ISO 13798:2020

Overview

ISO 13798:2020 - Ships and marine technology: Ship's mooring and towing fittings - Recessed bitts (steel plate type) defines requirements for steel plate type recessed bitts, the recessed mooring/towing fittings installed in a ship’s side shell where deck height makes conventional bitts impractical. The standard covers classification, nominal sizes, dimensions, materials, construction, manufacturing/inspection and marking to meet normal towing requirements.

Key topics and technical requirements

• Types & nominal sizes: Two manufacturing types - Type A (steel pipe) and Type B (steel plate). Nominal post diameters include 200, 250, 400A and 400B. Detailed dimensions and particulars are provided in the standard’s Table 1 and Figure 1.
• Materials: Type B uses weldable steel plates with yield ≥ 235 N/mm²; Type A uses weldable steel pipes with yield ≥ 215 N/mm².
• Strength assessment (Annex A): Strength is evaluated by simple beam theory. Load cases consider combined bending and shear from a towing force P at the upper position; reference rope diameters for load consideration are specified (e.g., 30, 35, 40, 60 mm depending on nominal size). Stress limits: bending ≤ 85% yield, shear ≤ 60% yield, combined ≤ 100% yield. A 2 mm wear allowance is deducted where rope rubs; corrosion margin is accounted for in stress criteria.
• Manufacturing & inspection: Surfaces (including welds) must be free of visible flaws; rope-contact surfaces must be smooth to avoid abrasion; external anti-corrosion coating is required.
• Marking & SWL: The Safe Working Load (SWL) must be noted in the shipboard towing and mooring plan (per IMO guidance) and permanently marked on the fitting (e.g., “SWL XXX t”). Actual on-board SWL must consider local reinforcement and shall not exceed the SWL in the standard.

Practical applications and users

ISO 13798:2020 is used for:

• Design and specification of recessed bitts during newbuild and retrofit work (naval architects, shipbuilders)
• Procurement and manufacturing of shipboard towing fittings (deck machinery manufacturers, fabricators)
• Survey, inspection and certification by classification societies and maritime authorities
• Ship operators and masters for safe mooring/towing planning and marking per IMO Circular MSC/Circ.1175

Practical benefits include consistent safety margins, reduced rope damage from well-designed recess boxes, and clear SWL marking to support safe towing and mooring operations.

Related standards

• IMO Circular MSC/Circ.1175 - Guidance on shipboard towing and mooring equipment
• ISO 2408 - Steel wire ropes - Requirements
• IACS UR A2 - Shipboard fittings and supporting hull structures associated with towing and mooring
• OCIMF Mooring Equipment Guidelines (MEG)

Keywords: ISO 13798:2020, recessed bitts, ship mooring, towing fittings, steel plate type, SWL, strength assessment, maritime equipment.