Section 3 Construction and design
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Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Ships, July 2022 - Part 5 Main and Auxiliary Machinery - Chapter 20 Azimuth Thrusters - Section 3 Construction and design

Section 3 Construction and design

Parent topic: Chapter 20 Azimuth Thrusters

3.1 Materials

3.1.1 Specification for materials of gears, shafts, couplings and propeller, giving chemical composition, heat treatment and mechanical properties are to be submitted for approval.

3.1.2 Specification for materials for the stock, struts, etc. are to be submitted for approval.

3.1.3 Where an ice class notation is included in the class of a ship, additional requirements are applicable as detailed in Pt 8 Rules for Ice and Cold Operations and Pt 3, Ch 9 Special Features.

3.2 Design

3.2.1 The requirements detailed in Pt 5, Ch 1 General Requirements for the Design and Construction of Machinery, Pt 5, Ch 5 Gearing, Pt 5, Ch 6 Main Propulsion Shafting, Pt 5, Ch 7 Propellers, Pt 5, Ch 8 Shaft Vibration and Alignment, Pt 5, Ch 9 Podded Propulsion Units, Pt 5, Ch 14 Machinery Piping Systems and Pt 5, Ch 19 Steering Systems are to be complied with where applicable.

3.2.2 For steerable thrusters with a nozzle, the equivalent rudder stock diameter in way of tiller, used in Table 19.1.1 Connection of tiller to stock in Chapter 19, is to be determined as follows:

where
V = maximum service speed, in knots, which the ship is designed to maintain under thruster operation
A N = projected nozzle area, in m2, and is equal to the length of the nozzle multiplied by the mean external vertical height of the nozzle and
xP = horizontal distance from the centreline of the steering tube to the centre of pressure, in metres. The position of the centre of pressure is determined for both ahead and astern cases fromPt 3, Ch 13, 2.7 Rudder torque for rudder blades without cut-outs 2.7.1.

The corresponding maximum turning moment, M T, is to be determined as follows:

MT = turning moment for conical couplings and is to be taken as the greatest of MF, M A or MW
MF = PL xP × 106 N mm (kgf mm) in the ahead condition
MA = PL xP × 106 N mm (kgf mm) in the astern condition
MW = the torque generated by the steering gear at the maximum working pressure supplied by the manufacturer, in N mm (kgf mm). M W is not to exceed the greater of 3,0M F or 3,0M A
PL = lateral force on rudder acting at centre of pressure, as defined in Pt 3, Ch 13, 2.6 Rudder force 2.6.1 (where A R equals 2A N), in kN (tonne-f).

3.2.3 The nozzle structure is to be in accordance with Pt 3, Ch 13, 3 Fixed and steering nozzles.

3.2.4 In addition to the requirements of Table 13.3.1 Nozzle construction, in Pt 3, Ch 13 Ship Control Systems, the scantlings of the nozzle stock or steering tube are to be such that the section modulus against transverse bending at any section xx is not less than:

where
a = dimension, in metres, as shown in Figure 20.3.1 Steerable thruster
TM = maximum thrust of the thruster unit in tonnes

Figure 20.3.1 Steerable thruster

3.2.5 The scantlings of nozzle connections or struts will be specially considered. In the case of certain high powered ships, direct calculation may be required.

3.2.6 For steerable thrusters without a nozzle the scantlings in way of the tiller will be specially considered.

3.3 Steering gear elements

3.3.1 These gears are to be considered for the following conditions:

  • a design maximum dynamic duty steering torque;
  • a static duty (≤103 load cycles) steering torque, and the static duty steering torque should be not less than M T.

Values for the above should be submitted together with the plans.

3.4 Components

3.4.1 The hydraulic power operating systems for each azimuth thruster are to be provided with the following:

  1. arrangements to maintain the cleanliness of the hydraulic fluid, taking into consideration the type and design of the hydraulic system;

  2. a fixed storage tank having sufficient capacity to recharge at least one azimuth power actuating system including the reservoir. The piping from the storage tank is to be permanent and arranged in such a manner as to allow recharging from within the thruster space.

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