Section 4 Rudder horns and appendages

4.1.1 Rudder horns and boss end brackets may be constructed of cast or forged steel or fabricated from steel plate. Where shaft brackets are fitted these may be either fabricated, cast or forged from steel.

4.1.2 In castings, sudden changes of section or possible constrictions to the flow of metal during casting are to be avoided. All fillets are to have adequate radii, which, in general, are to be not less than 50 to 75 mm, depending on the size of the casting.

4.1.3 Castings and forgings are to comply with the requirements of Ch 4 Steel Castingsand Ch 5 Steel Forgings of the Rules for Materials.

4.1.4 Rudder horns, shaft brackets, etc. are to be effectively integrated into the ship structure, and their design is to be such as to facilitate this.

4.2.1 The thickness of the propeller boss is to be not less than:

4.3.1 The requirements for the scantlings and arrangements of rudder horns will be subject to special consideration and may require to be determined by direct calculations.

4.4.1 Where the propeller shafting is enclosed in bossings extending back to the bearings supporting the propellers, the aft end of the bossings and the bearings are to be supported by substantially constructed boss end castings or fabrications. These are to be designed to transmit the loading from the shafting efficiently into the ship’s internal structure.

4.4.2 For shaft bossings attached to shaft brackets, the length of the boss is to be adequate to accommodate the aftermost bearing and to allow for proper connection of the shaft brackets.

4.4.3 Cast steel supports are to be suitably radiused where they enter the main hull to line up with the boss plating radius. Where the hull sections are narrow, the two arms are generally to be connected to each other within the ship. The arms are to be strengthened at intervals by webs.

4.4.4 Fabricated supports are to be carefully designed to avoid or reduce the effect of hard spots. Continuity of the arms into the ship is to be maintained, and they are to be attached to substantial floor plates or other structure. The connection of the arms to the bearing boss is to be by full penetration welding.

4.4.5 The scantlings of supports will be specially considered. In the case of certain high powered ships, direct calculations may be required.

4.4.6 The boss plating is generally to be radiused into the shell plating and supported at the aft end by diaphragms at every frame. These diaphragms are to be suitably stiffened and connected to floors or a suitable arrangement of main and deep web frames. At the forward end, the main frames may be shaped to fit the bossing, but deep webs are generally to be fitted not more than four frame spaces apart.

4.5.1 The scantlings of the arms of shaft brackets, based on a breadth to thickness ratio of about five, are to be determined from Vol 1, Pt 3, Ch 3, 4.6 Single arm shaft brackets (‘P’ – brackets) 4.6.1 or Vol 1, Pt 3, Ch 3, 4.7 Double arm shaft brackets (‘A’ – brackets) 4.7.2 as appropriate.

4.5.2 Where the propeller shafting is exposed to the sea for some distance clear of the main hull, it is generally to be supported adjacent to the propeller by independent brackets having two arms. In very small ships the use of single arm brackets will be considered.

4.5.3 Fabricated brackets are to be designed to avoid or reduce the effect of hard spots and ensure a satisfactory connection to the hull structure. The connection of the arms to the bearing boss is to be by full penetration welding.

4.5.4 Bracket arms are in general to be carried through the shell plating, they are to be attached to floors or girders of increased thickness. The shell plating is to be increased in thickness and connected to the arms by full penetration welding.

4.5.5 In the case of certain high powered ships direct calculations may be required.

4.5.6 For shaft brackets having hollow section arms, the cross-sectional areas at the root and the boss should be not less than that required for a solid arm which satisfies the Rule section modulus having the proportions stated in Vol 1, Pt 3, Ch 3, 4.5 Shaft brackets 4.5.1.

4.5.7 The length of the shaft bracket boss, l _b, is to be sufficient to support the length of the required bearing. In general, l _b is not to be less than 4d _t, where d _t is the Rule diameter of the screwshaft, in mm, see Vol 2, Pt 3, Ch 2, 4.4 Screwshafts and tube shafts. Proposals for a reduction in the required shaft bracket boss length will be considered in conjunction with details of the bearing material, allowable bearing operating pressure and installation arrangements, see Vol 2, Pt 3, Ch 2, 4.16 Sternbushes and sterntube arrangements 4.16.2. However, in no case is l _b to be less than the greater of:

1. 2d _t;

2. that recommended by the bearing manufacturer;

3. as required by Vol 1, Pt 3, Ch 3, 4.4 Shaft bossing 4.4.2.

4.5.8 Where the shaft and the shaft bracket boss are of the same material, the thickness of the shaft bracket boss is not to be less than d _t/4. Where the shaft and the shaft bracket boss are of dissimilar materials, the thickness of the boss, t _b, is to be not less than:

where

4.5.9 The design of the shaft brackets with regard to disturbance of the hydrodynamic flow into the propeller and rudders is outwith the scope of classification

4.6.1 Single arm shaft brackets are to have a section modulus, Z _xx, at the palm of not less than that determined from the formula:

where

The cross-sectional area of the bracket at the boss is to be not less than 60 per cent of the area of the bracket at the palm.

4.6.2 For single arm shaft brackets a vibration analysis may be required if deemed necessary by LR.

4.7.1 The angle between the arms for double arm shaft brackets is to be generally not less than 50°. Proposals for the angle between the arms to be less than 50° will be specially considered with supporting calculations to be submitted by the designers.

4.7.2 The arms of double arm shaft brackets are to have a section modulus, Z_xx, of not less than that determined from the formula:

where

d _up and f are as given in Vol 1, Pt 3, Ch 3, 4.6 Single arm shaft brackets (‘P’ – brackets) 4.6.1

4.8.1 The length and thickness of the shaft bracket boss are to be as required by Vol 1, Pt 3, Ch 3, 4.5 Shaft brackets 4.5.7 or Vol 1, Pt 3, Ch 3, 4.5 Shaft brackets 4.5.8 as appropriate. The scantlings of the arms will be specially considered on the basis of the Rules.

4.9.1 Fabricated supports are to be carefully designed to avoid or reduce the effect of hard spots. Continuity of the arms into the ship is to be maintained, and they are to be attached to substantial floor plates or other structure. The connection of the arms to the bearing boss is to be by full penetration welding.

4.10.1 The bottom shell thickness in way of the double arm propeller bracket palms is to be increased by 50 per cent. The bottom shell thickness in way of single arm propeller brackets palms is to be doubled in thickness. The insert plates are to be additionally supported by substantial floor plates or other structure.

4.10.2 Where shaft brackets are attached by bolts, they are to be provided with substantial palms securely attached to the hull structure which is to be adequately stiffened in way. Where bolts are used, the nuts are to be suitably locked.

4.10.3 The bracket palms may be bolted directly onto the shell using a suitable bedding compound. The palms may be bolted onto suitable shims or chocking compound, of an approved type, to facilitate alignment.

4.10.4 Where brackets are bolted onto resin chocks, plans indicating the following information are to be submitted for approval:

1. The thrust and torque loads, where applicable, that will be applied to the chocked item.

2. The torque load to be applied to the bracket mounting bolts.

3. The material of the bracket mounting bolts.

4. The number, thread size, shank diameter and length of the mounting bolts.

4.10.5 The minimum thickness of a resin chock is to be 12 mm.

4.10.6 The bracket palms are to have well radiused corners, and the faying surface to be dressed smooth. The palm thickness in way of the bolts is to be not less than the propeller bracket boss thickness from Vol 1, Pt 3, Ch 3, 4.5 Shaft brackets 4.5.7 or Vol 1, Pt 3, Ch 3, 4.5 Shaft brackets 4.5.8 as appropriate.

4.10.7 The diameter of the propeller bracket mounting bolts is to be not less than:

subject to d _bmin ≥ t _b mm

where

4.10.8 Where the shaft bracket and the shaft bracket mounting bolts are of dissimilar materials (which are galvanically compatible), the diameter of the propeller bracket mounting bolts, as determined from Vol 1, Pt 3, Ch 3, 4.10 Attachment of shaft brackets by bolting 4.10.7, is to be modified in proportion to the square root of the yield strengths of the particular materials. The corrected bolt diameter of the dissimilar material is to be not less than the propeller bracket boss thickness.

4.10.9 The propeller bracket palms are to have fitted bolts, and suitable arrangements provided to lock the nuts.

4.10.10 A washer plate is to be provided, generally of equal dimensions to the bracket palm with thickness t_b/6 mm, subject to a minimum of 3 mm.

4.11.1 Particular care is to be paid to the alignment of shaft brackets to minimise vibration and cyclic loadings being transmitted from the propulsion shafting and propellers into the hull structure.

4.11.2 Alignment of bolted shaft brackets may be by means of suitable metallic shims or chocking resin of an approved type, see Vol 1, Pt 3, Ch 3, 4.10 Attachment of shaft brackets by bolting 4.10.2 and Vol 1, Pt 3, Ch 3, 4.10 Attachment of shaft brackets by bolting 4.10.3.

4.11.3 The alignment of shaft brackets connected by welding or bonding may be facilitated by boring of the bracket boss after attachment of the shaft bracket and sterntube.

4.12.1 The sterntube scantlings are to be individually considered.

4.12.2 The bottom shell, in way of the sterntube, is to be additionally reinforced by means of an insert plate to increase the bottom shell thickness by 50 per cent.

4.12.3 The sterntube should in general be connected to the shell by welding. Bolted arrangements will be specially considered.

4.12.4 Where sterntubes are to be retained by bolting they are to be provided with a substantial flange securely attached to the hull structure. Where bolts are used, the nuts are to be suitably locked.

4.12.5 Where sterntubes are to be welded to hull insert plates full penetration welding is required.

4.12.6 Where sterntubes are to be installed using a resin system, of an approved type, the requirements of Vol 1, Pt 6, Ch 6 Material and Welding Requirements are to be complied with.

4.12.7 The region where the shafting enters the ship, and the bearing in way, is to be adequately supported by floors or deep webs.

4.12.8 The shaft bearings are to be secured against rotation within the sterntube.

4.12.9 A suitable gland arrangement is to be provided at the inboard end of sterntubes.

4.13.1 Skegs are to be efficiently integrated into the adjacent hull structure and their design is to facilitate this.

4.13.2 The scantlings of skegs are to be sufficient to withstand any docking forces imposed upon them.

4.14.1 Recommended minimum clearances between the propeller and the sternframe, rudder or hull are given in Table 3.4.1 Recommended minimum propeller hull clearances. These are the minimum distances considered desirable in order to expect reasonable levels of propeller excited vibration. Attention is drawn to the importance of the local hull form characteristics, shaft power, water flow characteristics into the propeller disc and cavitation when considering the recommended clearances.