Section 3 Construction and design
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Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Inland Waterways Ships, July 2022 - Part 5 Main and Auxiliary Machinery - Chapter 16 Azimuth Thrusters - Section 3 Construction and design

Section 3 Construction and design

Parent topic: Chapter 16 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 5, Ch 7 Strengthening for Navigation in Ice and Pt 3, Ch 9, 3 Strengthening for navigation in ice.

3.2 Design

3.2.1 For steerable thrusters with or without nozzle, the stresses in the nozzle stock or steering pipe are to be determined as follows:

σB =
τT =
σE =
where
σB = bending stress, in N/mm2
M B = bending moment at any section x-x, in Nmm and is to be determined as follows:
= M B = 106 T M a
where
T M = maximum thrust of the thruster unit, in kN
a = dimension from centreline propeller to plane of consideration, in metres, as shown in Figure 16.3.1 Steerable thruster
Z B = section modulus against bending, in mm3
τ T = torsional stress, in N/mm2
M T = the maximum torque at the relief valve pressure which is generally equal to the design torque as specified by the steering gear manufacturer, in Nmm
Z T = section modulus against torsion, in mm3
σ E = equivalent stress in N/mm2.

Figure 16.3.1 Steerable thruster

3.2.2  Permissible stresses:

  • Torsional stress
  • Equivalent stress
where

k o is a material factor as in Table 16.3.1 Material factor.

For all items calculated, a material factor, k o, may be used if the material has a better quality than regular carbon steel.

Table 16.3.1 Material factor

σo k o/k b/k s
For σo > 235
For σo ≤ 235
Symbols
σo = minimum yield stress in N/mm2
k o = higher steel correction factor
k b = coupling bolt material factor
k s = rudderstock or steering pipe flange material factor

Note 1. σo is to be taken not greater than 70 % of the ultimate tensile strength or 450 N/mm2, whichever is the lesser.

Note 2. For bolts, σo may be taken not greater than 70 per cent whichever is the lesser.

3.2.3 For coupling bolts, steering pipe/underwater gearbox the requirements of Pt 5, Ch 16, 3.2 Design 3.2.4 and Pt 5, Ch 16, 3.2 Design 3.2.5apply.

3.2.4  Tap bolts or bolts in clearance holes:

The minimum diameter of tap bolts or of bolts in clearance holes at the joining faces of connecting flanges, pretensioned to 70 per cent of the bolt material yield strength value, is to be not less than:

δb =
where
M T = maximum turning moment, in Nmm, see also Pt 5, Ch 16, 3.2 Design 3.2.1
n = number of bolts
σy = bolt material yield stress, in N/mm2
d c = pitch circle of the bolts, in mm.

3.2.5 Fitted bolts will be specially considered.

3.2.6 The minimum thickness, t f, in mm of the coupling flange:

t f =
where
δb = diameter of the coupling bolts, in mm

k b and k s are as defined in Table 16.3.1 Material factor.

The thickness of the coupling flange is in no case to be less than the actual diameter of the coupling bolts.

3.2.7 The fillet radius at the base of the connecting flange of the steering pipe is to be not less than 0,06 of the diameter of the steering pipe at the flange. A smaller fillet radius can be accepted based on a suitable calculation method in which the effects of stress concentration are to be taken into account.

3.2.8 The nozzle structure is to be in accordance with Pt 3, Ch 12, 3 Fixed and steering nozzles.

3.2.9 As an alternative to this Chapter, azimuth thrusters in full compliance with Pt 5, Ch 20 Azimuth Thrustersof the Rules for Ships are also acceptable for Inland Waterways Rules applications.

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

3.4.2 Where the lubricating oil for the azimuth thrusters is circulated under pressure, provision is to be made for the efficient filtration of the oil.

3.4.3 For flexible hoses, reference is made to Pt 5, Ch 10, 7 Flexible hoses.

3.5 Locking of thruster unit

3.5.1 Azimuth propulsion systems are to be capable of being locked in a fixed position, see also Pt 5, Ch 15, 1.6 Rudder, rudder stock, tiller and quadrant 1.6.3.

3.6 Prime movers

3.6.1 Engines intended for driving thrusters are to comply with the applicable requirements of Pt 5, Ch 2 Engines.

3.7 Gears

3.7.1 The gears are to be in compliance with the applicable requirements of Pt 5, Ch 3 Gearing with an application factor K A in Table 3.3.1 Values of K A and a factor of safety for contact stress and bending as intended for multiple screw applications, see Table 3.3.4 Factors of safety. As an alternative to Pt 5, Ch 3 Gearing, gear elements may be designed in accordance with ISO 6336, Parts 1, 2, 3 and 5.

3.8 Shafts

3.8.1 The diameter of screw shaft is to be not less than required by Pt 5, Ch 4, 3 Design.

3.8.2 Torsional vibration characteristics of the shaft system are to be in compliance with Pt 5, Ch 6 Shaft Vibration and Alignment.

3.8.3 Calculations of the lateral vibration characteristics of shafting systems incorporating cardan shafts are to be submitted in accordance with Pt 5, Ch 6 Shaft Vibration and Alignment.

3.9 Propellers

3.9.1 Propellers are to be in compliance with the requirements of Pt 5, Ch 5 Propellers.

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