GB1597364A - Propulsion units for marine vessels - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO PROPULSION UNITS FOR MARINE VESSELS (71) We, HYDROCONIC LIMITED, a British Company of Shipdesine House, East Quay, Ramsey, Isle of Man, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to propulsion units for marine vessels.

Where propellers or screws are used in ducts or nozzles which fully and closely enclose the propeller, running clearances are provide between the propeller blade tips and the duct or nozzle inner wall, and a problem that arises is the creation of vortices at the tips of the propeller blades.

These vortices have cavitation cores which subsequently collapse, and the collapse of such cavitation cores onto the inner wall of the duct or nozzle can cause erosion damage and other difficulties.

The object of the invention is therefore to overcome this problem.

According to the present invention, there is provided a method of inhibiting, in a marine propulsion unit comprising a bladed propeller situated in a nozzle or duct, the collapse of vortex cavitation cores onto the inner wall of the nozzle or duct, which method comprises the steps of: a) fabricating the propeller with a diameter less than the internal diameter of the nozzle or duct to leave a clearance gap between the blade tips and the nozzle or duct which is insufficiently small to prevent the shedding of span-wise vortices from the blade tips; b) forming slots or rebates in the blade tips; and c) securing into the slots or rebates vortex suppression vanes which serve to increase the propeller diameter and thereby reduce the gap between each blade and the nozzle or duct at least sufficiently to suppress the shedding of vortices from the blade tips.

Preferably, each vortex suppression vane is of a material such as nylon, that is lubricated by water and will cause no significant drag if it comes into light rubbing contact with the duct or nozzle wall surface.

Arrangements embodying the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 shows a propeller operating in a nozzle, with clearance gaps between the blade tips and the nozzle wall, Figure 2 shows a propeller blade of such an arrangement with a vortex suppression vane attached thereto, and Figures 3 to 6 show a number of forms of vortex suppression vane attachment.

In Figure 1, a propeller 10 operating in a nozzle or duct 11 has blades 12 the tips 13 of which are spaced from the interna wall surface 14 of the nozzle by a gap 15. This gap is chosen as the minimum that can be provided in manufacture without any risk of the blades fouling the nozzle.

In Figure 2, the gap 15 is substantially closed by a vortex suppression vane 16. The vortex suppression 'ane nay consist of a strip of nylon or some other appropriate material set parallel or at some suitable angle to the blade tip. Figures 3 to 6 show various ways of securing the vane 16 to the blade tip. Figure 3 shows the vane fitting into a slot 17 cut along the chord of the propeller blade and entering radially from the tip. The vortex suppression vane is secured in this slot by bolts 18. In Figure 4, the vortex suppression vane 16 is seated in a rebate 19 formed at the blade tip and secured by bolts 18 as before. In Figure 5, the blade tip is again slotted as in Figure 3 but with the bottom of the slot cut with a bulb section 20, and the vane 16 is locked in the slot by the formation at its inner edge of an enlarged bead 21 fitting the bulb section 20.

The bead 21 may be formed by providing a channel 22 through the vane near its inner edge and introducing fluid under pressure into this channel to bulge its surrounding walls and form the bead after the vane 16 has been inserted into the slot. In Figure 6, the vortex suppression vane is as wide as the blade tip throughout its height, except for a narrower shank 23 that is locked in position in a rebate 24 at the blade tip by a metal retaining plate 25 secured s bolts 26.

The vortex suppression -ane 16 is dimensioned and fitted so that the tip thereof is inside the boundary layer of the internal wall 14 of the nozzle or duct and it may come so close as to make rubbing contact with the nozzle itself. The matenal being water-lubricated in type, this latter condition is permissible provided the forces involved are not large, i.e. the contact is slight.

By fitting these vortex suppression vanes, the shedding of span-wise vortices from the blade tips is suppressed. Thus, the erosion and other problems that can arise due to the collapse of cavitation cores on to the duct or nozzle wall, as can occur with conventional propeller blades having clearance gaps, are avoided.

In our prior patent No. 1,131,071 there is disclosed and claimed a propulsion unit for a marine vessel comprising a bladed propeller situated in a nozzle or duct and whereof the tips of the blades of said propeller lie at a substantial distance from the internal wall surface of the nozzle or duct and the gap between each blade tip and said wall surface is substantially closed by attachment to the blade tip of an extension piece made from a material chosen such that no significant drag will result if it comes into light rubbing contact with said wall surface. We make no claim herein to the disclosure of that patent specification.

WHAT WE CLAIM IS: 1. A method of inhibiting, in a marine propulsion unit comprising a bladed propeller situated in a nozzle or duct, the collapse of vortex cavitation cores onto the inner wall of the nozzle or duct, which method comprises the steps of:a) fabricating the propeller with a diameter less than the internal diameter of the nozzle or duct to leave a clearance gap between the blade tips and the nozzle or duct which is insufficiently small to prevent the shedding of span-wise vortices from the blade tips; b) forming slots or rebates in the blade tips; and c) securing into the slots or rebates vortex suppression vanes which serve to increase the propeller diameter and thereby reduce the gap between each blade and the nozzle or duct at least sufficiently to suppress the shedding of vortices from the blade tips.

2. A method according to claim 1, wherein each vortex suppression vane is of a material, such as nylon, that is lubricated by water and will cause no significant drag if it comes into light rubbing contact with the duct or nozzle surface.

3. A method according to claim 1 or claim 2, wherein each vortex suppression vane is provided with a dove-tailed shank and the slot in the blade tip is correspondingly shaped to receive said shank.

4. A method according to claim 1 or claim 2, wherein the bottom of the slot in each blade tip is cut with a bulb section and the vane is locked in the slot by the formation at its inner edge of an enlarged bead fitting the bulb section.

5. A method according to claim 4, wherein the vane has a channel extending within its inner edge and the bead is formed by admitting fluid under pressure to said channel.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. retaining plate 25 secured s bolts 26. The vortex suppression -ane 16 is dimensioned and fitted so that the tip thereof is inside the boundary layer of the internal wall 14 of the nozzle or duct and it may come so close as to make rubbing contact with the nozzle itself. The matenal being water-lubricated in type, this latter condition is permissible provided the forces involved are not large, i.e. the contact is slight. By fitting these vortex suppression vanes, the shedding of span-wise vortices from the blade tips is suppressed. Thus, the erosion and other problems that can arise due to the collapse of cavitation cores on to the duct or nozzle wall, as can occur with conventional propeller blades having clearance gaps, are avoided. In our prior patent No. 1,131,071 there is disclosed and claimed a propulsion unit for a marine vessel comprising a bladed propeller situated in a nozzle or duct and whereof the tips of the blades of said propeller lie at a substantial distance from the internal wall surface of the nozzle or duct and the gap between each blade tip and said wall surface is substantially closed by attachment to the blade tip of an extension piece made from a material chosen such that no significant drag will result if it comes into light rubbing contact with said wall surface. We make no claim herein to the disclosure of that patent specification. WHAT WE CLAIM IS:

1. A method of inhibiting, in a marine propulsion unit comprising a bladed propeller situated in a nozzle or duct, the collapse of vortex cavitation cores onto the inner wall of the nozzle or duct, which method comprises the steps of:a) fabricating the propeller with a diameter less than the internal diameter of the nozzle or duct to leave a clearance gap between the blade tips and the nozzle or duct which is insufficiently small to prevent the shedding of span-wise vortices from the blade tips; b) forming slots or rebates in the blade tips; and c) securing into the slots or rebates vortex suppression vanes which serve to increase the propeller diameter and thereby reduce the gap between each blade and the nozzle or duct at least sufficiently to suppress the shedding of vortices from the blade tips.

2. A method according to claim 1, wherein each vortex suppression vane is of a material, such as nylon, that is lubricated by water and will cause no significant drag if it comes into light rubbing contact with the duct or nozzle surface.

3. A method according to claim 1 or claim 2, wherein each vortex suppression vane is provided with a dove-tailed shank and the slot in the blade tip is correspondingly shaped to receive said shank.

4. A method according to claim 1 or claim 2, wherein the bottom of the slot in each blade tip is cut with a bulb section and the vane is locked in the slot by the formation at its inner edge of an enlarged bead fitting the bulb section.

5. A method according to claim 4, wherein the vane has a channel extending within its inner edge and the bead is formed by admitting fluid under pressure to said channel.