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Figure 2 , Wave system around a ship top view
These waves are the result of high/low pressure field generated
by the ship moving through a body ofwater and its surface boundary.
In either case this pressure will pull a tugboat into a vessel, or
push itaway. Neither case is favorable when maintaining a safe- and
stable position near a ship.
For purposes of the below examples we assume a negative wake
field when steering away and
positive wake pressure field pushing a tugboat away from the
assisted vessel when transferring thepilot. A tugboat master has no
control of this wave system and it generally depends on the
assistedvessel length (on the waterline) and speed while boarding.
It is entirely possible that a tugboat ispulled towards the vessel
near its front and pushed away towards its stern and/or vice
versa.
Ship handling
Generally three types of propulsion configurations for tugboats
are accepted in todays marketenvironment. See also appendix 1 for
more information on these types and their respective
propulsionconfigurations
In order to safely perform pilot transfer operations were
looking for two key criteria in a tugboat
1) it should be able to steer away from the vessel at all times;
and
2) a tugboat should be able to maintain its position to enable
safe boarding by the pilot at highspeed (10 knots).
Figure 3 illustrates the respective tugboat types when steering
away from a vessel. Figure 3presumes a negative pressure pulling
the tugboat to the assisted vessel. This effect being the worstcase
scenario when steering away from an assisted vessel.
Figure 3, Steering clear with a tugboat
Tractor and Rotortug type tugboats can steer away in a safe
manner;
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Although these types are pulled towards the assisted vessel, the
tugboat master can safelysteer away using (one of) the forward
thrusters. The thruster unit pulls the tugboat out of thenegative
wake pressure field;
ASD tugs have a serious problem, the tugboats front is pulled
towards the assisted vessel bythe negative wave pressure field and
there is no propulsion unit available to pull the tugboatclear;
ASD tugboats risk collision on their aft side due to the
tugboats midship turning point;
Pilot transfer
Main key tugboat criteria for pilot delivery is maintaining a
stable position alongside an assisted vesselin order for the pilot
to safely board. As previously indicated the wake pressure field
alongside avessel can vary from pulling nearby bodies next to it,
to pushing them away.
In order for the tugboat master to have full control of his
vessel the tugboat should be able to exertforces towards its stern
and bow. This same criteria applies for example when operating at a
LNGcarriers side bitt . See also figure 4 for the respective
tugboat types.
Figure 4, Pilot transfer with tugboats
Tractor and ASD type tugboats experience difficulties
maintaining their position alongsideassisted vessel at10 knots;
Wave system (and pressure effects) increase with speed (fewer,
but higher waves);
Rotortugs are able to maintain position alongside and enable
safe pilot transfer due tothrusters fore and aft at 10 knots;
Rotortugs can safely establish a towing connection on LNG
carriers side bitts;
Large area hull appendages and skegs commonly found on ASD and
tractor escort tugsincrease susceptibility to the listed
effect.
Conclusion
Tractor - and Rotortugs can safely steer clear from assisted
vessels when operating inclose proximity to same (for example also
when operating near the bow of an assistedvessel);
Both tractor- and ASD-tugboats have great difficulty in
maintaining a position alongside anassisted vessel at higher
speeds;
Rotortugs can safely maintain a position alongside assisted
vessels enabling safe boardingof pilots at high speed;
To be in control of their vessel a tugboat should always have
the ability to operate in a safeand controlled manner;
Rotortugs offer a tugboat master full control of his vessel
during pilot transfers.
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Appendix 1Tug boat comparison
Figure 5, Azimuth stern driven tug Figure 6, Tractor tug boat
Figure 7, Rotor tug
Table 1, Comparison between an ASD -, Tractor - and Rotor tug
boat
Draught > 75 TBP
Draught < 75 TBP
Safe towing points
Towing over stern
Towing over bow
Push/Pull
Safe connecting to stern of speeding
ship in waves and current
Safe connecting to bow of speeding
ship in waves and current
BP sideways (pushing with the side)
Side stepping
Towline control at mooring in
confined or restricted areas
Positioning in current without force
on townline
Propulsion or winch brake down
while towing
good, no propeller ventilation. Extra: ability for
wave damping with fwd thruster units during pushingBehaveour in
swell during assistance
risk of propeller ventilation no propeller ventilation
good, also when speed decreasesrisk of capsizing at high speed
(10 knts)
assist ing strongly restri cted assisting st rongly rest
rictedstill 66 pct BP, good maoeuvrability
or via other winch
Escort capabilities good, less when speed decreasesgood, less
when speed decreases,
no need to reposition see
not possible not possiblegood, thruster configuration makes
dynamic
position possible
Towline control at narrow passagesgoes outside path width, goes
outside path width, stays within path width
need to reposition see need to reposition see
only in line with tow line only in line with tow line good, high
pulling in any direction (rotoring)
due to one end propeller configuration due to one end propeller
configuration due to unique triangle thruster configuration
limited BP limited BP high, approx 80% of max pull
approx 3-4 knts approx 3-4 knots high, approx 6-7 knts
unsafe, due to less control
good, safe over the stern good, safe over stern
with waves on aft deck
good, safe over the bow good, safe: if waves not too high good,
safe over bow
safe not possible safe
good, safe over the bow good, safe over the stern good, safe
over bow and stern
1 safe towi ng poi nt at the bow 1 safe towi ng poi nt at the
stern 2 saf e tow ing points at st ern as well as bow
risk of capsizing by girting safe safe
less than Tractortug more than Rotortug more than ASD and more
than Rotortug less than ASD and less than Tractortug
less than Tractortug and Rotortug more than ASD and more than
Rotortug more than ASD and less than Tr actortug
AZIMUTH STERN DRIVE TUG TRACTOR TUG (f.i. VSP) ROTOR TUG
