Section 7 Towing arrangements

7.1 Towing equipment

7.1.1 For tugs which normally tow over the stern with the main towline connection to the hull ahead of the propellers, the position of towline connection is normally to be five to 10 per cent of the ship's length abaft amidships, but in no circumstances is it to be sited forward of a position, five per cent of the ship's Rule length abaft the longitudinal centre of gravity of the tug in any anticipated condition of loading.

7.1.2 The attachment of the towline to the tug is to be located as low as practicable in order to minimise heeling moments arising from working conditions. Reliable slip arrangements which facilitate towline release regardless of the angle of the towline are to be provided.

7.1.3 It is recommended that the slip arrangements should also be operable from the bridge. The arrangements should be tested to the Surveyor's satisfaction. The breaking strength of the hook, or its equivalent, should generally be 50 per cent in excess of that of the towline, see Pt 3, Ch 13, 7 Equipment.

7.1.4 The emergency release systems provided on towing winches that are used for towing ships within close quarters, ports or terminals, are also to comply with the requirements specified in Pt 7, Ch 16 Towing Winch Emergency Release Systems.

7.2 Towing equipment foundations

7.2.1 Direct support for towing equipment by means of pillars and/or pillar bulkheads is to be arranged as far as this is practicable.

7.2.2 The design load to be considered for the strength assessment of the towing equipment and the associated supporting structures is to be as given in Table 3.7.1 Tug towing equipment design loads, unless a higher design load is specified by the designer/owner.

Table 3.7.1 Tug towing equipment design loads

Class Notation	Design Force or Bollard Pull as appropriate (kN)	Design Load (kN)
tug protected waters service	T_BP ≤ 200
	200 < T_BP < 800
	T_BP ≥ 800
tug	T_BP ≤ 400
	400 < T_BP < 1000
	T_BP ≥ 1000
escort tug protected waters service	T_DF ≤ 500
	500 < TDF < 1000
	T_DF ≥ 1000
escort tug	T_DF ≤ 500
	500 < T_DF < 1000
	T_DF ≥ 1000
Where T_BP = Bollard pull of the tug as submitted by the designer and represents the towline maximum continuous force (also called quasi-static towline force). T_DF = Design force as submitted by the designer and represents the towline maximum continuous force for escort operations (also called quasi-static towline force).
Note 1 The Design loads shown in this table include dynamic effects.

7.2.3 Scantlings of pillars and pillar bulkheads are to be in accordance with Pt 4, Ch 1, 4.4 Deck supporting structure.

7.2.4 The scantling requirements for towing arrangements are detailed in the following subsections:

Towing winches Pt 4, Ch 3, 7.3 Towing winches
Towline guiding fittings Pt 4, Ch 3, 7.4 Towline guiding fittings
Towing hooks: Pt 4, Ch 3, 7.5 Towing hooks
Towing equipment supporting structure: Pt 4, Ch 3, 7.6 Supporting Structure

7.2.5 Generally, the foundations of towing fairleads are to be carried through the deck and integrated into suitable underdeck structure.

7.2.6 On tugs which utilise an indirect method of towing, attention is drawn to the increased out-of-plane forces that occur in towing fairleads.

7.3 Towing winches

7.3.1 The scantlings of towing winches (including winch drums, drum shafts, brakes, support frames and connections to the hull strucuture) are to be determined by direct calculations using the Design Loads given in Pt 4, Ch 3, 7.2 Towing equipment foundations 7.2.2 and are to be able to sustain the following:

The Design Loads (in the most unfavourable anticipated position of the towline) without permanent deformation i.e. σ_e ≤ 1,σ_y.
The winch BHL (in the most unfavourable anticipated position of the towline), without exceeding σ_e ≤ 0,80σ_y.
The anticipated maximum RP (in the most unfavourable anticipated position of the towline), without exceeding σ_e ≤ 0,40σ_y.

Where

BHL is the brake holding load, i.e. the maximum towline force the towing winch can withstand without slipping of the (activated) brake, considering the towline at the first inner layer.

RP is the rated pull, i.e. the winch maximum hauling-in load considering the towline at the first inner layer.

σ_e	=	equivalent stress, in N/mm²
σ_y	=	specified minimum tensile yield stress of the material, in N/mm²

7.4 Towline guiding fittings

7.4.1 Towline guiding fittings, such as fairleads, staples, towing pins, stern rollers and equivalent components which guide the towline, shall be able to sustain the force exerted by the towline loaded under a tension equal to the design load as specified in Table 3.7.1 Tug towing equipment design loads, in the most unfavourable anticipated position of the towline. The fittings shall not exceed the following permissible stress levels:

σ ≤ 0,75_ref;
τ ≤ 0,47_ref;
σ_e ≤ 0,85_ref

Where

σ	=	normal stress, in N/mm²
τ	=	shear stress, in N/mm²
σ_e	=	equivalent stress, in N/mm²
σ_ref	=	== Reference stress of the material in N/mm², but may be taken as σ_y for fittings not made of welded construction
k_L	=	as defined in Pt 3 Ship Structures (General), Table 2.1.1 Values of k L

7.4.2 Towline guiding fittings used for guiding the towline when towing on a towing winch shall be able to sustain the force exerted by the towline loaded under a tension equal to the winch BHL (with the most unfavourable anticipated position of the towline) without exceeding the specified permissible stress criteria.

7.4.3 Where a towline guiding fitting has been designed for a specific Safe Working Load (SWL), defined as the maximum static working load, the fitting shall be able to sustain a force equal to 2 times the SWL without exceeding the above-specified permissible stress criteria.

7.5 Towing hooks

7.5.1 The scantlings of towing hooks, their load carrying attachments (transferring load from the hook to the hull structure) are to be determined by direct calculations using the Design Loads given in Pt 4, Ch 3, 7.2 Towing equipment foundations 7.2.2 and are to be able to sustain the Design Load (in the most unfavourable anticipated position), without exceeding σ_e ≤ 0,80σ_y.

7.6 Supporting Structure

7.6.1 The scantlings of the supporting structures of towing equipment shall be such that they able to sustain the force exerted upon them with the towing equipment under the action of the towline (with the most unfavourable anticipated position) loaded under a tension equal to the DL as specified in Table 3.7.1 Tug towing equipment design loads, without exceeding the following permissible stress levels:

σ ≤ 0,75_ref;
τ ≤ 0,47_ref;
σ_e ≤ 0,85_ref

Where

σ	=	normal stress, in N/mm²
τ	=	shear stress, in N/mm²
σ_e	=	equivalent stress, in N/mm²
σ_ref	=	== Reference stress of the material in N/mm²,
k_L	=	as defined in Pt 3 Ship Structures (General), Table 2.1.1 Values of k L

7.6.2 In addition, supporting structures of towing equipment used for escort or towing on a winch services shall be such that they able to sustain the force exerted upon them with the towing equipment under the action of the towline (with the most unfavourable anticipated position) loaded under a tension equal to the BHL of the associated winch without exceeding the permissible stress criteria in Pt 4, Ch 3, 7.6 Supporting Structure 7.6.1.

7.6.3 Where a towline guiding fitting has been designed for a specific Safe Working Load (SWL), defined as the maximum static working load, the supporting structure shall be able to sustain a force equal to 2 times the SWL without exceeding the permissible stress criteria in Pt 4, Ch 3, 7.6 Supporting Structure 7.6.1.