House D.J. Ship Handling Theory and Practice
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Bollard pull
Charter rates for tugs are based on the ‘Bollard pull’ that the vessel can exert. This is a
measure that is determined from trials when the vessel and equipment are new. The
towline being secured to a land based anchor point and a load cell measures the strain
exerted by the tug when pulling against the fixed point – the higher the bollard pull,
the greater the charter rate. Generally speaking, the more powerful the tug, the more
the hirer will have to pay. Tugs tend to be hired out for a minimum time period, e.g.
3 hours. The bollard pull is defined by the amount of force expressed in tonnes that a
tug could exert under given conditions. A static bollard pull test would be affected by
the depth of water, the tugs propulsion rating and the type of propellers fitted.
Safety precautions – handling towlines
Handling and securing of towlines is always a hazardous task and especially so
when young or inexperienced seafarers are involved. Everyone concerned on the
mooring deck, when tugs are engaged, should be fully aware of the operation and
the various stages that the assisting tug is at. Bridge Officers should be aware that
from their remote position, they will be relying on two-way communications between
mooring stations, and sudden engine movements without adequate warning and
timings to deck personnel could have serious consequences.
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Harbour Tractor Tugs. An example of a large harbour tug, laying starboard side to the berth in
Lisbon, Portugal. Tugs tend to be well fendered, all round, with a bow pudding fender arrange-
ment which permits pushing as well as pulling. A towing hook arrangement is situated just aft
of the amidships position, aft of the accommodation block.
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The following list provides some guidance for deck personnel while Bridge
Officers should be thoroughly familiar with the nature of activities when engaging
tug assistance.
1. Ship to tug communications should be established well in advance of the desig-
nated rendezvous.
2. The designated command authority should be established and each party should
be familiar with appropriate manoeuvring signals.
3. Towlines (tug or ship’s lines) should be connected by use of good quality heaving
lines and/or messengers. The towlines themselves should be of the highest quality.
4. The eyes of towlines should not be placed directly onto ‘Bitts’, but figure ‘eighted’
to leave the eye on top. Once secured, the figure ‘8’ wire turns should be lashed to
prevent turns springing loose.
5. Personnel should be advised to stand well back from secured towlines. Particularly
important once the signal of all fast is made to the tug master. It is normal proce-
dure for a test weight to be taken on the towline, once the line is on the bitts.
6. Personnel should avoid standing in bights or near sharp leads of towlines when
the tug is actively engaged in pulling.
7. When letting go tugs, it is normal practice for the tug to manoeuvre to ease the
weight on the towline, prior to release of the line. This should be carefully let go,
under full control and not just discarded, which could cause injury to persons
below on the deck of the tug.
8. Where the ship’s towline is used aft and has to be released, an ahead engine
movement can usually be beneficial. The wake from the screw race would stream
the towline right aft, following release from the tug. This action would expect to
provide ample time for the officer on station to land the towline aboard without
running the risk of the rope fouling the propellers.
NB. Towlines carry substantial weight and all personnel, especially young seafarers, should
gain experience in their safe handling without being foolhardy. Mooring Deck Officers
should actively carry out onboard training in this subject to ensure future safe operations for
their personnel.
Tug towlines
When tugs are engaged, unless designated to push, they will secure with either their
own towing spring or a ship’s line (usually the best line the vessel has on station).
Obvious hazards exist when making lines fast, as in the fact that lines may enter the
water in close proximity to turning propellers. To this end, mooring officers are
expected to keep the bridge fully informed when running lines overside, especially
if and when the ship’s engines are still turning propellers. The problem is not as
great forward as clearly as it is aft, in a position of the main propulsion. However,
bow thrust operation at the wrong moment has also been known to foul ropes.
OPERATIONS WITH TUGS 117
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118 SHIP HANDLING
A towline is passed down from the aft mooring deck to the stern of the tug. The dangers from
interaction and the proximity of the two sterns is clearly seen. The manhandling of the tow
line is also heavy work and personnel are expected to take all reasonable precautions when
securing towlines, prior to a towing operation.
The Panamanian vessel ‘Etilico’ seen navigating stern first with the tug ‘Montsacopa’ providing
the motive power to a berth in Barcelona, Spain.
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Tug operations about the pivot point
OPERATIONS WITH TUGS 119
X
Y
Pivot point
Following a turn off the berth the ‘Etilico’ is seen berthing Port side to the berth in the
Spanish Port. The tug’s line from the aft position is eased to permit the forward moorings of
the ship to be landed. Once the ship is secured alongside, the tug would be dismissed.
When the vessel is stopped in the water, the pivot point is in approximately the mid-
ship’s position near to the ship’s Centre of Gravity. If tugs are secured at the respec-
tive ends fore and aft to turn the vessel, the forces acting on the parent vessel are
equivalent to: That force exerted by the aft tug x distance X, while the force exerted
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by the forward tug x distance Y. These two forces acting about the pivot point gener-
ate a couple and cause the vessel to turn about the amidships position; the gener-
ated forces being greater than water resistance being experienced by the hull.
NB. The diagram shows tugs pulling from their respective positions. It should be realized
that if the tugs were pushing with equivalent forces the movement would be the same.
Where a single tug is engaged – say forward – and the vessel is moving ahead by
engines, the pivot point will move to a forward position. The force then exerted to
turn the vessel would then equate to that force exerted by the tug x distance Z.
120 SHIP HANDLING
W
Z
If a tug is engaged aft an opposite force would be exerted equating to: That forces
exerted by the aft tug x distance W.
The greater lever W is seen to be far greater than the lever Z. This effectively means
that the forward tug would need to exert greater power than the aft tug, in order to
achieve the same turning force as the aft tug (if required), at the forward end.
Clearly, the turning capability of an engaged tug on the parent vessel will be
directly related to the position of the ship’s pivot point and the length of the turning
lever (force of the couple) generated.
Multiple tug use
Large vessels like the VLCC and ULCC tankers find it essential to engage several
tugs to exercise control of the vessel when in restricted waters as with berthing and
unberthing. Tug use is often a combination of pulling and pushing units to assist in
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turning, steerage and checking adverse external elements like eddies, currents,
wind and inactive forces effecting the movement of such massive hulls.
OPERATIONS WITH TUGS 121
The BP Tanker British Reliance assisted by four tugs. Two bow tugs are engaged in pulling,
while two other stern tugs are just visible operating off the port and starboard quarters.
Tugs engaged in pushing
The majority of tugs have the dual capability to engage in pushing as well as in tow-
ing a vessel. Pushing has its own merits and is often used to assist large vessels to
manoeuvre through dock entrances and narrow access positions. The tug is employed
to push the parent vessel off concrete knuckles at the corners of dock entrances.
The tugs which are so engaged are well fendered around the stem and the bow
regions to present a soft cushioning pad against the ship’s side shell plate. The push-
ing action is particularly useful when a river tidal stream is causing the parent vessel
to set down onto an obstruction.
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122 SHIP HANDLING
The ‘Jahre Viking’ the largest manmade transport in the world (seen prior to being converted
into a FSOP) employing a tug off the port bow at the entrance to the Dubia Dry Dock
complex.
Girting the tug
Tug Operations – securing the towline. The aft deck of an operational tug engaged in stretching
a towline. The line is passed over the towing rails and secured by the ‘soft eye’ onto the amid-
ships towing hook. A gob wire/rope arrangement is also set up in the slack condition, being led
directly from the centre line winch through an aft lead, to a heavy duty shackle over the towline.
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Any towing operation is coupled with inherent hazards and therefore carries an ele-
ment of risk. One of the main concerns by the tug is where the angle of the towline
leads towards the beam away from the natural position over the stern. Should
the lead of the towline traverse towards the tug’s beam it could generate a capsize
motion instead of the desired pulling action, the movement is known as ‘girting’
the tug.
OPERATIONS WITH TUGS 123
2
1 Pulling
motion
Parent vessel turns to
starboard and the towline
angles to the beam of the tug
causing girting
Capsize
motion
In order to avoid girting the tug, it is normal practice to employ a ‘gob rope’
(sometimes referred to as a gog rope). It is generally used at the aft end of the tug’s
deck to change the towing position (fulcrum point) from the amidships position to a
position right aft. This effectively generates a turning motion as opposed to a cap-
size motion, causing a slewing action on the tug. Such a change in motion eliminates
the immediate danger of capsize.
The gob rope is rigged in conjunction with a pipe lead or stag horn bitts for the
anchored end, with the bight of the rope being led to a winch or capstan. Alternative
rigging may employ a wire with a heavy duty shackle about the towline. The wire is
led to a winch or capstan at the after end of the tug to draw the towline down to a
position away from the amidships position.
Use of the gob rope
Where wire tow ropes are employed and rigged for use in conjunction with a ‘Gob
Rope’ a heavy duty haul wire with a bow shackle is an alternative rig. It is usually
secured from a winch drum with the towing wire passing through the shackle at the
other end.
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124 SHIP HANDLING
Control of wire towing line by means of gob wire and heavy duty shackle.
The gob rope is seen anchored to the ‘bitts’ and the bight is passed over the towline and led
back through the pipe lead onto a capstan. The gob rope can be payed out and slackened off
or heaved in close to draw the towline to the aft lead by the capstan operator.
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Tugs towing fitments
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Tugs towing fitments. The aft towing deck area of the tug ‘Montado’ of Lisbon, Portugal. The
operational area of the tug shows clearly the towing rail across the breadth of the vessel. This
is somewhat of a misnomer as the purpose of the rail is not meant to take the weight of the
towline, but to permit the towline to ride over it, so protecting the heads of the deck crew sit-
uated lower than the height of the tow rail. The eye of the combined rope/wire towing spring
is over the amidships towing hook, while the eye and bight are coiled to the starboard side of
the centre line capstan.
Sister tugs lie moored alongside each other. The aft decks show centre line ‘stag horn’ leads for
passing towlines onto the towing hooks set aft of the accommodation block. The deck also
shows a mooring capstan on the starboard quarter and an electric centre line winch positioned
forward of the stag horn.
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