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Considerations regarding tug towline forces onto ship’s deck fittings

MARKINGS ON SHIP’S FITTINGS

Types of markings stamped on ship’s fittings:

SWL = The safe working load for mooring lines.

TOW = The safe working load for a tug’s towline.

ETB = Emergency Towing Bitts (usually found on tankers & LNG Carriers)

kN = Rule of thump is insert one decimal place to convert to Tonnes: 1000kN = 100 tonne

DOUBLING THE SWL TONNAGE ON SHIP’S BITTS

If the tug’s towline is placed over a set of Bitts so the eye of the towline is near the base of the ship’s Bitts then the stated SWL on the Bitts can be doubled.

  • NB: This does not apply to the ship’s other fittings: Fairleads, Roller Leads etc.

 

CONSIDER SHIP’S AGE

The construction age of a ship should to be considered when estimating the towline force that can be safely applied to or through its deck fittings.

  • For ships built prior to 2007 there is possibly no Class notations for deck fittings.
  • For ships built prior to 2012 there is possibly no Class notations for the underpinning structure the deck fittings are connected to.
  • For ships with rated Emergency Towing Bitts (ETB) the deck fittings SWL are likely more trustworthy.

 

POINTS TO CONSIDER

  • When the towline leads at a steep angle down from the ship’s Fairlead this equates to a crushing force, hence more destructive force is applied to the Bitts than the Fairlead; So focus on the Bitts SWL than the Fairleads SWL.
  • When the towline force is near horizonal to waterline there is virtually no towline angle multiple factor to be applied to the force the towline is generating.
  • If the towline leads at an obtuse (soft) angle from the Bitts through the Fairlead to the tug less force will be applied onto the ship’s Fairleads. Hence focus on Bitt’s SWL.
  • If the towline leads at an acute (sharp) angle from the Bitts through the Fairlead to the tug more force will be applied to the Fairlead. Hence focus on the Fairlead’s SWL.
  • For tugs with towline tonnage Load Cell measure-ment equipment the readout includes the factor for towline angle up to the ship and the actual bollard pull the tug is creating. But the readout does not make allowance for the fiction percentage caused by the towline leading around the tug’s towing staple or ship’s fairlead. This can be + 10% to the Load Cell readout.
  • The % Factor to apply to the tug’s Bollard Pull a is guidance for a worst case possibility for tugs without towline Load Cell measuring equipment. It takes into account estimates for snatch loadings due to sea state, steep towline angles and acute angles from the CLA to the ship’s bitts.
  • The Pilot should always inform the tug as to the SWL of both the Bitts & Fairlead the tug is to attached it’s towline to. Note: The SWL could be rated at different tonnages.

 

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