Section 3 Sternframes and appendages

3.1.1 Sternframes, rudder horns and boss end brackets may be constructed of cast or forged steel, cast or forged aluminium alloy, fabricated from aluminium or steel plate or moulded from fibre reinforced plastic dependent upon the material of construction of the craft. Where shaft brackets are fitted these may be either fabricated, cast or forged from steel or aluminium alloy as applicable to the material of construction of the main hull.

3.1.2 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.

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

3.1.4 Sternframes, rudder horns, shaft brackets, etc. are to be effectively integrated into the craft structure, and their design is to be such as to facilitate this.

3.2.1 The scantlings of sternframes are to be determined from Table 3.3.1 Sternframes. In the case of very large craft, the scantlings and arrangements may be required to be verified by direct calculations.

Table 3.3.1 Sternframes

Item	Parameter	Requirement
(1) Propeller posts		Cast steel (see Figure 3.3.1 Propeller posts)	Forged steel (see Figure 3.3.1 Propeller posts)	Fabricated mild steel (see Figure 3.3.1 Propeller posts)
	l	165	-	200 mm
	r	20	-	18 mm
	tw	8	-	6 mm
		(need not exceed 38 mm)		(need not exceed 30 mm)
		(see Notes 1 and 2)		(see Notes 1 and 2)
	t1	12 (min 19 mm)	-	12 mm
	t2	16 (min 25 mm)	-	-
	W	115 mm	40 mm	140 mm
	A	-	(10 + 0,5L R)T cm2	-
			where L R ≤ 60 m
			40T cm2
			where L R ≤ 60 m
(2) Propeller boss (see Note 3 and Figure 3.3.2 Propeller boss)	tb	mm, but need not exceed
(3) Rudder posts or axles		Single screw with integral solepiece, see Figure 3.3.5 Solepiece	Single screw with bolted rudder axle, see Figure 3.3.3 Rudder axle	Twin screw, integral with hull, see Figure 3.3.4 Rudder post for twin screw craft
	n	-	6 (see Note 4)	-
	r	-	-	20 mm
	rb	-	Amm	-
	tF	-	bmm	-
	t1	-	-	12 mm
	t2	-	-	15 mm
	t3	-	-	18 mm
	w	-	-	120 mm
	zPB1, zPB2	-	mm	-
	ZT		-	-
	A	-	mm	-
			but need not exceeded
			mm
	b	-	mm or	-
			mm
			whichever is the greater
	PL1, PL2	-	As for rudder pintles	-
	bearing pressure and pintle clerance		(see Table 3.2.10 Pintle requirements)
4) Solepieces (see Notes 5,6 and 7)		With integral rudder post, see Figure 3.3.5 Solepiece	With bolted axle, see Figure 3.3.5 Solepiece	Open type (no rudder post), see Figure 3.3.5 Solepiece
(a) Cast Steel	ZT	0,50W cm3	0,95W cm3	1,00W cm3
	ZV	0,35W cm3	0,40W cm3	0,50W cm3
(b) Fabricated mild steel	ZT	0,42W cm3	0,81W cm3	0,85W cm3
Symbols
L R, T as defined in Pt 3, Ch 1, 6.2 Principal particulars a,b,c = distances, in metres, as shown in Figure 3.3.5 Solepiece
n = number of bolts in palm coupling
r b = mean distance of bolt centres from centre of palm, in mm
t b = finished thickness of boss, in mm
x = distance, in metres, from centre of rudder stock to section under consideration
A = cross-sectional area of forged steel propeller post, in cm2
A R = total rudder area, in m2
L 1 = L R, but is to be taken not less than 90 m
V = maximum service speed, in knots, with the craft in the loaded condition
W =
Z T = section modulus against transverse bending, in cm3
Z V = section modulus against vertical bending, in cm3
δb = diameter of coupling bolts, in mm
δTS = diameter of tail shaft, in mm
Note 1. Where scantlings and proportions of the propeller post differ from those shown in Item 1, the section modulus about the longitudinal axis of the proposed section normal to the post is to be equivalent to that with Rule scantlings. t 1 is to be not less than 8 (minimum of 19mm for cast steel sternframes)
Note 2. On sternframes without solepieces, the modulus of the post below the propeller boss, about the longitudinal axis may be gradually reduced to not less than 85% of that required by Note 1, subject to the same thickness limitations.
Note 3. In fabricated sternframes the connection of the propeller post to the boss is to be by full penetration welds.
Note 4. If more than six bolts are fitted, the arrangements are to provide equivalent strength.
Note 5. In fabricated solepieces, transverse webs are to be fitted spaced not more than 760 mm apart. Where the breadth of the solepiece exceeds 900 mm, a centreline vertical web is also to be fitted.
Note 6. Solepieces supporting fixed or movable nozzles will be specially considered (see Pt 3, Ch 13, 3 Fixed and steering nozzles of the Rules and Regulations for the Classification of Ships, July 2022).
Note 7. For dredging and reclamation craft in restricted service Groups G1, G2 or G3, the scantlings of an `open' type solepiece are to be such that: (a) Z T = 0,625W cm3 (b) The cross-sectional area is not less than 18 cm2. (c) The depth is not less than two-thirds of the width at any point.

3.2.2 Fabricated and cast propeller posts and rudder posts of twin screw craft are to be strengthened at intervals by webs. In way of the upper part of the sternframe arch, these webs are to line up with the floors.

3.2.3 Rudder posts and propeller posts are to be connected to floors of increased thickness. See Pt 6, Ch 3, 5 Single bottom structure and appendages and Pt 7, Ch 3, 5 Single bottom structure and appendages for steel and aluminium alloy construction respectively.

3.2.4 The requirements for sternframes of composite craft are to be in accordance with Pt 8, Ch 3, 5.9 Sternframes.

3.3.1 The requirements for the scantlings and arrangements of rudder horns are given in Pt 6, Ch 3, 5.9 Rudder horns and Pt 7, Ch 3, 5.9 Rudder horns for steel and aluminium alloy construction and Pt 8, Ch 3, 5.8 Rudder horns for composite construction respectively.

3.4.1 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 craft's internal structure.

3.4.2 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.

3.4.3 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 craft. The arms are to be strengthened at intervals by webs.

3.4.4 Fabricated supports are to be carefully designed to avoid or reduce the effect of hard spots. Continuity of the arms into the craft 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.

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

3.4.6 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.

3.5.1 The scantlings of the arms of shaft brackets, based on a breadth to thickness ratio of about five, are to be determined from Pt 3, Ch 3, 3.6 Single arm shaft brackets (`P' - brackets) 3.6.1 and Pt 3, Ch 3, 3.7 Double arm shaft brackets (`A' - brackets) 3.7.2.

3.5.2 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 craft the use of single arm brackets will be considered.

3.5.3 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.

3.5.4 Where bracket arms are 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.

3.5.5 In the case of certain high powered craft direct calculations may be required.

3.5.6 For shaft brackets having hollow section arms, the cross-sectional areas at the root and the boss should not be less than that required for a sold arm which satisfies the Rule section modulus having the proportions stated in Pt 3, Ch 3, 3.5 Shaft brackets 3.5.1. Hollow sections are to have a continuous central main piece connecting the shells at or near the location of greatest width, alternative arrangements will be specially considered.

Figure 3.3.1 Propeller posts

Figure 3.3.2 Propeller boss

Figure 3.3.3 Rudder axle

3.5.7 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 Pt 11, 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 Pt 11, 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 ; or

2. that recommended by the bearing manufacturer; or

3. as required by Pt 3, Ch 3, 3.4 Shaft bossing 3.4.2.

Figure 3.3.4 Rudder post for twin screw craft

3.5.8 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:

3.5.9 The design of the shaft brackets with regard to hydrodynamic effects causing vibrational excitations as well as disturbance of the hydrodynamic flow into the propeller and rudders is outside the scope of classification. However, it is recommended that the effects of periodic excitation caused by vortex shedding or other sources be carefully examined in order to prevent excessive structural vibration. The responsibility for such investigation rests with the designer.

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

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.

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

Figure 3.3.5 Solepiece

Figure 3.3.6 Single arm shaft bracket

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

3.7.2 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

n	=	the minimum thickness, in cm, of a hydrofoil section obtained from:
	=
a d	=	the length of the longer strut, in mm, see Figure 3.3.7 Double arm shaft bracket

d _up and f are as given in Pt 3, Ch 3, 3.6 Single arm shaft brackets (`P' - brackets) 3.6.1.

Figure 3.3.7 Double arm shaft bracket

3.8.1 The length and thickness of the shaft bracket boss are to be as required by Pt 3, Ch 3, 3.5 Shaft brackets 3.5.7 or Pt 3, Ch 3, 3.5 Shaft brackets 3.5.8 as appropriate. The scantlings of the arms will be specially considered on the basis of the Rules.

3.9.1 Fabricated supports are to be carefully designed to avoid or reduce the effect of hard spots. Continuity of the arms into the craft 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.

3.10.1 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, or reinforced shell laminate in FRP craft, are to be additionally supported by substantial floor plates or other structure.

3.10.2 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.

3.10.3 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.

3.10.4 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.

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

3.10.6 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 Pt 3, Ch 3, 3.5 Shaft brackets 3.5.7 or Pt 3, Ch 3, 3.5 Shaft brackets 3.5.8 as appropriate.

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

and not less than the shell plate thickness in way of the palm or 12 mm, whichever is greater

3.10.8 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 Pt 3, Ch 3, 3.10 Attachment of shaft brackets by bolting 3.10.7, 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.

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

3.10.10 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.

3.11.1 Proposals to connect shaft brackets to FRP hulls by bonding will be the subject of special consideration. Details of the following are to be submitted:

1. Preparation of the hull penetration and internal bonding surface.

2. Details of transverse through pinning of the shaft bracket strut.

3. Details of over bonding of strut and pin arrangement and subsequent integration of strut into primary hull structure.

3.12.1 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.

3.12.2 Alignment of bolted shaft brackets may be by means of suitable metallic shims or chocking resin of an approved type. See Pt 3, Ch 3, 3.10 Attachment of shaft brackets by bolting 3.10.2 and Pt 3, Ch 3, 3.10 Attachment of shaft brackets by bolting 3.10.3.

3.12.3 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.

3.13.1 The sterntube construction may be of aluminium alloy, steel, bronze or fibre reinforced plastic.

3.13.2 The sterntube scantlings are to be individually considered.

3.13.3 For steel and aluminium hulls, 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.

3.13.4 For FRP hulls, the bottom shell laminate, in way of the sterntube, is to be locally increased by 50 per cent by gradual tapering of the laminate. The increased thickness in way of the sterntube need not exceed the Rule keel thickness requirement.

3.13.5 For FRP sandwich hulls the shell in way of the stern tube connection is to be either:

1. Reduced from sandwich hull construction to single skin laminate by removal of the core and by combining the inner and outer skins. The single skin region is then to be additionally reinforced by a minimum of 50 per cent of the sum of the inner and outer sandwich laminate. The increased thickness in way of the sterntube need not be greater than the Rule keel thickness requirement.

2. Reduced from the sandwich hull construction to a single skin laminate by removal of the core and combining the inner and outer skins. After bonding in the sterntube to the single skin laminate the foam core and the inner skin are to be reinstated.

3. Proposals to replace the sandwich core with a core having higher core shear strength and compressive strength than that of the adjacent structure prior to bonding the tube to the inner and outer skins will be the subject of special consideration.

3.13.6 The sterntube may be connected to the shell by bonding, bolting or welding as applicable depending upon the construction material of the shell.

3.13.7 When bonding in sterntubes the bonding angle laminate weight is to be not less than the Rule minimum bottom weight. FRP tubes are to be thoroughly abraded and degreased prior to installation and laminating. Bonded in metallic tubes are to be knurled in way of the bonding material and thoroughly degreased prior to installation. During the bonding operation particular care is to be given to maintaining the sterntube alignment.

3.13.8 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.

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

3.13.10 Where sterntubes are to be installed using a resin system, of an approved type, the requirements of Pt 11, Ch 2, 4.16 Sternbushes and sterntube arrangements are to be complied with.

3.13.11 The region where the shafting enters the craft, and the bearing in way, are to be adequately supported by floors or deep webs.

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

Figure 3.3.8 Propeller clearance

3.13.13 A suitable gland arrangement is to be provided at the inboard end of sterntubes in accordance with Pt 11, Ch 2, 4.15 Intermediate bearings.

3.13.14 Where difficulty in welding or bonding of sterntube to the hull is anticipated due to small clearances or tight angles, a local cofferdam or watertight compartment of moderate volume is to be fitted enclosing the hull penetration and the sterntube. Where the sterntube penetrates the inboard boundary of the watertight compartment or cofferdam, it is to be suitably supported and sealed.

3.14.1 The requirements for solepieces are as indicated in Table 3.3.1 Sternframes.

3.15.1 Recommended minimum clearances between the propeller and the sternframe, rudder or hull are given in Table 3.3.2 Recommended 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.

Table 3.3.2 Recommended propeller hull clearances

Number of Blades	Hull clearances for single screw, in metres,				Hull clearances for twin screw
	see Figure 3.3.8 Propeller clearance				in metres, see Figure 3.3.8 Propeller clearance
	a	b	c	d	e	f
3	1,20Kδ	1,80Kδ	0,12δ	0,03δ	1,20Kδ	1,20Kδ
4	1,00Kδ	1,50Kδ	1,12δ	0,03δ	1,00Kδ	1,20Kδ
5	0,85Kδ	1,275Kδ	0,12δ	0,03δ	0,85Kδ	0,85Kδ
6	0,75Kδ	1,125Kδ	0,12δ	0,03δ	0,75Kδ	0,75Kδ
Minimum value	0,10δ	0,15δ	t R	-	3 and 4 blades	0,15δ
					0,20δ
					5 and 6 blades
					0,16δ
Symbols
L R and C b as defined in Pt 3, Ch 1, 6.1 General			t R = thickness of rudder, in metres measured at 0,7R p above the shaft centreline P s = designed power on one shaft, in kW R p = propeller radius, in metres δ = propeller diameter, in metres
Note The above recommended minimum clearances also apply to semi-spade type rudders.