing tip to the other.

The number of comparisons that have been made of calculated and measured blade-frequency thrust, torque and other force and moment components are very few because of the paucity of data. In this chapter comparisons of theoretical predictions with experimental data will be given. Results obtained by various theories will also be compared. The chapter concludes with presentation of a simple procedure, based upon the KT-J curve of the steady case, for a quick estimate of the varying thrust at blade frequency.

The measurement of blade-frequency forces on model propellers requires great care in the design of the dynamometer which must have both high sensitivity and high natural frequencies well above the model blade frequency. After a number of failures a successful blade-frequency propeller dynamometer capable of measuring six components (three forces, three moments) was evolved at David Taylor Research Center (DTRC) about 1960.

Measurements were made with a triplet of three-bladed propellers of different blade-area ratio designed to produce the same mean thrust. This set was tested in the DTRC 24-inch water tunnel alternately abaft threeand four-cycle wake screens which produced large harmonic amplitudes of the order of 0.25-U in order to obtain strong output-to-“noise” levels! The three-cycle screens give rise to blade-frequency thrust and torque whereas the four-cycle wake produces transverse and vertical forces and moments about the y- and z-axes which in general come from the fourth and second harmonic orders of blade loading on a three-bladed propeller, as was demonstrated on p. 367.

The non-symmetrical flow generated by flat and cambered laminae at angles of attack is at first modelled by vorticity distributions via classical linearized theory. Here, in contrast to the analysis of symmetrical sections, we encounter integral equations in the determination of the vorticity density because the local transverse component of flow at any one point depends upon the integrated or accumulated contributions of all other elements of the distribution. Pressure distributions at non-ideal incidence yield a square-root-type infinity at the leading edge because of the approximations of first order theory. Lighthill's (1951) leading edge correction is applied to give realistic pressure minima at non-ideal angles of incidence.

Our interest in pressure minima of sections is due to our concern for cavitation which can occur when the total or absolute pressure is reduced to the vapor pressure of the liquid at the ambient temperature. Since cavitation may cause erosion and noise it should be avoided or at least mitigated which may possibly be done by keeping the minimum pressure above the vapor pressure. This corresponds to maintaining the (negative) minimum-pressure coefficient Cpmin higher than the negative of the cavitation index.

At this point we shall not go deeper into the details of cavitation which is postponed until Chapter 8. Instead we shall continue our theoretical development with flat and cambered sections.

THE FLAT PLATE

We now seek the pressure distributions and the lift on sections having zero thickness but being cambered and, in general, set at any arbitrary (but small) angle of attack to the free stream, U. Consider a flat plate at small angle α.

Criteria for the design of blade sections may be selected to include:

1. i. Minimum thickness and chord to meet strength requirements;

2. ii. Sufficient camber to generate the design lift;

3. iii. Distribution of thickness and camber to yield the least negative pressure coefficient to avoid or mitigate cavitation;

4. iv. Thickness- and loading-pressure distributions to avoid boundary layer separation with least chord to yield minimum drag consistent with requirements i. and iii.;

5. v. Leading and trailing edges to satisfy strength and manufacturing requirements.

The first part of this chapter follows from linearized theories developed by aerodynamicists more than 50 years ago, placing emphasis on the use of existing camber and thickness distributions yielding least negative minimum pressure coefficients, Cpmin at ideal angle of attack. At nonideal angles (which always occur in operation in the spatially and temporally varying hull wake flows) we are required to seek sections having greatest tolerance to angle deviations and at the same time having negative minimum pressure coefficients exceeding the level that indicates occurence of cavitation. This tolerance depends critically upon the leading edge radius and the forebody shape as well as upon the extent of the flat part of the pressure distribution. Thus we are led to the more recent findings of researchers who have developed profiles having greater tolerance to angle of attack. When cavitation is unavoidable the latest approach is to use blunter leading edges to generate shorter, more stable cavities thereby avoiding “cloud” cavitation which causes highly deleterious erosion or pitting of the blades.

As a preparation for determining unsteady forces on propellers in ship wakes, we first consider two-dimensional sections beset by travelling gusts. Our development of the unsteady force on such sections differs from that given in the seminal work of von Kármán & Sears (1938) by adopting a procedure which is easily extended to wings and propellers. Their formula for unsteady sectional lift is recovered, being that of lift at an effective angle of attack which varies with the parameter k = ωc/2U, the “reduced” frequency. Turning to hydrofoils of finite span, we derive results for low aspect ratio in steady flow. For wings in gusts there is no analytical inversion of the integral equation which involves a highly singular kernel function. Graphical results are given from numerical solutions for a range of aspect ratios which reveal diminishing unsteady effects with decreasing aspect ratio.

Corresponding reduced frequencies for propeller blades in terms of expanded- blade-area ratio are shown to be high relative to aerodynamic experience. This indicates that two-dimensional, unsteady section theory cannot be applied to wide-bladed (low-aspect-ratio) propellers.

TWO–DIMENSIONAL SECTIONS

The blades of a propeller orbit through the spatially non-uniform flow of the hull wake and consequently experience cyclic variations in the flow normal to their sections. For blades of small chord-to-radius, this is analogous to the case of a two-dimensional section moving at constant speed through a stationary, cyclic variation in cross flow distributed as a standing wave along the course of the moving section.

In this chapter the pressure fields induced by the loading and thickness distributions on a single blade will be derived. They are found to be composed of pressure dipoles (for the loading) distributed with axes normal to the fluid helical reference surface in way of the blade and tangentially directed dipoles along this surface for the thickness. The expressions are then expanded in exponential Fourier series to facilitate determination of the total contributions from Z blades. This reveals that the pressure “signature” contains only components at integer multiples of blade number. The behaviour at large axial distances at blade frequency is examined analytically and the variation of the weighting functions in the integrals for small axial distance is displayed graphically via computer evaluation. The chapter concludes with comparisons with measurements and with various approximate evaluations of the integrals involved made in the past.

PRESSURE RELATIVE TO FIXED AXES

We shall derive the induced pressures in a fixed coordinate system. In this system the propeller sees an axial inflow (in the negative direction) while it rotates about the x-axis. The pressure is derived at a fixed point so this situation corresponds to finding the pressure induced by the propeller at a point which travels along with the ship and is fixed, for example on the ship surface. Note however that neither the varying wake, mirror effects of the ship surface nor the influence of the free surface will be considered at present as they will be dealt with later (in Chapters 15, 21 and 22).

Here, following a brief account of early observations of the effects of cavitation on ship propellers, we present methods of estimation of conditions at inception of cavitation followed by an outline of the development of linearized theory of cavitating sections. Application of this theory is made to partially cavitating sections, employing the rarely used method of coupled integral equations. The chapter concludes with important corrections to linear theory and a brief consideration of unsteady cavitation.

HISTORICAL OVERVIEW

Cavitation or vaporization of a fluid is a phase change observed in high speed flows wherein the local absolute pressure in the liquid reaches the vicinity of the vapor pressure at the ambient temperature. This phenomenon is of vital importance because of the damage (pitting and erosion) of metal surfaces produced by vapor bubble collapse and degradation of performance of lifting surfaces with extensive cavitation. It is also a source of high-frequency noise and hence of paramount interest in connection with acoustic detection of ships and submarines. Both “sheet” and “bubble” forms of cavitation are shown in Figure 8.1.

One of the earliest observations of the effects of extensive cavitation on marine propellers was made by Osborne Reynolds (1873) when investigating the causes of the “racing” or overapeeding of propellers. The first fully recorded account of cavitation effects on a ship was given by Barnaby (1897) in connection with the operation of the British destroyer Daring in 1894.