GB2194295A

GB2194295A - Screw propeller boss cap

Info

Publication number: GB2194295A
Application number: GB08718165A
Authority: GB
Inventors: Michihito Ogura; Hajime Koizuka; Toshinori Takeshita; Yoshio Kohno; Kazuyuki Ouchi; Takashi Shiotsu
Original assignee: MIKADO PROPELLER; WEST JAPAN FLUID ENG; Mitsui OSK Lines Ltd
Current assignee: MIKADO PROPELLER; WEST JAPAN FLUID ENG; Mitsui OSK Lines Ltd
Priority date: 1986-07-31
Filing date: 1987-07-31
Publication date: 1988-03-02
Anticipated expiration: 2007-07-31
Also published as: HK85092A; AU7630187A; GB2194295B; AU593670B2; FR2619352A1; SG60592G; CA1289821C; GB8718165D0; EP0255136A1

Description

GB2194295A 1

SPECIFICATION

A screw propeller boss cap with fins The present invention relates to a boss cap of a screw propeller, particularly to a screw 5 propeller boss cap with fins.

In order to improve the characteristics of a screw propeller, particularly the propeller efficiency, extensive and intensive researches have already been made by engineers with respect to the technical design of number, shape, developed area, pitch, etc. of blades and now their fruits materially have been brought forth to a nearly maximum extent. Thus it is extremely difficult to 10 expect any future drastic improvement of the propeller characteristics through researches on these items.

On the other hand, it has been known that the propeller efficiency of a screw propeller is low in the proximity of its boss. For this reason, it has been proposed several times to provide a small diameter propeller at the rear stream side of the main propeller so that the propeller 15 efficiency in the proximity of its boss may be raised, for example, in the Japanese Utility Model Laid-opens Nos. 30,195/81 and 139,500/82. It seems however that such idea in fact was not successful, probably for the reason that the thrust does not so much increase as the torque increases and thus the propeller efficiency is not so improved as expected.

An object of the present invention therefore is to provide a new technique which enables to 20 improve the propeller characteristics particularly the propeller efficiency to a considerably high extent through addition of a. boss cap with fins to a propeller.

As shown in the attached drawing of Fig. 3 (prior art technique), an ordinary screw propeller

31 comprises a plurality of blades 33 provided in a equidistance around the periphery of a boss 32 and connected through the boss 32 to a rotational drive shaft 34. On an end opposite to the 25 drive shaft 34 of the boss 32, a conical boss cap 35 is mounted in order to reduce vortices generated downstream of the boss 32 as much as possible.

The present inventors have focused their attention on the fact that even in the rear stream of such propeller boss cap, a considerable hub vortex 36 is generated and, in the thought that the prior art small diameter additional propeller would have increased such hub vortex, have made 30 intensive researches seeking to find any other means for reducing such hub vortex. Finally it has been found that the addition of a boss cap with fins to a propeller can reduce such hub vortex and in effect can increase the propeller efficiency.

Thus the present invention presents a cap to be mounted on a boss of a screw propeller; which cap has fins satisfying the following conditions: 35 (i) the same number is associated with each propeller plade, (ii) they have the inclination---Alpha-from -20' to +30' relative to the geometric pitch angle---Epsilon-of a propeller blade root, that is, 20':5---Alpha"- "Epsilon" ---5 30', and the leading edge located between the adjacent propeller blade roots, and (iii) they have the maximum diameter larger than the diameter of the cap- mounting end of the 40 boss and not larger than 33% of the propeller diameter.

Thus, where N is an integer and is the number of fins per blade, there will be 4N fins in the case of four propeller blades and so on. This means the number of fins, corresponding to each space between propeller blade roots is the same.

The fins provided according to the present invention are not those for generating a thrust by 45 themselves, but for guiding the water stream rearward of the boss cap to a direction to reduce the generation of the hub vortex.

Owing to such guide effect, the hub vortex in the rearward of the boss cap is diffused and thus the drag force induced by the vortex on a propeller blade plane is reduced and as the result the propeller characteristics particularly the propeller efficiency are greatly improved without 50 remarkable increase of the torque.

Accordingly, as a general tendency, the present invention gives a particularly higher effect to a propeller having a higher pitch ratio (H/D) which generates a stronger hub vortex.

As shown in the embodiments of the present invention hereinafter, the fins may be provided to have a rake angle or a positive or negative camber against the boss cap. 55 Figure 1 is a front view of a propeller on which an embodiment of the propeller boss cap with fins of the present invention is mounted, and Fig. 2 is a side view of the Fig. 1.

Figure 3 is a side view similar to the Fig. 2, but shows a propeller and a boss cap of prior art technique without fins together with a hub vortex generated rearward of the boss cap.

Figure 4 is a side view partly shown in the section of a propeller characteristics measurement 60 apparatus used in the experiments.

Figure 5 is a plan view showing plane shape of the fins used in the experiments and Fig. 6 is a side view showing the mounting positions of the fins to the propeller boss cap.

Figure 7 shows the propeller characteristics curves obtained in the Experiment No. 1 and Figs.

8-10 show the schematic illustrations representative of the relative positions of the propeller 65 2 GB2194295A 2 blades roots and the fins in the Experiments Nos. 2-4, respectively.

Figure I I is a side view to show the rake angle of the fins in the Experiment No. 5 and Fig.

12 is an A-A line sectional view of the Fig. 11.

Figure 13 is a schematic illustration similar to the Figs. 8-10, but showing the relative positions of the propeller blades roots and the fins in the Experiment No. 6. 5 Figure 14 is a diagram showing the results of the Experiment No. 7.

Some embodiments of the present invention will be explained in detal with reference to the attached drawings.

Tests are -made in a water tank, using models of propellers having the data as shown in the following table 1. The water tank is of a circular stream type and has an observational part of 10:6 scales 5.0 m (length) x 2.0 m (width) x 1.0 m (depth). The maximum flow rate is 2.0 m/sec and the uniformity of the flow rate is within 1.5%.

Table I

15 Tyl?e CP24 CP26 Diameter (mm) 2.20.0 220.0 20 Pitch ratio 0.8 1.2 Developed blade. are& ratio 0.55 0.55 25 Boas ratio 0.18 0.18 Blade thikness ratio 0.05 0.05 30 Blade cross section shape MAU MAU Blade number 4 4 In Fig. 4, a side view of a propeller characteristics measurement apparatus is shown partly by 35 a section. This apparatus is located in the observational part of said water tank by securing its propeller open boat 41 to a rigid carrier (not shown) placed above the water tank. The boat 41 has a drive mechanism 43 to rotate a propeller 42 which may detachably be attached to its tip end, a thrust detector 44 and a torque detector 45.

Although not shown in the Fig. 4, a propeller rotating speed is measured by a digital counter 40 TM-225 (product of Ono Measurement Instruments company, Japan) and a flow rate by a combination of a JIS type Pitot tube and a differential pressure converter DLPU-0.02 (product of Toyo Boldwin company, Japan). The analogue signals of such thrust, torque and flow rate represented by the differential pressure, etc. are converted to digital signals through an A/D converter provided in a microprocessor located in a separate controller and processes into 45 physical data which then are printed by a pinter or plotted by a plotter.

A thrust coefficient (KT) and a torque coefficient (KQ) are measured under different advance I coefficients (J) adjusted by changing the flow rate while keeping the propeller rotating speed approximately constant within the range of 7.5-9.0 r.p.s.. The depth in water of the propeller center is 300 mm and the direction of water flow is as shown by an arrow in the Fig. 4. 50 As a boss cap to be mounted on the propeller models, a cap of a rounded conical shape having a base diameter of 35 mm and a height of 25.6 mm is prepared. The cap may be mounted on the propeller by any known means and in these experiments a bolt-nut securing is employed.

As fins to be provided on the boss cap, those having six different triangular shapes (A)-fl 55 shown by a plan view in Fig. 5 are prepared from flat plates of 1 mm thickness to have the dimensions as shown in the following table 2.

3 GB2194295A 3 Table 2

Fin shape Width Height 5 (X-axis direction) (Y-axis diiection) (A) 20 MM 20 Imm 10 (B) 26 MM 16. 5 (C) 26 MM 21 mm 15 (D) 26 MM 28.5 mrn (E) 26 MM 34 mm (P) 26 MM 39.5 mm 20 Fig. 6 shows the relative positions of the fins and the boss cap. In this Fig. 6, a rear end 0 of a root 62 of a propeller blade 61 is set on the propeller axis 63 as a reference point. In this specification, a peripheral distance from the front end of a fin 64 to the plane including the 25 reference point 0 and the propeller axis 63 is called -a- (positive in the direction of propeller rotation shown by an arrow). A surface distance from the front end of the fin 64 to a periphery including the reference point 0 is called---b-. The angle of the fin 64 against plane normal to the propeller axis is called "Alpha". The geometric pitch angle of the propeller blade root 62 is called "Epsilon". 30 In this specification, the geometric pitch angle "Epsilon" of a propeller blade root is one based upon a nose-tail line of the propeller blade root. More precisely, two surfaces of a cylindrical surface having an axis on the propeller axis and a radius equal to the boss radius and a propeller blade surface or its extension as a suspected surface are considered. A cylindrical surface intercepted by a crossing line between these two surfaces, that is, a cylindrical section, is 35 developed on a plane. In the developed view, an angle between a nose-tail line of the blade section defined by the developed cylindrical section and a line normal to a generatrix of the cylindrical surface corresponds to the "Epsilon".

The fin 64 is mounted on the boss cap in the direction perpendicular to the sheet of the Fig.

6, when no rake angle is given. The mounting is made, in these experiments, by cutting a 40 groove on the boss cap, inserting the lower portion of the fin into the groove and fixing by means of an adhesive, but it is of course possible and in actual cases it is preferred to form the boss cap and the fins integrally as one body. The broken lines shown in the lower portions of fins in the Fig. 5 indicate crossing lines between the fin surface and the boss cap surface after mounting of the former on the latter. 45 Experiment 1 Water tank tests have been made by using the propeller of the type CP26 (Epsilon=67.4') and the fins of Fig. 5(C). The fins of total number 4, one for each propeller blade, are mounted on the boss cap in positions of a= 10 mm, b=5 mm and Alpha=66'. In this instance, the 50 maximum diameter of the fins, that is, a doubled distance J2r) between the radially remotest end of a fin (from the propeller axis) and the propeller axis (after mounting of the fins on the boss cap) and a propeller diameter (2R) stand in a ratio r/R=0.23. Fig. 1 shows a front view of thus composed propeller 1, boss 2, propeller blades 3, shaft 4, boss cap 5 and fins 6; and Fig. 2 shows a side view thereof. For comparison, tests have been made also as to cases without 55 fins. The thrust coefficient (KT) and the torque coefficient (KQ) have been measured under various advance coefficients (J) of 0.0-1.1 and the propeller efficiency (Ra=J.KT/2PiKQ) has been calculated. Then an increase ratio (dEta) of the propeller efficiency increased from the cases without fins to the cases with fins has been calculated by percents. The results are shown in the following tables 3 and 4. 60 4 GB2194295A 4 Table 3 (C&Ses Without fins) No KT KQ X 10 Eta 5 0 0.000 0.4816 0.9376 0.0000 1 0.050 0.4715 0.9092 0.0413 10 2 0.100 0.4606 0.8823 0.0831 3 0.150 0.4489 0.8565 0.1251 15 4 0.200 0.4363. M316 0.1670 0.250 0.4230 0.8071 0.2085 6 0.300 0.4088 0.7829 0.2493 20 7 0.350 0.3940 0.7586 0.2893 8 0.400 0.3785 0.7342 0.3282 25 9 0.450 0.3623 0.7094 0.3657 0.500 0.3454 0.6839 0.4019 30 11 0.550 0.3281 0.6578 0.4365 12 0.600 0.3102 09 0.4695 0.63 1 35 13 0.650 0.2919 0.6031 0.5007 14 0.700 0.2731 0.5743 0.5299 1.5 0.750 0.2541 0.5445 0.5571 40 16 0.800 0.2349 0.5138 0.5820 17 0.850 0.2154 0.4820 0.6046 45 18 0.9 - DO 0.1959 0.4494 0.6244 19 0.950 0.1764 0.4159 0.6413 50 - 1.000 0.1570 0.3817 0.6547 21 1.050 0.1378 0.3468 0.6639 - 55 22 1.1 0.1189 0.3115 0.6680 00 GB2194295A 5 Table 4 (caees with fins) No. j KT KO X 10 Eta dEte(t) 5 0 0.000 0.4985 0.9154 0.0000 1 0.050 0.4894 0.8914 0.0437 5.55 2 0.100 0.4785 0.8677 0.0878 5.35 3 0,150 0.4660 0.8440 0.1318 5.09 15 4 0.200 0.4522 0.8204 0.1755 4.81 0.250 0.4373 0.7965 0.2184 4.54 6 0.300 0.4215 0.7724 0.2605 4.29 20 7 0.350 0.4050 0.7479 0.3016 4.09 a 0.400 0.3880 0.7229 0.3417 3.96 25 9 0.450 0.3705 0.6973 0.3806 3.90 0.500 0.3528 0.6710 0.4184 3.95 30 11 0.550 0.3349 0.6441 0.4552 4.09 12 0.600 0.3168 0.6164 0.4909 4.35 35 13 0.650 0.2986 0.5879 0.5255 4.73 14 0.700 0.2803 0.5585 0.5590 5.21 is 0.750 0.2617 0.5284 0.5913 5.78 40 16 0.800 0.2430 0.4974 0.6220 6.42 17 0.850 0.2239 0.4655 0.6506 7.07 45 is 0.900 0.2044 0.4328 0.6762 7.66 19 0.950 0.1843 0.3994 0.6976 8.07 50 1.000 0.1634 0.3652 0.7123 8.09 21 1.050 0.1417 0,3303 0.7168 7.38 55 22 1.100 0.1187 0.2947 0.7053 5.29 6 GB2194295A 6 Fig. 7 illustrates the results of the tables 3 and 4 showing the advance coefficient (J) in abscissa and the thrust coefficient (KT), the torque coefficient multiplied by ten (KQx 10 and the propeller efficiency (Eta) in ordinate. In this Fig. 7, curves T2, Q2 and P2 represents KT, KG x 10 and Eta in the table 3 and curves T3, Q3 and P3 represents KT, KQ X 10 and Eta in the table 4.

From this Fig. 7 and the table 4, it is understood that the propeller efficiency increases about 5 4-8% in the overall range of J0.05-1.10 and particularly 7.66% at the usually employed J=0.9.

Further, in these tests, a needle pipe is manually put into the water from above the water tank to the close proximity of the rear end of the boss cap to supply air bubbles. It has been found that in the cases without fins, a large number of air bubbles align along the propeller axis, but in 10 the cases with fins, air bubbles are diffused to disappear. It is considered that a hub vortex is greatly reduced by the merit of the fins.

Experiment 2 Tests similar to those shown in the Experiment 1 have been made, but using different 15 positions of fins, that is, different a, b and Alpha. The propeller efficiency increase ratio obtained under the usually employed advance coefficient (J)=0.9 is shown in the following table 5.

Table 5

20 No. a b Alpha Alpha- r/R dEta (mm) (0) - Epsilon(') ( t) 25 1 10 0 64 -3.4 0.25 5.49 2 is 0 61 -6.4 0.25 7.32 30 3 10 5 66 1.4 0.23 7.66 4 14 5 59 -8.4 0.23 6.39 35 from the data in the Experiment 1 40 The relative positions of fins and the propeller blade roots are illustrated in Fig. 8, wherein the rear end 0 of one propeller blade root shown in the Fig. 6 is placed on the base line X, and the blade position is shown as a line segment starting from the base line X with an angle Epsilon and having a length corresponding to the length of the nose-tail line of the propeller blade root.

Another propeller blade root adjacent to said one also is shown is a similar manner, but at a 45 peripheral distance between the rear ends of them taken in the direction of the base line X. The 1 positions of the fins are shown by taking -a- of the Fig. 6 in the direction of the base line X and -b- of the Fig. 6 in the direction of a base line Y which passes through the reference point 0 normally to the base line X. The lengths of the fin segments correspond to the lengths of the 1 1 crossing lines between the fins and the boss cap as shown in the Fig. 5 by broken lines. From 50 the table 5 and the Fig. 8, it is understood that a considerable improvement of propeller efficiency can be obtained when the front ends of fins are placed between the adjacent propeller blade roots, that is, within a space between extended nose-tail lines of the adjacent propeller blade roots.

55 Experiment 3 Tests similar to those of Experiment 1 have been made, but changing only Alpha. The propeller efficiency increase ratio obtained at the advance coefficient (J)=0.9 is shown in the following table 6.

7 GB2194295A 7 Table 6

No. A12ha() Al]2ha-Epsilon(0 r/R dEta(%) 5 1 45 -22.4 0.24 0.34 2 so -17.4 0.235 3.46 10 3 66 - 1.4 0.23 7.66 4 85 17.6 0.22 3.80 15 90 22.6 0.22 2.19 6 95 27.6 0.22 2.38 7 100 32.6 0.22 0.77 20 37.6 0.22 0.47 25 .. from the data of Experiment 1 The results of the table 6 are shown in Fig. 9 similarly to the Experiment 2. It is understood 30 from the table 6 and the Fig. 9 that a considerable improvement of propeller efficiency can be obtained within the range of -20'--,!5A]pha-Epsilon:-!530'.

Experiment 4 Tests similar to those of Experiment 2 have been made, but using the propeller of the type 35 CP24 (Epsilon=57.4') and the fins of the Fig. 5(A) and 5(C).--- Thepropeller efficiency increase ratio obtained at the advance coefficient (J)=0.6 usually employed for such propeller is shown in the following table 7.

8 GB2194295A 8 Table 7

No. a b Alpha Alpha- r/R Fin shape dEta 5 (MM) (=) (0) EpsilonC) (%) 1 0 5 80 22.6 0.21 (A) 2.03 10 2 10 17 63 5.6 0.18 (A) 3.02 3 5 12 63 5.6 0.20 (A) 2.06 15 4 5 9.5 63 5.6 0.21 (A) 2.32 5 7 63 5.6 0.215 (A) 2.84 6 10 7 63 5.6 0.23 (C) 3.93 20 7 10 7 57 -0.4 0.22 (C) 2.32 8 6 7 63 5.6 0.22 (C) 2.57 25 1 9 4 5 35 -22.4 0.235 (C) -0.09 4 5 90 32.6 0.22 (C) -0.19 30 The results of the table 7 are shown in Fig. 10 similarly to Experiment 2. It is understood that there is no material difference between the fin shapes (A) and (C) and that the fin positions should satisfy the conditions that the front end of the fin is located between the adjacent propeller blade roots and the inclination of the fin stands within the range of -20'-:5Alpha Epsilon--530', as in the case of said Fig. 9. 35 Experiment 5 The test of Experiment 4, No. 6 has been repeated, adding rake angles of 30, to the fins.

The rake angles are measured from the direction perpendicular to the sheet of the Fig. 6 to the direction of rotation.of the propeller. The results are shown in the following table 8. 40 Table 8

45 No. a b Alpha Alpha- r/R Rake Angle dEta (%) (MM) (m) ( a) Epsilon P) ( 11) 6F 10 7 63 5.6 0.21 +30 1.46 50 6M 10 7 63 5.6 0.23 0 3.93 6B 10 7 63 5.6 0.21 -30 4.47 55 ... from th6 data of EXperiment 4 60 From the table 8, it is understood that there is a tendency of further improvement of dEta when a rake angle opposite to the rotation direction of the propeller is added to fins. The mounting positions of fins are shown in Fig, 11 by way of a side view similar to the Fig. 6 and in Fig. 12 which is an A-A line section of the Fig. 11. - ---- 9 GB2194295A 9 Experiment 6 The tests of Experiment 4, No. 7 has been repeated, but by changing the number and positions of the fins, the results are shown in the following table 9.

Table 9 5

Nd. b Alpha Alpha- r/R Fin dEta 10 (MM) (mm) (") _Epsilon() Number M 7-N2 10 7 57 -0.4 0.22 2 -0.12 15 7-N3 10 7 57 -0.4 0.22 3 0.49 7-N4 10 7 57 -0.4 0.22 4 2.32 7-N5 10 7 57 -0.4 0.22 5 -1.12 20 from the data of Experimant 4 25 .. value of one specific fin; values of the other fins correspond to the positions 30 determined by the quotient of 360" divided by the number of fins 35 The relative positions of the propeller blade roots and the fins are shown in Fig. 13 by way of X-Y plane as in the Figs. 8-10. Relative to the four propeller blade roots 131-134, the fins are located, in the case of fin number two, at the positions 1/F and 2/2; in the case of fin number three, at the positions 1/F, 2/3 and 3/3; in the case of fin number four, at the positions 1/F, 2/4, 3/4 and 4/4; and in the case of fin number five, at the positions 1/F, 2/5, 3/5, 4/5 and 40 5/5, as shown in the Fig. 13. It can be seen therefrom that there is no fin located, in the case of fin number two, between the propeller blade roots B2 and B3 and between B4 and 131; in the case of fin number three, between B3 and B4. Further, there are two fins between B3 and B4 in the case of fin number five. Thus the fins are not evenly positioned in the cases of fin number two, three and five. 45 It is understood from the table 8 that the number of fins should be even for each space between the adjacent propeller blade roots.

Experiment 7 Tests similar to those of Experiment 1 have been made, but by using various fins of the Fig. 50 5(B)-5(F) having the same width but different heights. Total four same shape fins, one for each propeller blade, are mounted in the positions determined by a=10 mm, b=5 mm and Ak pha=66. The propeller efficiency increase ratio obtained at the advance coefficient (J)=0.9 is shown in the following table 10. - GB2194295A 10 Table 10

No. a b Alpha Alpha- r/R Fin shape dEta 5 (=) (MM) (11) -zilon () (%) 1 10 5 66 -1.4 0.2 4.12 10 2 10 5 66 -1.4 0.23 (C) 7.66 3 10 5 66 -1.4 0.3 (D) 6.08 1 - 15 4 10 5 66 -1.4 0.35 (E) 0.87 10 5 66 -1.4 0.4 (F) -0.50 20 .from the data of Experiment 1 25 The results of the table 10 are illustrated in Fig. 13, taking r/R in abscissa and dEta in ordinate.

In view of the fact that the boss ratio of the propeller of type CP26 is 0. 18, it is understood that the maximum diameter of the fins should be greater than the diameter of the cap-mounting end of the boss and not be greater than 33% of the propeller diameter, in order to obtain a 30 considerable improvement of the propeller efficiency.

Experiment 8 Tests similar to those of Experiment 1 have been made, but by using fins of the Fig. 5(C) bended to an arc of radius 50 mm. Two kinds of fins, one bended to the arc convex in the 35 direction of propeller rotation (=C-out) and the other to the are concave in the direction of propeller rotation (=C-in), are used. Total four same shape fins, one for each propeller blade, are mounted in the positions determined by a= 10 mm, b=5 mm and Alpha=66' (=angle of the direction of the chord of the are). The propeller efficiency increase ratio as obtained at the advance coefficient (J)=0.9 is shown in the following table 11. 40 Table 11

No a b Alpha Alpha- r/R Fin shape dEta 45 (mm) (mm) EpiBilon(o) 1 10 5 66 -1.4 0.23 C-out 6.46 50 2 10 5 66 -1.4 0.23 c 7.66 3 10 5 66 -1.4 0.23 C-in 6.94 55 .. from the data of Experiment 1 60 From the above data, it is understood that the shape of fins is not limited to plane and may have a positive or a negative camber.

As explained in detail above, it is possible to improve the propeller characteristics particularly the propeller efficiency without increasing torque, by the effect of guiding the water stream rearward of the boss cap to a direction of reducing generation of hub vortex, through the 65 11 GB2194295A 11 provision of fins on a boss cap to be mounted on a screw propeller in accordance with the present invention.

According to such invention, further merits are obtained, for example, the propeller character- istics may greatly be improved only by a slight modification of a rather small boss cap and not by a drastic change of the screw propeller itself, to which the boss cap is appended, necessitat- 5 ing difficult work and high cost. In effect, the present invention is applicable to screw propellers already mounted on existing ships, simply by exchanging or working the boss cap without incurring high cost.

Claims

CLAIMS 10

1 1. A cap to be mounted on a boss of a screw propeller; which cap has fins satisfying the following conditions:

(i) the same number is associated with each propeller blade, (ii) they have the inclination "Alpha" from -20' to +30' relative to the geometric pitch angle "Epsilon" of a propeller blade root, that is, - 20:-5 "Alpha"-" Epsilon - :-55 300, and the leading 15 edge located between the adjacent propeller blade roots, and (iii) they have the maximum diameter larger than the diameter of the cap- mounting end of the boss and not larger than 33% of the propeller diameter.

2. A cap according to claim 1 wherein the fins are cambered.

3. A cap according to claim 1 or 2 wherein the have a rake angle opposite to the direction 20 of rotation of the propeller.

4. A cap according to claim 1, 2 or 3 wherein there is one fin per propeller blade.

5. A cap to be mounted on the boss of a screw propeller substantially as hereinbefore described with reference to and as illustrated in Figs. 1, 2, 5, 6 and 11 of the accompanying drawings. 25

6. A screw propeller provided with a cap according to any preceding claim.

Published 1988 at The Patent Office, State House, 66/71 High Holborn. London WC1R 4TP. Further copies maybe obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BF15 3RD. Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.