Section 8 Towing fittings

8.1.1 This sub-Section applies to the typical Smit-Type towing brackets and closed fairleads installed on board manned or unmanned non-self-propelled ships which are part of the envisaged towing arrangement.

8.1.2 The design load for Smit-Type brackets and closed fairleads shall be no less than 1,3 times the minimum breaking strength of the provided towline or of the weakest component in the envisaged towing arrangement if that is lower, applied at all possible angles of tow.

Figure 5.8.1 Illustration of apex angle in a towing arrangement

8.1.3 When towed using a bridle arrangement, the angle at the apex of the bridle shall normally be between 45° and 60° as shown in Figure 5.8.1 Illustration of apex angle in a towing arrangement. If it exceeds 60°, then the design load for Smit-Type brackets and closed fairleads specified in Pt 4, Ch 5, 8.1 Smit-Type towing brackets and closed fairleads 8.1.2 shall be increased to account for the increased resolved load in the bridle.

8.1.4 The Smit-Type brackets and closed fairleads shall be selected from an industry standard accepted by LR based on the design load specified in Pt 4, Ch 5, 8.1 Smit-Type towing brackets and closed fairleads 8.1.2 or Pt 4, Ch 5, 8.1 Smit-Type towing brackets and closed fairleads 8.1.3 as appropriate, applied at all possible angles of tow.

8.1.5 When the Smit-Type brackets or the closed fairleads are not selected from an industry standard accepted by the society, the strength of fittings is to be adequate for the design load specified in Pt 4, Ch 5, 8.1 Smit-Type towing brackets and closed fairleads 8.1.2 or Pt 4, Ch 5, 8.1 Smit-Type towing brackets and closed fairleads 8.1.3as appropriate, applied at all possible angles of tow. Under the design load and with net scantlings, the stresses are not to exceed those given in Table 5.8.1 Permissible Stress Values. Non-linear analysis is considered acceptable subject to special consideration.

Table 5.8.1 Permissible Stress Values

	Permissible stress N/mm2
Normal stress	σ0
Shear stress
Combined stress	σ0
Symbols
σ0 = specified minimum yield stress, in N/mm2 Note Normal stress is defined as the sum of bending and axial stresses, with the corresponding shearing stress acting perpendicular to the normal stress.

8.1.6 Prototype testing will be required to confirm the strength unless the Smit-Type bracket or the closed fairlead is selected from a recognised National or International Standard. The fitting shall be tensioned to its design load as specified in Pt 4, Ch 5, 8.1 Smit-Type towing brackets and closed fairleads 8.1.2 or Pt 4, Ch 5, 8.1 Smit-Type towing brackets and closed fairleads 8.1.3 as appropriate, accounting for all possible angles of tow. After the test, the prototype needs to be examined for any failures including unacceptable deformations. The prototype which was strength-tested shall not be installed on board.

8.1.7 The weld between the fittings and the deck shall be capable of resisting the design load specified in Pt 4, Ch 5, 8.1 Smit-Type towing brackets and closed fairleads 8.1.2 or Pt 4, Ch 5, 8.1 Smit-Type towing brackets and closed fairleads 8.1.3 as appropriate.

8.1.8 The deck and the supporting structure in way of the fittings are to be suitably reinforced to resist a design load specified in Pt 4, Ch 5, 8.1 Smit-Type towing brackets and closed fairleads 8.1.2 or Pt 4, Ch 5, 8.1 Smit-Type towing brackets and closed fairleads 8.1.3 as appropriate, applied at all possible angles of tow. Stresses induced in the supporting structure, determined using the design load and with net scantlings are not to exceed the permissible values given in Table 5.8.1 Permissible Stress Values. The capability of the structure to withstand buckling is also to be assessed, and an appropriate insert plate needs to be provided where necessary.

8.1.9 An allowance for corrosion is to be added to the net thickness derived as indicated below:

• For the supporting deck structure, a corrosion addition of 2 mm is to be added to the net thickness derived.
• For fittings not selected from an accepted industry standard, 2 mm is to be added to the net thickness derived.

8.1.10 In addition to the corrosion requirements specified in Pt 4, Ch 5, 8.1 Smit-Type towing brackets and closed fairleads 8.1.9, the wear allowance for the shipboard fittings not selected from an accepted industry standard is not to be less than 1,0 mm, added to the surfaces which are intended to be in regular contact with the towline.