Escort Tugs

7/28/2019 Escort Tugs

1/10THIS DOCUMENT MAY BE FREELY USED PROVIDED THAT IT BEARS THE WORDINGCOPYRIGHTPORTREVEL.

PORT REVELSHIPHANDLING

ESCORT TUGS

NOVEMBER 2011


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Escort Tug Port Revel with Voith Schneider Propulsion:

Sogreah - Port Revel Shiphandling Escort Tugs 1


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ASD Escort Tug with Z-Peller:


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Escort tugs are meant to cope with potential emergencies. They are usually tethered

to the assisted ship for speeds up to around 10 knots.

In case of emergency, their action may be to brake the assisted ship as quickly as

possible, and/or to steer the assisted ship away from an obstacle.

Braking can be carried out in a direct mode, i.e. by pulling in line with the assisted

ship, or in an indirect mode, i.e. by using the tugs hull as a paravane to produce a

towline force.

When should they use one mode or the other ?

Crowleys Nanuq



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At slow speed in a narrow canal with a bridge, direct braking and steering will be used.



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Example of indirect braking of a 200 000 dwt tanker with a large escort tug. The

paravane effect of the tug induces large towline forces which increase with speed.



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Obviously, the steering component increases with a larger towline angle.



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In a direct steering mode, the tug is pushed back into the wake of the assisted ship by

the water flow W when the speed is more than 4-5 knots. Above this speed the tug is

unable to move sideways against the water flow W.

The steering component is thus reduced due to the small towline angle.

At higher speeds the indirect steering mode is increasingly effective .



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The steering force is obtained with the VSP going astern with the levers and applying

wheel to generate a side thrust so as to rotate the tug.

Hydrodynamics of the hull and skeg generate a significant towline force at speeds

above 4-5 knots.

Modern tug design provides high indirect pulling forces at speeds as low as

2-3 knots. Hence indirect pulling can nowadays be used over a wide range of speeds.



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Trials performed with the Port Revel manned models (see p. 5) have demonstrated

the trends as shown above:

Braking:

>> Direct mode: at speed V=0, the braking force is the Bollard Pull, set for these

trials at 60 t. The braking force increases slightly with V; this reflects the increase

of the drag of the tug pulling in line .

>> Indirect mode: the braking force increases more significantly with V as the drag

of the tug is much larger because of its moving sideways and with a small towlineangle.

Steering:

>> Direct mode: at speed V=0, the steering force is again the Bollard Pull. The

steering force reduces with V as the tug cannot stay at a large angle because of

the water flow pushing it back into the wake of the assisted ship.

>> Indirect mode: at speed V=0, the steering force is obviously nil. The steering

force increases significantly with V as the drag of the tug is much larger because of

its moving sideways and with a large towline angle.

Conclusion for steering:V < 4 knots: go for direct mode

V > 4 knots: go for indirect mode


Escort Tugs

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