Section 2 Propulsion units

2.1.1 Propulsion unit structure scantling requirements are to comply with Pt 5 Main and Auxiliary Machinery of the Rules for Ships and the additional requirements below.

2.1.2 The propulsion unit structure and components shall be designed to withstand the loads derived from the standard load scenarios.

2.1.3 A risk assessment is required to be performed for ships operating stern first in ice equipped with single propulsion units to identify components where a single failure could cause loss of all propulsion and/or steering capability and the proposed arrangements for preventing and mitigating the effects of such a failure.

2.2.1 The local scantlings of the propulsion unit body and strut are to be derived using the PC Rules using the appropriate area factors in Table 3.2.1 PC Rule Area Factors for bow and bow intermediate regions applied to the stern of SFIC ships in Chapter 3 for the propulsion unit location. For FS Rule application, the forward region structural requirements are to be applied using the factors in Table 3.2.2 FSICR design height for bow region applied to the stern of SFIC ships in Chapter 3.

2.2.2 Global loads, as required in Pt 5 Main and Auxiliary Machinery of the Rules for Ships, are to include longitudinal and transverse forces from ice interaction, in accordance with the standard load scenarios. See Figure 2.1.1 Propulsion unit standard load scenario in Chapter 2.

2.2.3 Global loads associated with the standard scenarios are based on the contact area with the keel of an ice ridge that is dependent on the propulsion unit orientation.

2.2.4 The load from the ice ridge is to be considered as two components the force on the propulsion unit strut, F _C, and the force collected by the propeller or propulsion unit body, F _K. The total moment, M _TOTAL, is defined as a combination of F _C and F _K. See Table 4.2.1 Global ice loads for propulsion unit.

2.2.5 Global loads are to be applied to the propulsion unit and strut at the point of assumed action for the dimensioning of primary members.

2.2.6 Determination of the load area A _C and A _K is to be based upon the standard load scenario for the propulsion unit. In general, the height of the load area (contact height) for the strut is recommended to be taken at 0,25 of the design ice thickness h, associated with the ice class. See Table 4.2.2 Nominal ice thicknesses. For the application of FS Rules to the above requirements, Ice Class 1A FS and 1AS FS are to be considered as Ice Class PC7 and PC6 respectively. The width of the load area is recommended to follow Figure 4.2.1 Load areas for propulsion unit global force derivation.

Figure 4.2.1 Load areas for propulsion unit global force derivation

2.2.7 Coefficients k ₁ and k ₂ are included to acknowledge the varying methods that can be utilised to dimension the global ice loads for the propulsion unit. For the determination of F _C, the pressure-area relationship is also expected to include the load aspect ratio considering the propulsion unit strut geometry. For the determination of F _K, the properties of the ice ridge are to be consistent with expected areas of operation for the ice class notation assigned.

2.2.8 The determination of F _C and F _K will be specially considered for the assignment of SFIC to ships with ice classes PC1, PC2 and PC3.

2.2.9 A pyramid of strength approach is to be adopted for the propulsion unit. The propeller is to be designed to fail before the propeller shafting.

2.2.10 Propulsion unit appendages, such as a pod fin, are to be designed to fail before the propulsion unit support structure.

2.3.1 A Finite Element analysis of the propulsion unit structural integration is to be submitted. See Ch 6, 1 Stern area structural analysis.

2.4.1 The requirements of this Section are to be applied to the propulsion unit mounting block, if applicable.

2.4.2 The stresses in the propulsion unit mounting block are to be verified by direct calculation. See Ch 6, 1 Stern area structural analysis.

2.4.3 The loads on the mounting block are to be derived from the maximum bearing reactions from the ice loads applied.

2.5.1 Calculations employed for the verification of the propulsion unit and support structure are to be in accordance with Ch 4, 2.2 Propulsion unit structure, Pt 5 Main and Auxiliary Machinery of the Rules for Ships and the requirements of this Section.

2.5.2 The derivation of the global loads F _x, F _y, and M _x in Pt 5 Main and Auxiliary Machinery of the Rules for Ships are to include ice loads based on the propulsion unit operating scenarios. In general, consideration should be given to the following:

1. Bollard pull;

2. Longitudinal ice load, FI _L;

3. Transverse ice load, FI _T;

4. Breaking force on one blade FI _P;

5. Propeller open water thrust;

6. Self weight.

2.5.3 For the propulsion unit structural integration and support structure, the longitudinal ice load (FI _L) and transverse ice load (FI _T) are to be applied at their resultant point on the propulsion unit strut. These may be determined by the summation of ice ridge forces F _C and F _K in Table 4.2.1 Global ice loads for propulsion unit or from the blade break force FI _p, whichever is the greater.

2.5.4 For initial dimensioning purposes, the breaking force on one blade FI _p, at a radius of 0,8R, is to be taken as F _T or F _A, whichever is the greater, may be considered as an alternative to the relevant blade break load cases of the PC Rules and FS Rules. See Table 4.2.3 Bearing axial and tangential ice loads using simplified blade break force for initial dimensioning.

Table 4.2.3 Bearing axial and tangential ice loads using simplified blade break force for initial dimensioning

Tangential ice load	FT = F2 sin αR2 kN
Axial ice load	FA = F2 cos αR2 kN
Symbols
where R1 = radius at root section, in mm R2 = mm R = propeller radius, in mm cr tr2 = actual value of blade section (chord length and thickness) at R1 cm3 σM = minimum specified tensile strength of propeller material, in N/mm2 αR1, αR2 = see Figure below
where Section X-X is the propeller section at R1 Section Y-Y is the propeller section at 0,8R

2.5.5 Propulsion unit structure global ice loads are to be used for the dimensioning of primary members of the propulsion unit and strut.

2.6.1 The bearing reactions on the propulsion shaft are to be based on the following load cases:

1. Bollard pull;

2. Breaking force on one blade (axial and tangential components);

3. Propeller load components for open water operation;

4. Propulsion unit self weight.

2.6.2 The bearing life calculations are to include an assumed proportion of time in ice, according to the ship’s intended operational profile.

2.6.3 The axial (F _A) and tangential (F _T) ice loads associated with blade break load, used for calculating bearing reaction forces on the propulsion shaft for initial dimensioning purposes, may be in accordance with Table 4.2.3 Bearing axial and tangential ice loads using simplified blade break force for initial dimensioning. Dynamic analysis utilising the relevant load cases of the PC and FS Rules is required for deriving the final design reaction forces.

2.6.4 Reactions for the radial bearings supporting the propeller shaft line are to be calculated using the following forces:

1. Propeller open water thrust and ice impacts (dynamic case).

2. Ice loads F _A and F _T from Ch 4, 2.6 Propulsion shafting 2.6.3 (static case).

2.6.5 The bearing dimensions are to be determined for the following load cases:

1. Dynamic (for lifetime loads).

2. Static.

2.6.6 The dimensioning for cases Ch 4, 2.6 Propulsion shafting 2.6.5 and Ch 4, 2.6 Propulsion shafting 2.6.5.(b) in 2.6.5 should be in accordance with the bearing manufacturer’s or other recognised method.

2.6.7 Axial loads at the thrust bearing are to be calculated from the following:

1. Bollard pull and ice torque contribution from rotating propellers (dynamic), F _D

   

2. Axial ice load (static), if required for initial dimensioning purposes F _A

3. Axial ice load derived from the PC Rules or FS Rules load case, whichever is applicable

   where

   T B	=	bollard pull, in kN
   N	=	number of blades
   Q I	=	maximum ice torque
   	=	m(2R)2
   R	=	propeller radius
   m	=	as defined in Table 4.2.4 Determination of factor, in metres
   F A	=	as defined in Ch 4, 2.6 Propulsion shafting 2.6.3.

2.6.8 Requirements for the dimensioning of propulsion shafting are to be in accordance with the pyramid of strength philosophy applicable to both FS Rules and PC Rules application.

2.6.9 Shaft coupling bolts are to be dimensioned to account for the following in combination:

1. Pre-stress due to tightening torque.

2. External loads due to bolt array bending moment and blade break ice load, per bolt. Coupling bolts are to be dimensioned so that the maximum stress, σ _B derived from F _B does not exceed the bolt yield strength σ _YB by a safety factor of 1.2:

   

   where

   σ B	=	F B/A a
   F B	=
   A a	=	cross sectional area of bolt in mm2
   F PRE	=	pre-stress due to tightening torque
   K 1	=	bolt elastic constant
   K 2	=	foundation elastic constant
   n	=	number of bolts in flange
   F E	=	maximum load due to bending moment, in kN
   F A	=	as defined in Ch 4, 2.6 Propulsion shafting 2.6.3.

2.7.1 The strength of propellers are to be verified using the PC Rules or FS Rules, as applicable.

2.7.2 Alternative methods for assessing the propeller strength to those prescribed by ice class rules will be considered. The formulations for pushing type propellers in the PC Rules may be used as guidance in determining the strength requirements for pulling propellers of Stern First Ice Class ships.

2.7.3 The propeller loading in Ch 4, 2.7 Propeller 2.7.1 should take into account the propulsion unit load scenarios derived in Ch 2, 1 Standard load scenarios, where applicable.

2.7.4 In general, the following cases are to be considered when determining loads in Ch 4, 2.7 Propeller 2.7.2:

1. Propeller blade contact and non-contact loads during ice milling;

2. Propeller stopped in ice condition.

2.8.1 Propeller fixing bolts are to be designed with a factor of safety of 1,5.

2.8.2 The loading for propeller bolts should consider the following:

1. Pre-stretch loads;

2. Centrifugal loads;

3. Ice loads.

2.8.3 The ice loads in Ch 4, 2.8 Propeller bolts 2.8.2.(c) are to be derived from the torque to break the blade root due to an ice strike.

2.9.1 The slewing ring bearing reactions are to be calculated from the load cases in Ch 4, 2.5 Load calculations 2.5.2. The factor of safety is to be taken from Pt 5, Ch 9 Podded Propulsion Units of the Rules for Ships.

2.10.1 For Stern First Ice Class Ships, ice model tests are to be used to verify ice performance with propulsion unit power if minimum power is a requirement of the assigned ice class.