CHAPTER 39. ADDITIONAL REQUIREMENTS FOR TUGS

1. Objective
2. Definitions
3. Stability requirements
4. Heeling moment
5. Special requirements

In this chapter additional requirements are given for tugs.

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2. Definitions

2.1. Tug

2.1.1. For the purpose of these Rules a tug is a craft which is intended for towing other vessels, timber rafts or floating objects with a cable and which for this purpose is equipped with a hook, winch, bollard or similar device.

2.1.2. A craft, the propulsion machinery of which has a power of less than 150 kW, and which is not arranged for towing of vessels which use their own propulsion machinery or for towing of a vessel in co-operation with a tug, is not considered a tug.

3. Documentation

To verify that the requirements presented in this chapter are met, the following documentation is required:

• Stability calculations that take into account heeling moment from towing. General stability requirements are given in Chapter 4.
• Documentation of the towing cable release under load

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4. Stability

4.1. Requirements

4.1.1. To assess the stability of a tug, the righting arm curve and the heeling arm curve, calculated in accordance with 5.1 for the most unfavourable towing condition shall be plotted in the same diagram. The resulting area above the heeling arm curve between the heeling and the righting arm curves, calculated up to a heeling angle of 40 degrees shall be at least 0,01 metre-radians.

4.1.2. For tugs intended for assistance of vessels which use their own propulsion machinery or which assist vessels in co-operation with other tugs, a special approval of the stability is required.

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5. Heeling moment

5.1. Heeling moment arm

5.1.1. The curves for heeling arms shall be calculated in accordance with the following formula:

[m]

Where

C= 4 × l/Lwl but shall not be taken greater than 1,0

r= radius of the towing arc in metres. If the radius varies: the distance from the centreline of the craft to the centre of effort of the hook when the pull is acting athwarthships..

d= the mean draught of the craft in metres

h= the height of the towing point above the waterline in metres

k= nominal heeling arm in metres

Lwl= waterline length in the appropriate loading condition.

l= the horizontal distance between the towing point and the aft end of the waterline in metres.

T= the static bollard thrust in kilonewtons

D= the displacement of the craft in the appropriate loading condition in metric tons.

f= heeling angle, degrees

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6. Special requirements

6.1. Towing cable

6.1.1. The towing cable shall be capable of being released at full load.

6.1.2. The towing hook with winch system shall be dimensioned on the basis of the maximum traction force with a factor of safety of 5,0 in relation to the tensile strength of the material.

6.2.1. 5.2.1. A unobstructed view to the towing hook or winch on the afterdeck from the steering position in the wheelhouse is to be arranged..

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