GB2119732A

GB2119732A - A flow guide surface for the stern of a propeller ship

Info

Publication number: GB2119732A
Application number: GB08312024A
Authority: GB
Inventors: Herbert Schneekluth
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-05-04
Filing date: 1983-05-03
Publication date: 1983-11-23
Also published as: JPS58194691A; SE8302038D0; FI831512A0; FI74675B; GB2119732B; BE905069Q; JPH0366197B2; HK39087A; SE455491B; SE8302038L; FI74675C; FI831512L; GB8312024D0; DE3216578C1

Abstract

In order to provide a flow guide surface for the stern of a single or multi-propeller ship, a guide surface nozzle 1 is provided in front of the propeller 5 at a horizontal spacing of up to one propeller diameter. The centroid of the enclosed cross- sectional area lies above the propeller axis 7. The nozzle may comprise two half shells which may have different axial directions or may be formed as brackets fixed to the hull of the ship. <IMAGE>

Description

SPECIFICATION A flow guide surface for the stern of a propeller ship The invention relates to a flow guide surface for the stern of a propeller ship to be fixed to the ship in front of the propeller.

An arrangement of this general type is known from the United States Patent Specification 4,309,172, in which a nozzle is disposed coaxially or approximately coaxially with the propeller axis slightly in front of the propeller. This arrangement requires relatively large dimensions with corresponding constructional costs, and moreover there is an additional frictional resistance from the nozzle. In addition the British publication "Motor Ship", of April 1982 on pages 36 and 37 discloses a guide surface system disposed around the propeller shaft approximately in the shape of a star and a combination of this guide surface system with a ring nozzle limiting this guide surface.

The asymmetry of this system relates to the radial or approximately radial guide surfaces, in which the nozzle is disposed in a centrally symmetrical manner with respect to the shaft.

Although in the case of this arrangement the initial twist of the propeller is taken into account by the guide surfaces, the potential flow direction determined by the shape of the ship which rises slightly in most cases towards the stern is not taken into account.

In the case of ships' screw propellers which are in most cases disposed on the stern as single propellers, it is known that the flow to the propeller rises towards the stern in accordance with the shape of the ship and that the propeller flow therefore includes an upwardly directed component. As a result of the converging shape of a ship towards its stern the flow also converges at this point with respect to the midships plane. In addition the propeller does not only produce a twist in the wash but also in the area in front of itself. This means, in the case of a propeller rotating clockwise -- seen from behind -- that in the upper area on the portside the upward component of the oncoming flow is further amplified. On the starboard side the upwardly directed oncoming flow component acts against the propeller.

The flow component in the direction of the propeller rotation effects a pressure relief on the propeller on the portside. The counterflow component on the starboard side causes additional ioading of the propeller. The centre of pressure of the propeller is disposed on the starboard side of the propeller in the case of a propeller rotating clockwise when seen from behind.

The flow to the propeller is also very nonuniform in terms of speed as a result of the boundary layer flowing towards the propeller and as a result of cavitation effects. This effect is particularly marked in the upper halves of the propellers in the case of full afterbodies. The mean speed of flow to the propeller is, in the case of single-screw merchant ships, approximately 20% to 40% lower than the speed of the ship. This wash has a favourable effect on the power requirement. However an extremely non-uniform wash distribution has an unfavourable effect on the efficiency of the propeller. In conventional merchant ships the oncoming flow speed via the propeller surface may vary from 95% to 30% of the ship's speed. Even greater differences may be encountered in the case of ioaded tankers.

In order to compensate the speed of flow to the propeller the afterbody may be correspondingly formed. The so-called stern seams are for example suitable in the first instance for this purpose. It is also known to dispose substantially horizontal flow guide surfaces in front of the propeller. These guide surfaces cause a more axial flow to the propeller and can also serve to compensate the oncoming flow speed. If a carrier or guide surface which is inclined between the direction of flow and horizontal is disposed in this flow which moves obliquely upwardly, this carrier surface is subject to lifting with a forwardly directed force component, i.e. it produces an additional forward drive. The flow behind the carrier surface is forced downwardly by the carrier surface and then moves in a more horizontal manner.These carrier surfaces therefore modify the direction of flow in the desired manner and the speed of flow to the propeller is simultaneously compensated. A carrier surface of this type may improve the propulsion efficiency of a ship, i.e. in this way it is possible to reduce the propulsion power required for achieving a specific speed or to achieve a higher speed with the same power. The following effects may be differentiated in this respect: (1) The production of a propulsive force directly on the carrier surface. The additional viscous resistance of the guide surface is slight and may be partially recovered in the propeller.

(2) The increase in efficiency of the propeller as a result of the direction of the oncoming flow being more horizontal.

(3) The increase in efficiency of the propeller as a result of the compensation of the speed of the oncoming flow over the propeller disc surface.

(4) The flow compensation reducing the vibrational stimulus. This effect may be used for a reduction of vibration at the stern or for the purpose of increasing the propeller diameter in conjunction with a reduction of the upper propeller blade tip spacing from the counter. Afurther improvement in efficiency of the propeller may be achieved with a diameter increase in accordance in each case with the speed.

These facts are widely known. Attempts have been made to utilize their concept by providing carrier surfaces in front of the propeller on both sides of the propeller post. As carrier surfaces of this type lead to additional construction costs they have been used only infrequently up to now even when the additional cost is lower than that which would have been required for the savings in propulsion power in terms of capital and operation.

The above-mentioned effect is however quite different on the two sides, even when it is ensured that the angle of incidence of the carrier surfaces is separately optimised for each side. On the starboard side two effects combine together in the case of propellers rotating clockwise as seen from behind, these effects supplying the propeller in most cases with a substantially horizontal flow, i.e.

the displacement flow of the ship's body having an upwardly directed flow component and the initial twist of the propeller with a downwardly directed flow component. These vertical components, which oppose one another on the starboard side, act in the same direction of the portside, i.e. they reinforce one another on this side. It is therefore often suitable to restrict the arrangement of carrier surfaces to one side. This is normally the side with the propeller blade which beats upwardly in the forward direction, i.e. the portside in the case of propellers rotating clockwise. The carrier surface may in this respect comprise a symmetrical profile or a non-symmetrical profile, in which the suction side is disposed at the top.

In heavy seas the carrier surface may be subject to conditions in which it is temporarily in or out of the water. The immersion is then accompanied by a slamming effect which subjects the structure to high static stresses. In order to avoid water shock effects of this type the carrier surface may be disposed at a somewhat deeper location than is optimum from a hydrodynamic point of view.

In addition it is known to arrange Kort nozzles as an annular casing of the propeller with a carrier surface cross-section. These Kort nozzles produce a propulsion which reinforces the propeller action.

However a propulsive benefit occurs only with relatively large propeller loads as otherwise the nozzle resistance is predominant. Kort nozzles of this type effect both a compensation of the oncoming flow with respect to direction and a speed distribution.

If nozzles of the Kort type are disposed concentrically in front of the propeller, it is possible to achieve some of the advantages which may be achieved for cased propellers. Kort nozzles disposed concentrically in front of the propeller and having an external diameter which is about as large as the propeller diameter are known.

The object of the invention is to provide an improved guide surface nozzle of the Kort ring type disposed in front of a propeller.

According to the invention there is provided a flow guide surface for a propeller ship which is adapted to be fixed to the ship in front of the propeller, wherein the centroid of cross-sectional area enclosed by a guide surface nozzle lies above the propeller axis.

Preferably the horizontal spacing of the guide surface nozzle from the propeller is less than one propeller diameter.

Preferably the centroid of the cross-sectional surface enclosed by the guide surface nozzle lies above the propeller axis by at least 10% of the propeller diameter and the cross-sectional surface is smaller than half the area of the propeller blade tip circle.

In order to achieve a further improvement of the propulsion efficiency, the axis of the guide surface or ring nozzle is preferably disposed such that it rises slightly to the rear. In this way an additional propulsion effect is achieved in the same way as for guide surfaces of the carrier surface type. The optimum angle of incidence of the axis is in most cases smaller than the flow thread angle without the nozzle ring with respect to the horizontal.

As the optimum angle of incidence of the annular nozzle is different on the two sides of the ship, it is proposed to form the ring nozzle of half shells with a dividing surface interposed therebetween and to associate one half shell in each case with the sides of the ship with different axial directions. The lateral angle of the half shell axes can produce a back twist to the propeller flow and be additionally used to produce a control moment in order thereby to compensate different control effects of the propeller. In this case the rudder can be disposed amidships for sailing in a straight line.

For the case in which the horizontal distance between the propeller post and the propeller is small, it is proposed for the half shells to form a closed ring in each case with the outer skin of the ship. In this way the flow is also accelerated laterally with respect to the centre.

An alternative embodiment of the invention is provided in that a guide surface formed as a closed ring is only disposed on one side of the ship.

The further advantageous features of the invention are the result of the following: In contrast to Kort nozzles which actually surround the propeller, it is possible in the case of front nozzles to select their diameter at will and to position the nozzle stream cross-sectional area and to make it of a size such that the compensation of the flow to the propeller is as extensive as possible.

The wash is particularly heavy in the upper area of flow to the propeller and in the vicinity of the midships plane. The nozzle effect is also concentrated at this area.

In this area the flow separation is also strongest on the outer skin of the ship as a result of an angle of incidence with respect to the longitudinal axis of the ship which is large or too large. In this respect the nozzle may cause improved contacting of the flow with the ship's body. At this point the angle of flow is in most cases particularly large with respect to horizontal, i.e. it is easiest at this point to achieve a propulsion component in the case of deflection of the flow in a more horizontal direction using a guide surface.

If the size and arrangement of the front nozzle is limited to this area, it has a considerably lower resistance than conventional front nozzles having an inner diameter of approximately the propeller diameter and is simpler from a constructional point of view.

A good effect from the point of view of propulsion, which is achievable with a flow guide surface of the invention, is the production of an initial whirl in the upper sector of the propeller.

This effect may be combined with a "curved hull" in the lower sector. It is therefore proposed in accordance with the invention for the extension of the water lines of the afterbody structure below the propeller axis to lie outside of the midships plane, As the water lines in the lower area extend in the midships plane at an angle which is more acute than in the upper region, there is less risk of flow separation in the lower area than in the upper area. This means that the hull connection below the shaft may be limited to a small area in accordance with length.

For twin-screw ships it is proposed to form the guide surfaces as propeller brackets. Propeller brackets of this type are conveniently designed with rearwardly converging axes in the case of propellers beating upwardly outwardly.

In the case of greater convergence of the propeller bracket axes this may increase manoeuvrability such that in the case of rotation the screw which is beating back acts on the spot on the propeller bracket such that a transverse component amplifying the rotation is produced.

In the case of double screw arrangements the power saving is much greater than for single screw arrangements as the additional resistance and disturbing effect of the actual propeller bracket is not present.

The advantages of the guide surface are further increased in sea travel as the propeller flow behind a guide surface is subject to less considerabie fluctuations than would be the case without this guide surface. The guide surface also causes a reduction in pitch fiuctuation, although this is low. In addition the guide surface contributes to course stability.

Guide surfaces are not only suitable for inclusion in new ship constructions. They may be readily installed in existing ships and may be used for the correction of defects, for example in the following cases: (1) When the speed conditions are not satisfied during sea trials, (2) When the screw action has become too heavy over time, (3) When course stability is in need of improvement, (4) When the afterbody is inclined to vibrate.

To help understanding of the invention two specific embodiments thereof will now be described with reference to the accompanying drawings in which: Figure 1 shows a side view of a stern portion of a ship's hull incorporating a flow guide surface of the invention; Figure 2 is a horizontal cross-section of the arrangement shown in Figure 1, taken approximately half way up the ring nozzle consisting of two half shells, and Figure 3 is a cross-sectional diagrammatic view of a twin-screw arrangement taken at guide surfaces in front of the propellers and seen from behind.

Referring first to Figures 1 and 2, the ship's hull portion 4 is provided with a propeller 5 which is driven via a shaft guided by means of a stern tube 9 and which has a blade tip circle 10. A rudder 6 is provided downstream of the propeller 5. Upstream of the propeller a ring nozzle is provided which comprises two half shells 1,2 which are separated by an optional dividing surface 3. The centroid of the cross-sectional surface enclosed in this way lies above the propeller axis 7. As may be seen in Figure 2 the two half shells 1,2 differ and axes may in this respect assume different axial directions. The axes rise slightly towards the stern.

In the embodiment of Figure 3 with a twinscrew arrangement flow guide surfaces 1 are formed as propeller brackets. In this respect the frame contour in the plane of cross-section is numbered 1 the main frame contour 12, the constructional water line 13 and the midships plane 14 for better comprehension.

With the above described flow guide surfaces, it will be appreciated that an increase in propulsion efficiency and a compensation in the speed of flow to the propellers can be simply provided. The arrangements enable fastening over a large area. In this respect slamming effects are less of a danger and the water shock effects are less concentrated as a result of the vertical disposition of the guide surfaces.

If desired, the flow guide surface may be mounted so that it can be used as an additional rudder for the ship to which it is fitted.

Claims

1. A flow guide surface for a propeller ship which is adapted to be fixed to the ship in front of the propeller, wherein the centroid of crosssectional area enclosed by a guide surface nozzle lies above the propeller axis.

2. A flow guide surface as claimed in claim 1, wherein the horizontal spacing of the guide surface nozzle from the propeller is less than one propeller diameter.

3. A flow guide surface as claimed in claim 1 or claim 2 wherein the centroid of the cross-sectional area enclosed by the guide surface nozzle lies above the propeller axis by at least 10% of the propeller diameter and the cross-sectional area is smaller than half the area of the propeller blade tip circle.

4. A flow guide surface as claimed in claim 1, claim 2 or claim 3 wherein the axis of the guide surface nozzle is disposed such that it rises slightly towards the stern.

5. A flow guide surface as claimed in any preceding claim, wherein the guide surface nozzle is a ring formed by two half shells, each of the half shells being associated with its side of the ship in different axial directions.

6. A flow guide surface as claimed in claim 5, including a dividing surface between the two half shells.

7. A flow guide surface as claimed in any one of claims 1 to 4, wherein the guide surface nozzle is closed into a ring with the ship's skin.

8. A flow guide surface as claimed in any one of claim 1, claim 2 and claim 3, wherein the guide surface nozzle, formed as a closed ring, is disposed on one side only of the ship.

9. A flow guide surface as claimed in any preceding claim, wherein the extension of the water lines of the afterbody structure and for the end of the dividing surface lie below the propeller axis outside of the midships plane.

1 0. A flow guide surface for a twin-propeller ship as claimed in any one of claims 1 to 4, wherein the guide surface nozzle for each propeller is also formed as a propeller bracket.

11. A flow guide surface substantially as hereinbefore described with reference to Figures 1 and 2 or Figure 3 of the accompanying drawings.

12. A ship incorporating a guide surface as claimed in any preceding claim.

1 3. A flow guide surface as claimed in claim 1 adapted for mounting on a ship for use as an additional rudder.