Section 4 Design and performance

4.1 General

4.1.1 Power-operated steering units are to be provided with positive arrangements, such as limit switches, for stopping the unit before the mechanical stops are reached. These arrangements are to be synchronized with the unit itself and not with the steering unit control mechanism.

4.1.2 The steering unit is to be secured to the seating by fitted bolts, and suitable chocking arrangements are to be provided. The seating is to be of substantial construction.

4.1.3 All welded joints within the pressure boundary of an actuator or connecting parts transmitting mechanical loads are to be of full penetration type or of equivalent strength.

4.1.4 Steering devices involving variable geometry of the craft or its lift system components are to be so constructed that any failure of the drive linkage or actuating system will not significantly hazard the craft.

4.2 Actuating systems

4.2.1 Actuators are to be designed in accordance with the relevant requirements of Pt 15 Piping Systems and Pressure Plant for Class I pressure vessels (notwithstanding any exemptions for hydraulic cylinders).

4.2.2 Accumulators, if fitted, are to comply with the relevant requirements of Pt 15 Piping Systems and Pressure Plant.

4.2.3 The design pressure for calculations to determine the scantlings of piping and other steering components subjected to internal hydraulic pressure shall be at least 1,25 times the maximum working pressure to be expected under the operational conditions specified taking into account any pressure which may exist in the low pressure side of the system. Fatigue criteria may be applied for the design of piping and components, taking into account pulsating pressures due to dynamic loads.

4.2.4 The permissible primary general membrane stress is not to exceed the lower of the following values:

where

σ_B	=	specified minimum tensile strength of material at ambient temperature
σ_y	=	specified minimum yield stress or 0,2 per cent proof stress of the material, at ambient temperature

A and B are given by the following Table:

	Wrought steel	Cast steel	Nodular cast iron
A	3,5	4	5
B	1,7	2	3

4.2.5 Oil seals between non-moving parts, forming part of the external pressure boundary, should be of the metal upon metal type or of an equivalent type.

4.2.6 Hydraulic power operated steering units are to be provided with the following :

Arrangements to maintain the cleanliness of the hydraulic fluid taking into consideration the type and design of the hydraulic system;
A fixed storage tank having sufficient capacity to recharge at least one power actuating system including the reservoir. The storage tank is to be provided with a contents gauge and be permanently connected by piping in such a manner that the hydraulic systems can be readily recharged from a position within the steering unit compartment, if applicable.

4.3 Rudder systems

4.3.1 For the requirements of rudder and rudder stock, see Pt 3, Ch 3 Control Systems.

4.3.2 Tillers and quadrants are to comply with the requirements of Table 1.4.1 Connection of tiller to stock.

4.3.3 On double rudder installations, where the two tillers are connected by mechanical means (tie-bar), the strength and stability of the tie-bar is to be assessed using the maximum steering torque applied to the stock.

4.3.4 Where higher tensile steel bolts are used on bolted tillers and quadrants, the yield and ultimate tensile stresses of the bolt material are to be stated on plans submitted for approval, together with full details of the methods to be adopted to obtain the required setting-up stress. Where proprietary nuts or systems are used, the manufacturer's instructions for assembly are to be adhered to.

4.3.5 All steering components transmitting mechanical forces to the rudder stock, which are not protected against overload by structural rudder stops or mechanical buffers, are to have a strength at least equivalent to that of the rudder stock in way of the tiller.

Table 1.4.1 Connection of tiller to stock

(1) Dry fit - tiller to stock for M _T (see Notes)	(a) For keyed connection, factor of safety against slippage = 1,1 (b) For keyless connection, factor of safety against slippage = 2,2 (c) Coefficient of friction = 0,17 (d) Grip stress not to be less than 20 N/mm²
(2) Hydraulic fit - tiller to stock for M _T (see Notes)	(a) For keyed connection, factor of safety against slippage = 1,1 (b) For keyless connection, factor of safety against slippage = 2,2 (c) Coefficient of friction = 0,12 (d) Grip stress not to be less than 20 N/mm²
(3) Bolted tiller and quadrant (see Symbols and Notes)	Shim to be fitted between two halves before machining to take rudder stock, then removed prior to fitting Minimum thickness of shim: For 4 connecting bolts: t _s = 0,0014d _SU mm For 6 connecting bolts: t _s = 0,0012d _SU mm Key to be fitted Diameter of bolts, Distance from centre of stock to centre of bolts should generally be equal to Thickness of flange on each half of the bolted tiller
(4) Key (see Symbols and Notes)	Effective sectional area in shear ≥ 0,25d _SU ² mm² Key thickness ≥ 0,17d _SU mm Keyway is to extend over full depth of tiller and is to have a rounded end. Corners are to be provided with suitable radii to avoid high stress at the keyway root.
(5) Section modulus - tiller arm (at any point within its length about vertical axis) (see Symbols and Notes)	To be not less than the greater of: (a) (b) If more than one arm is fitted, combined modulus is not to be less than the greater of (a) or (b). For solid tillers, the breadth to depth ratio is not to exceed 2.
(6) Boss (see Symbols and Notes)	Depth of boss ≥ d _SU Thickness of boss in way of tiller ≥ 0,4d _SU
Symbols
b _s = distance between the section of the tiller arm under consideration and the centre of the rudder stock, in mm	Z _TA = section modulus of tiller arm, in cm³
NOTE: b _T and b _s are to be measured with zero rudder angle	d _SU = see Pt 3, Ch 3 Control Systems
b _T = distance from the point of application of the load on the tiller to the rudder stock, in mm	δ_T = diameter of bolts securing bolted tillers and quadrants, in mm
n _T = number of bolts in coupling, but generally not to be taken greater than six	σ_o = minimum yield stress or 0,5 per cent proof stress of the tiller bolt material, in N/mm²
t _s = thickness of shim for machining bolted tillers and quadrants, in mm
Note 1. If d _SU > 400 mm, higher tensile steel bolts are to be used for bolted tillers. A predetermined setting-up load equivalent to a stress of approximately 0,7 σ_o should be applied to each bolt assembly. A lower stress may be accepted provided that two keys, complying with item (4) are fitted. Note 2. Where M _T, the maximum turning moment applied to the stock, is to be taken as the greater of the following: (a) 11,1d _SU ³ Nmm, where d _SU is to be determined from Table 3.2.6 Rudder stock diameter in Pt 3, Ch 3 with σ_o taken as 235 N/mm² and N= 0. (b) The torque generated by the steering gear at the maximum working pressure, see Pt 14, Ch 1, 1.3 Definitions 1.3.5.

4.3.6 In bow rudders having a vertical locking pin operated from the deck above, positive means are to be provided to ensure that the pin can be lowered only when the rudder is exactly central. In addition, an indicator is to be fitted at the deck to show when the rudder is exactly central.

4.4 Performance

4.4.1 The main steering system is to be:

Of adequate strength and capable of steering the craft at all speeds and conditions for which the craft is designed and this shall be demonstrated during trials,
operated by power where necessary to meet the requirements of (a) and in any case when the Rules require a rudder stock over 120 mm diameter in way of the tiller; and
so designed that it will not be damaged at maximum astern speed.

4.4.2 The auxiliary steering system is to be:

Of adequate strength and capable of steering the craft at navigable speed and of being brought speedily into action in an emergency;
Operated by power where necessary to meet the requirements of (a) and in any case when the Rules, require a rudder stock over 230mm diameter in way of the tiller.
Where manual operated steering units are proposed, these are acceptable when the operation does not require an effort exceeding 160N under normal conditions.

4.4.3 Main and auxiliary steering power units are to be:

Arranged to re-start automatically when power is restored after power failure;
Capable of being brought into operation from a position at the control station. In the event of a power failure to any one of the steering power units, an audible and visual alarm is to be given on the control station;
Arranged so that transfer between units can be readily effected.

4.4.4 For high speed craft, in the event of total power failure, either:

emergency power for steering systems/drives is to be restored automatically within five seconds. To achieve this an interim fast acting system may be required to come into operation until such time as auxiliary/emergency power source comes on line. (Note: starting arrangements are to comply with the requirements relating to starting arrangements of emergency generators); or
means are to be provided to bring the craft to a safe condition.

4.4.5 Where the steering unit is so interconnected that more than one power system, or control system, can be simultaneously operated, the design is to be such that hydraulic locking caused by a single failure cannot occur.

4.4.6 Steering systems, other than of the hydraulic type, will be accepted provided the standards are considered equivalent to the requirements of this Section.