CA1036873A - Ducted propellers - Google Patents

Abstract

ABSTRACT

A ducted propeller assembly which operates by detaching the boundary layer flow and hence the propeller wake from one or other side of that region of the propeller duct downstream of the propeller. This deflects the propeller wake horizontally and thereby provides a sideways force and steering moment on the ship on which the assembly is mounted. Embodiments are described in which the boundary flow is detached by introducing secondary flows through apertures in the duct walls or by introducing a mechanical obstruction into the boundary layer flow at one or other side of the propeller wake. The assembly can also be operated with the propeller stationary to act as a side-thruster unit.
An apparatus for use with a submersible is also described in which the control means is operable to deflect the propeller wake vertically, horizontally, or in directions lying between the vertical and the horizontal thereby to provide a corresponding steering force on the submersible.

Description

1036~3 The present invention relates to a ducted propeller assembly for marine applications and a method of operating such an assembly. The advantages of mounting a ship's marine propeller in a duct to improve its effectiveness are well known.
The duct is normally followed by a conventional rudder but it has also been proposed to have the duct unted so that it can swivel about a roughly vertical axis to direct the propeller wake to one or other side of the ship's centre line, and thereby provide a sideways force and steering moment on the ship.
According to the present invention, an apparatus, for use in manoeuvring a ship, comprises a ducted propeller assembly and spoiler means situated in an intermediate region of the duct between the propeller and one end of the duct, said spoiler means being located downstream of the propeller and operable to deflect the propeller wake horizontally thereby to provide a sideways force and steering moment on the ship, said duct diverging downstream of said spoiler means whereby a significant overall sideways movement of the propeller race is produced during steering.
The method of the invention consists of a method of operating an apparatus comprising a ducted propeller assembly and control means situated in the duct between the propeller and one end of the duct and operable to deflect the propeller wake horizontally thereby to provide a sideways force and steering moment on the ship on which the apparatus is mounted, the control means comprising apertures in the duct walls and means for introducing interfering secondary flows through the apertures into one or other side of the propeller wake to detach the boundary layer flow and hence the propeller wake from one or o~her side of that region of the propeller duct downstream of the propeller, and the velocity of the secondary flow when it issues from a control aperature being up to two or three times the velocity of the main flow leaving the duct.
The terms "front" and "rear" used throughout the specification .. *~, ~", ~.~36873 assume the usual "straight ahead" motion of the duct. It follows that when~ for example, the propeller is operating in reverse, then the "rear" end of the duct or of any other item referred to will precede its "front" end.
The duct preferably includes a parallel-sided portion and the propeller is then situated in this portion. The control means is also preferably situated in the parallel-sided portion where present e.g. at the rear or front extremity of the parallel-sided portion if this is confined to a central portion of the duct.
The control means may operate by introducing the interfering secondary flow through slots or rows of holes in the duct walls.
In other embodiments, the control means achieves the same effect by introducing a small mechanical obstruction into the boundary layer flow at one or other side of the propeller wake.
The control means preferably consists of two separate controls symmetically arranged one on one side and one on the other side of an imaginary vertical reference plane containing the axis of rotation of the propeller. The transverse reference plane, referred to below, is an imaginary reference plane that contains the axis of rotation of the propeller but is perpendicular to the vertical reference plane.
Reference in the specification to the control means being "inoperative" is intended to cover any situation in which the control means is not operating to deflect the propeller wake to one or other side of the vertical reference plane.
When the control means is inoperative, the apparatus will behave like a conventionalducted propeller in so far as the

-2-1036~73 propeller wake is symmetrically disposed in the duct and attaching to all or part of the downstream region of the duct above referred to. When the control on just one side of the duct is operating, however, this will destroy or reduce the attachment of the boundary layer of the propeller wake on that side of the duct and as a consequence the propeller wake as a whole will swing more towards the other side of the duct.
The cross-section of the duct region to the rear of the propeller, as viewed in the transverse reference plane, may be increasingly divergent as it approaches the rear end of the duct.
In such a case, when the control means is operated to deflect the propeller wake to one or other side of the vertical reference plane, then it is thought that the natural tendencyfor the propeller wake to attach to the walls of the duct will result in the propeller wake being increasingly deflected in the transverse reference plane as it is pulled by this wall attachment effect on to increasingly divergent portions of the duct. The result of this is that the final horizontal deflection of the wake may be significantly greater than it could be using a duct with a conical cross-section in the transverse reference plane and having the same inlet and outlet diameters in this plane as the preferred version just referred to.
However, in the cross-section of the duct in the vertical reference plane, it is advantageous to have the duct region downstream of the propeller straight-walled e.g. parallel sided so as to reduce diffusion of the propeller wake flow in the ~036B73 vertical direction. Alternatively the duct walls might be slightly divergent or slightly convergent in this plane.
The walls of the duct may be hollow. Thus where secondary flows are used to detach the propeller wake, the thickness of the &ct wall may be used to house associated ducting. Where, instead, obstructions are used for this purpose, then the actuating mechanism for moving the associated members into or out of the propeller wake can be housed in the thickness of the duct wall.
In one embodiment, where the duct walls are hollow, baffles are provided in the duct wall to separate the interior of the wall into two control chambers. Each control chamber is apertured at the front edge of the duct and at a region further towards the rear of the duct.
A valve is included in each control chamber between the two apertured regions to permit or prevent flow of ambient liquid through the chamber.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which Figures 1 and 2 respectively show an end view, looking forward, and a simplified vertical section of one embodiment.
Figures 3 and 4 show schematically, with simplified horizontal sections, the operation of the assembly for the 'straight-ahead' situation and the 'turn to port' situation;
Figure 5, which appears on the same sheet as Figure 3 and Figure 6 show diagrammatically the overall lay-out for the embodiment of Figures 1-4;
Figures 7 and 8 show alternative embodiments to Figures 1-4;
Figure 9 shows a simplified hori~ontal section of a preferred embodiment;
Figures lOa and lOb show a vertical section and an end view (looking forwards) of an apparatus according to the present invention fitted on to a ship's hull;

J. , ~

~036~73 Figure 11 shows a diagrammatic horizontal section illustrating the use of a suction effect to deflect the propeller wake;
Figures 12 and 13 show similar sections illustrating how the apparatus can be used as a side thruster (Figure 12) and with a pinched propeller wake (Figure 13);
Figures 13-17 show modifications of earlier embodiments;
Figure 18 shows a further general modification;
Figures 18a and 19 show diagrammatic end viewsof two apparatus embodiments;
Figure 20 shows a vertical section depicting the common features of the embodiments shown in Figures 18a and 19; and Figure 21 shows a submersible with the apparatus fitted in place.
m us referring first to Figures 1-3, a ducted propeller assembly 10 comprises a propeller 12 situated in a parallel sided portion 13 of a hollow-walled duct 14.
Reference numeral 16 indicates the region of the duct down-stream of the rear face of the propeller 12. In Figures 2 and 3, plane A-A is the plane containing this face. Upstream of plane A-A, the duct 14 has an intake and converging section of a suitable form conventionally used in ducted propeller assemblies. The downstream region 16 of the duct differs from such assemblies, however, in its shape and in the provision, at the downstream end of portion 13 of two slots lô, 20 through which secondary flows can be introduced into the propeller wake. m e slots in this embodiment preferably each subtend an angle of at the axis 21 of the duct (as may be seen from Figure 1) but obviously, if desired, the design can be modified by having the slots longer or shorter than shown in Figure 1. The general form of the slots can best be seen from Figure 2.
As may be seen from Figures 1-3, region 16 of the duct 14 has internal walls which define a passage which is increasingly divergent in cross-section in the transverse reference plane (Figure 3) and parallel sided in the vertical reference plane (Figure 2). According to a preferred feature of the invention the divergent walls in the transverse reference plane lie on the axis of two circles preferably having their centres lying in the transverse reference plane on a line containing or closely adjacent to the rear face of the propeller. Figures 1-3 show one such embodiment in which the circles have a radius of 3.75 times the diameter of the duct along this line (Figure 3). The duct section (as viewed in Figure 3) should also have a large trailing edge exit angle (30-35 from the duct axis) in the transverse reference plane. With the illustrated embodiment the waIl attachment effect associated with the curvature of the side walls is expected to make sideways deflections of + 35 or so possible for the propeller wake.
In vertical sections perpendicular to both the two reference planes above referred to, the duct varies, in preferred embodiments, smoothly from a substantially circular cross-section at the rear face of the propeller to a cross-section at the rear end of the duct which includes two side portions lying on a common circle and top and bottom portions lying on chords of that circle and parallel to the transverse reference plane. In the embodiment shown in Figures 1-3 the side portions each subtend about 90 at the centre of the circle and the top and bottom portions each subtend about 60 there, the remaining "corner" portions of the duct being curved so as to lead smoothly from the side portions of the duct into the top and bottom portions (Figure 1). As already explained the flattened shape of the duct section shown in Figure 1 reduces vertical diffusion and hence improves the efficiency of the assembly.

~o36e73 Baffles 22, 24 (Figure 2) divide~the duct wall into two control chambers 26, 28 (Figure 3) respectively associated with slots 18, 20, and liquid for the secondary flows is pumped to control chambers 26, 28 through ducting 30 shown diagrammatically in Figure S which illustrates the ducted propeller assembly in place in a ship 31. Reference numerals 32 and 33 indicate the pump and its motor, and numerals 34, 35 indicate the supply intake and by-pass pipes for the secondary flow liquid. In a typical case, the pump might be a single stage axial low head pump e.g. operating at less than 30 feet of water.
Figure 6 shows the control system for the pump. This comprises valves 36, 38 connected in the ducting leading to chambers 26, 28, a master valve 40 to control flow upstream of valves 36, 38 and a valve 42 in the by-pass pipe 35. All the valves are hydraulically or pneumatically actuable from the ship's control room.
When no side force and steering moment is required on the ship assembly 10 will be working as a normal ductedopropeller and pump 32 will not be used. In such circumstances valves 40, 42 will be closed. Valves 36, 38 may also be closed but it is thought preferable that they be maintained open to equalise the pressure in the duct slots 18, 20. Figure 3 illustrates the symmetrical distribution of the propeller wake (43) in the 'straight ahead' mode of operation just described.
If, for example, it is desired to turn the ship to port, valve 40 is opened, and valves 36 and 38 are operated so that valve 36 is closed and valve 38 is open. Pump 32 will now operate to suck sea water in through the intake pipe 34 and via open valves 40, 38 to the control chamber 28 associat-ed with slot 20. This liquid, issuing from slot 20 as a secondary flow into duct 14 will detach the duct flow from that part of region 16 containing the slot whereupon the propeller wake as a whole will veer towards the other wall of region 16 as shown, diagrammatically, in Figure 4. The deflected propeller wake will of course produce a corresponding turning moment on the ship to port. To turn the ship to starboard, valve 38 is closed and valve 36 opened.
It will be appreciated that varying the amount of secondary flow will vary the amount of side force produced so as to result in an analogous effect to that produced in conventional ducted propeller systems when the appliedrudder angle is varied or the angle of the duct is changed as the case may be.
For the ~straight ahead~ situation valves 36, 38 are open, and valve 40 is closed after by-pass valve 42 has been opened to allow liquid from pump 32 to be returned to the sea via pipe 35.
Figures 7 and 8 are sections of other embodiments and correspond to the section of the first embodiment shown in Figure 3.
In the Figure 8 embodiment, ambient liquid is drawn through apertures 100, 101 for the secondary control flows. Remotely controlled mechanically-hydraulically-(or pneumaticaIly-) operated flap valves 102, 103 determine which of the two control chambers 26, 28 is in operation at any given moment.
According to a preferred feature, the two control chambers are interconnected by a valve controlled loop (not shown) operating in exactly the same way as above described with reference to the Figure 6 embodiment to prevent the occurrence of unintentional deflection of the propeller wake when neither control is operating. It will be noted that the parallel sided duct section of the earlier embodiments is absent.
In the Figure 7 embodiment, the slots of the earlier embodiments have been replaced by small obstructions in the form of flaps. Four such flaps (48) are shown in Figure 7 in place of slots 20. The flaps are hinged at their front edges and mechanical, hydraulic (or pneumatic) rams (not shown) contained within the wall thickness of the duct are operable to de-flect the flaps into the propeller wake or to hold them substantially flush 1036~73 with the duct walls, as desired. The opening of the flaps is analogous to the provision of a secondary flow at the slots they replace.
In variations of the two embodiments above described, duct region 16 is modified to be slightly divergent or slightly convergent in the vertical reference plane. A divergence or convergence of a few degrees, e.g. one or two degrees, is envisaged at present. When duct region 16 is convergent in this plane, it is preferably never so convergent that it is not possible to extract the propeller 12 through theduct exit e.g. for maintenance or repair work.
Referring now to Figure 9, this shows a longitudinal section of a preferred embodiment of the invention in which the duct of assembly 10 is symmetrical aboutits longitudinal axis 60 and the control slots of the earlier embodiments have been re~laced by rows of holes 62 at the rearward extremity of the par~llel sided duct portion 13. The Figure actually shows a section of the well known "NSMB nozzle number 37" adapted in accordance with the present invention. The principle advantage of using holes instead of slots is that they are ea9ier to form. The holes are fed from rectangular-section manifold chambers 64, 65 in the duct which are sealed by cover plates 66 which allow access to the manifold chamber and holes.
Figures lOa and lOb show how the embodiments of the earlier Figures are fitted to the hull 67 of the vessel. The controls may take the form of pipes (68) leading through the duct waIl into the manifold cha~bers (for the embodiment shown in Figure 9), to the control chamber (for the embodiment shown in Figures 3 and 7) or to mechanical pneumatic, or hydraulic means housed in the thickness of the duct waIls (for the embodiment shown in Figure 8~. As Figure 10 clearly shows, in each case the controls, whatever their form~ may be housed wholly within the hull of the vessel. This greatly facilitates maintenance work on the apparatus.
Variations in the apparatus illustrated and described may be 1036~73 considered under two separate headings, namely the design of the duct, on the one hand and the design of the control equipment on the other. Consider-ing the duct first, although, if desired, ducted propeller assemblies may be specially designed for a particular case, the principles involved in the present invention are applicable to any of the conventional fixed-duct assemblies at present available. Thus the normal design p~ocedure will be to select the ducted propeller assembly according to the usual criteria and then to modify the assembly in accordance with the present invention.
Modification will of course invo~ve the provision of some sort of appropriate control means but this presents little or no practical difficulties especially where control apertures are to be used. The Figures show various designs of duct and in view of what has been written above, it will be clear that the duct of one embodiment may e~ually well be substituted for that of another embodiment with~ut significantly affecting the performance of the control means involved.
The rudder conventionally required with fixed duct assemblies may be dispensed with as steering can be satisfactorily accomplished according to the present invention in the manner already described above.
Unlike the two alternatives offered at present, (fixed duct follow-ed by a rudder, or pivotted duct3, the apparatus of the present invention involves no movement of any buIky part of the assembly relative to the vessel on which it is mounted. The advantages of this will be obvious to those versed in marine constructional design.
Considering now the apparatus of the present invention under the second heading, the design of the control means, it will again be o~vious to those skilled in the art that many minor variations in the described embodi-ments are possiblè within the scope of the present invention. For example the control apertures shown in Figure 9 could equally well be used with the ducts of Figures 1 to 4, and 8 instead of the control slots shown, or vice 1036~73 versa. Similarly, as above indicated, different designs of duct can be used.
Useful guidelines can be given however when considering what design of control means to aim at. The most satisfactory method must never-theless be to check conclusions experimentally. Where control apertures are used, for example, then (i) the area of each control slot or, if more than one control aperture is present, the aggregate area of these apertures, on one side of the duct (and of the propeller), should preferably lie between 2% and~5~ of the area of the duct passage at its narrowest bore (i.e.
the parallel sided portion in the embodiments illustrated). (ii) The ends of each slot, or if more than one aperture is present, of the group of apertures, should subtend an angle at the longitudinal axis of the duct lying in the range 60 ~o 90 . Values in the middle part of this range are normal-ly to be preferred. In the embodiment of Figure 9, for example, the ang~e subtended is 75 . (iii) Where the control means takes the form of apertures and these are supplied from manifold chambers or other control chambers in the walls of the duct, then the cross sectional area (or the average cross-sectional area) of each chamber in planes intersecting the axis of the duct, should preferably be at least twice the area (or the aggregate area) of the aperture~s) fed by that chamber (Figure 9 again illustrates this feature).

The procedure for operating the various "control flow" embodiments is exactly the same in each case and has been already described above.
Typically the velocity of the secondary flow when ~ issued~from a control aperture will be up to two to three times the velocity of the main flow through the duct. Deflections of 25 - 35 of the propeller race can be achieved, for example with the embodiment of Figure 9.
Other modes of operation are also possible. For example it may be desirable where secondary flows are used to detach the propeller wake, to provide a control flow (70) on one side of the duct and to apply suction (71) at the control aperture(s)(by connecting these to the pump inlet 301) on the 1036~73 other side of the wake (illustrated diagrammatically in the horizontal section shown in Figure 11). It may also be advantageous with such embodiments to provide secondary flows (73, 74 in Figure 12) through the control aperture(s) on both sides at once soas to pinch the propeller wake with the purpose of improving he performance of the propeller duct in the ahead condition.
Another possibility is to use the apparatus, with the propeller stationary, as a side thruster (as shown diagrammatically in the horizontal section of Figure 13). Figure 13 shows the case where this is done by providing a secondary flow 75 on one side of the duct and suction 76 to a control aperture on the other side. However the apparatus may be made to work satisfactorily as a side thruster by having the "suction" duct closed and relying entirely on the action of the "control flow" duct.
It has been found that when the control means are situated between the propeller and the rear end of the duct, they are effective when the propeller is driven in reverse to deflect the propeller wake leaving the forward end of the duct. This means that it is not usually neces~ary to provide a 9econd set of control means between the propeller and the forward end of the duct. Indeed model tests have indicated that the control means may be even more efficent in turning the propeller wa~e when the propeller is operating in reverse than they are when it is operating normally. This has indicated an alternative design of apparatus in which the control means are situated on the upstream side of the propellers. Figures 14, 15, 16 are diagrammatic horizontal sections illustrating, by way of example, how the embodiments of Figures 3, 7, 9 may be modified by repositioning the control means in this way.
As already stated, it will not normally be necessary to include two sets of control means but if it is desired, this may be done very simply.
Figure 17 shows, by way of example, one such apparatus in which the control slots shown in Figures 3 and 14 have been included in the one embodiment.

~036B73 Obviously other embodiments may be similarly combined.
Figure 18 shows a modification which can be applied to any of the above described embodiments or variations of these embodiments, namely the addition to duct 14 of a terminal portion 150 including two sets of fixed turning vanes 152, one on~ either side of the duct centre line. When the propeller wake leaves duct 14 substantially undeflected, then vanes 152 have little or no effect~ butwhen the wake has already been deflected by operation of the detachment means (not shown) in duct 14, then the vanes will be effective to turn the flow even further, thereby increasing the sideways force produced.
Apart from its other ad~antages, the apparatus of the present invention is expected to give the vessel a smaller tD~ning circle than is at present possible with presently available marine steering systems.
Although so far the invention has been described in respect of surface vessels, there is of course no reason why the same principles should not be made use of to provide an apparatus for manoeuvring submersibles.
Thus in an apparatus according to another aspect of the invention, the control means is additionally operable to deflect the propeller wake vertically and/or in directions lying between the vertical and the horizontal thereby to provide a corresponding steering force on the submersible.
Figures 18a and 19 show somewhat diagrammatic end views of two such apparatuses and Figure 20 shows a vertical section depicting the common features of the two embodiments. Figure 21 shows a submersible with the apparatus fitted in place.
The apparatus shown in Figures 18a and 20 differs in two important respects from that shown in earlier Figures. First the control apertures 62 go completely around the duct and secondly there are four manifold chambers each occupying a respective 90 sector 165, 166, 167, 168 (Figure 18a) of the duct. If it is desired to steer the submersible upwards, then the control 10;~ 73 apertures in sector 167 and/or sector 165 are used in an exactly analogous way to the way in which the apertures associated with manifold chambers 65, 66 are used in the Figure 9 embodiment. Of course for left and right steerage the apertures in sectors 168 and/or 166 are used instead. Steering in intermediate directions can be achieved by using first one set of control apertures (e.g- in sector 165 and/or 167) and then the other (in sector 168 and/or 166), or byusing the two sets differenti~lly.
In the embodiment of Figures 19 and 20, each of the control apertures 62 is fed by an appropriate feed pipe e.g. as indicated by reference numeral 170. With this embodiment, if it is desired to steer the submersi-~le in a direction lying between the vertical and horizontal this can be done directly by using only the appropriate control apertures. The arrows 172 in Figure 19 indicate by way of example the control flows present~*o produce a steering force F.
It will be obvious that the various modifications, substitutions and design guidelines already discussed with reference to the earlier embodiments will be equally applicable to the embodiments of Figures 18a to 20.
As Figure 21 makes clear, these latter embodiments will also have the advantage of allowing the controls to be housed in the hull of the vessel (submersible 174).

Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. For use in manoeuvring a ship, an apparatus comprising a ducted propeller assembly and spoiler means situated in an intermediate region of the duct between the propeller and one end of the duct, said spoiler means being located downstream of the propeller and operable to deflect the propeller wake horizontally thereby to provide a sideways force and steering moment on the ship, said duct diverging downstream of said spoiler means whereby a significant overall sideways movement of the propeller race is produced during steering.

2. An apparatus as claimed in Claim 1 in which the intermediate region of the duct is parallel-sided and the spoiler means is situated at one extremity of this portion.

3. An apparatus as claimed in Claim 1 in which the spoiler means comprises apertures in the duct walls and means for introducing interfering secondary flows through the apertures into one or other side of the propeller wake to detach the boundary layer flow and hence the propeller wake from one or other side of that region of the propeller duct downstream of the propeller.

4. An apparatus as claimed in Claim 1 in which the spoiler means comprises mechanical obstructions and means for introducing said mechanical obstructions into the boundary layer flow at one or other side of the propeller wake.

5. An apparatus as claimed in Claim 1 in which the spoiler means consists of two separate controls symmetrically arranged one on one side and one on the other side of an imaginary vertical reference plane containing the axis of rotation of the propeller.

6. An apparatus as claimed in Claim 1 in which the spoiler means comprises apertures in the duct walls and means for introducing interfering secondary flows through the apertures into one or other side of the propeller wake to detach the boundary layer flow and hence the propeller wake from one or other side of that region of the propeller duct downstream of the propeller, the spoiler means comprising controls on either side of the duct and chambers with which the spoiler means are associated formed in the thickness of the duct, and the chambers being interconnected by a valve controlled loop operable to prevent the occurrence of unintentional deflection of the propeller wake when neither control is operating.

7. An apparatus as claimed in Claim 1 in which the spoiler means comprises apertures in the duct walls and means for introducing interfering secondary flows through the apertures into one or other side of the propeller wake to detach the boundary layer flow and hence the propeller wake from one or other side of that region of the propeller duct downstream of the propellor, pumping means in the hull of the vessel for providing secondary flows to the apertures and valve means actuable from the vessel to control said flows.

8. An apparatus as claimed in Claim 1 in which the spoiler means comprises apertures in the duct walls and means for introducing interfering secondary flows through the apertures into one or other side of the propeller wake to detach the boundary layer flow and hence the propeller wake from one or other side of that region of the propeller duct downstream of the propeller, the area of each control aperture, or, if more than one control aperture is present, the aggregate area of these apertures, on one side of the duct and of the propeller, lying between 2% and 5% of the area of the duct passage at its narrowest bore.

9. An apparatus as claimed in Claim 1 in which the spoiler means comprises apertures in the duct walls and means for introducing interfering secondary flows through the apertures into one or other side of the propeller wake to detach the boundary layer flow and hence the propeller wake from one or other side of that region of the propeller duct downstream of the propeller, the ends of each aperture, or if more than one aperture is present, of the group of apertures, on one side of the duct and of the propeller, subtending an angle at the longitudinal axis of the duct lying in the range 60° to 90°.

10. An apparatus as claimed in Claim 1 in which the spoiler means com-prises apertures in the duct walls and means for introducing interfering secondary flows through the apertures into one or other side of the propell-er wake to detach the boundary layer flow and hence the propeller wake from one or other side of that region of the propeller duct downstream of the propeller, the apertures being backed by control chambers in the walls of the duct, and the cross-sectional area, or the average cross-sectional area, of each chamber in planes intersecting the axis of the duct, being at least twice the area, or twice the aggregate area, of the apertures fed by that chamber.

11. A method of operating an apparatus comprising a ducted propeller assembly and control means situated in the duct between the propeller and one end of the duct and operable to deflect the propeller wake horizontally thereby to provide a sideways force and steering moment on the ship on which the apparatus is mounted, the control means comprising apertures in the duct walls and means for introducing interfering secondary flows through the aper-tures into one or other side of the propeller wake to detach the boundary layer flow and hence the propeller wake from one or other side of that region of the propeller duct downstream of the propeller, and the velocity of the secondary flow when it issues from a control aperture being up to two or three times the velocity of the main flow leaving the duct.

12. A method according to claim 11 in which, with the propeller stationary, a secondary flow is provided to cause the apparatus to act as a side thruster.

13. An apparatus as claimed in claim 1 for use with a submersible and in which the spoiler means is additionally operable to deflect the propeller wake vertically and/or in directions lying between the vertical and the horizontal thereby to provide a corresponding steering force on the sub-mersible.

14. For operating an apparatus for use with a submersible, a method as claimed in claim 11 or 12 in which the control means is operated to detach the boundary layer flow and hence the propeller wake from some part of the propeller duct to deflect the propeller wake vertically and/or in directions lying between the vertical and the horizontal thereby to provide a corresponding steering force on the submersible.

15. A submersible provided with an apparatus according to claim 13.