Day 1
Paper 6
ITS
2018
MARSEILLE
Organised by The ABR Company Ltd
Ships’ Deck Fittings Utilised for Towage
Capt Arie Nygh (speaker/author), SeaWays Consultants, Australia
SYNOPSIS
There has been a move around the world to use high-powered escort tugs to ensure the safe
passage of ships in restricted waterways. What has become apparent is that while these tugs can
render significant assistance, there is an Achilles Heel – the ship’s fittings to which the towline is
attached in many cases are unable to handle the forces generated by the tug (see Figures 1a1c). Investigation has found classification society regulations are confusing and lead to significant
misunderstanding by pilots, tug masters and indeed ship owners. This potentially renders the whole
exercise of escort towage a waste of time. This paper presents the issues, the underlying history
of the situation, and comes up with some pragmatic guidelines to assist all concerned in making
informed decisions.
INTRODUCTION
In January 2017, the author was undertaking annual
competency assessments on a client’s tugs operating
in an Australian port. When ordered in to connect the
towline, the pilot informed the tug master that the safe
working load (SWL) of the ship’s bitts the tug was to
connect to was 50 tonnes. It was noted that throughout
the towage operation the tug master endeavoured to
keep the tug’s bollard pull to a maximum of 50 tonnes,
which for that tug was only three-quarters power.
The pilot service management was asked about the
logic behind this procedure. The reply was that this was
common in the port, and was designed to ensure there
was no likelihood of damage or failure to the ship’s
fittings. Mention was made at this point that the SWL of
Figures 1a, 1b, 1c: Images of damaged ship’s fittings
1
mooring bitts on ships
a ship’s bitts has nothing to do with the forces created
by a tug’s towline during towage operations. Rather,
the SWL is referencing the forces created by a ship’s
mooring line when connected to the wharf.
A classification society will assign a vessel an
equipment number (EN) based on design criteria such
as vessel dimensions, displacement and windage.
These criteria are the main components that determine
the tidal and wind forces that will act on the vessel and
must be counteracted to moor the vessel safely. The
EN is then used to determine the number, length and
strength of mooring lines that must be supplied onboard
the vessel. NB: The EN is also used to determine
the strength of anchoring and emergency towage
arrangements (see Table 1).
Because the mooring line is belayed in figure-ofeight turns around the legs of the bitts, the SWL is a
calculation based on the collapsing force of the vertical
levers created by the figure-of-eight turns nearing the
top of the bitt’s legs. With this in mind, it is accepted
practice in the towage industry, though little known, to
effectively double whatever the SWL of the bitts is when
placing the towline at the base of one of the ship bitt’s
legs. For example, if the bitts are rated at a SWL of 50
tonnes, from a tug’s perspective this can be read as 50
tonnes x 2, giving the tug a SWL of 100 tonnes.
Strength requirements: mooring line
IACS
MBL as determined by EN
OCIMF
MBL as determined by Class EN
Table 1: Mooring line strength requirements
Subsequently the author contacted colleagues in the
Brisbane Pilot Service and suggested undertaking work
to investigate the subject of ships’ fittings utilised in
towage operations. At that stage we had no idea where
the research programme was to lead us, and the issues
we were to uncover.
Ship fittings are marked with a safe working load
(SWL). This is the maximum load that can be applied
to a line attached to or passing around the fitting for
mooring purposes. Note that the resultant load on the
fitting may exceed the load on the line (see Table 2).
Joining us in this research was a small team of
exemplary industry professionals: Capt Henk Hensen,
author of publications such as Tug Use in Port: A
Practical Guide and Tug Stability: A Practical Guide
to Safe Operations; Capt Brenton Winn, senior pilot
at Brisbane Marine Pilots, and Gijsbert de Jong, of
classification society Bureau Veritas.
Strength requirements: bitts
IACS
2 x MBL
SWL = MBL
OCIMF
2 x MBL
Table 2: Bitts’ strength requirements
For mooring purposes, it is assumed that the mooring
line is attached to the ship’s bitts in a figure-of-eight
fashion. Turns are taken around each post in turn to
secure the line. The load imposed on each post by a
line attached in this manner is twice the load on the line
(see Figures 2 and 3).
DeSIgN AND CONSTRUCTION STANDARDS
OF ShIPS’ BITTS AND FAIRleADS
It is important to clearly understand that ship deck
fittings are primarily provided for mooring the ship to the
wharf, not towage operations. Their required strength is
determined by the breaking strain of the mooring lines
deemed necessary by classification societies. Ships’
mooring fittings and their foundations should be designed
to carry at least the force imposed on the fitting and
foundation by an attached mooring line to the wharf at
the line’s minimum breaking load (MBL).
OCIMF guidelines state that: “Belaying figure-of-eight
tends to pull the two posts together inducing a higher
stress in each barrel than that produced by an eye laid
around a single post.” The design load of the bitts must
be twice the maximum expected load of an attached
line. Remembering that the SWL of a fitting is defined
as the maximum load of a line attached to the fitting,
then the design load of the bitts must be 2 x SWL.
Gaining an understanding regarding ship deck fittings
used for towage requires an understanding of ships’
mooring fittings design regulations.
Reference throughout this paper is made to the
following documents:
•
•
Requirement Concerning Mooring, Anchoring and
Towing, International Association of Classification
Societies (IACS), 2017
Mooring Equipment Guidelines, 3rd ed,
Oil Companies International Marine Forum
(OCIMF), 2008
This paper does not include detailed consideration
of safety factors when discussing design or breaking
loads. In general terms, the safety factor allowed is
around 15-25 per cent.
Figure 2: SWL when belaying a mooring line
2
Figure 4: Prefabricated deck fittings
SWL of the associated bitts, but caution should be
exercised, as this is not always the case.
Towing
The load calculation on the fairlead is the same,
irrespective of whether the line is being used for
mooring or towing (notwithstanding the additional
dynamic loads that may occur on a towing line) (see
Table 3, Figures 5a and 5b).
Figure 3: Figure-of-eight belaying a mooring line to
the wharf
Strength requirement: fairlead
Bitts are an attachment to the vessel and are often
provided ‘off the shelf’. Manufacturers produce bitts
complying with relevant international or national industry
standards (ie, ISO 13795, DIN, JMSA), Type-tested
to demonstrate compliance to strength requirements.
(Note that the standards of design differ between the
various industry standards.)
IACS
Up to 2 x
MBL
OCIMF
2 x MBL
Dependent on wrap
angle, as shown on the
Mooring Arrangement
Plan. SWL = MBL
Worst case: wrap angle
180 degrees
MBL = Minimum Breaking Load
Table 3: Fairlead strength requirements
Fairleads on ships
A fairlead redirects a line passing through it from the
wharf or tug to the ship’s bitts. The angle of the line
inboard of the fairlead is determined by the layout of
the fittings on the deck that the line may pass around or
be secured to. These working angles are shown on the
ship’s mooring arrangement plan.
Outboard of the fairlead, the line will pass to a
bollard on a quay to moor the ship, or, in the case of
towing, will be secured to a tug. The angle of the line
is variable, depending on the freeboard of the ship and
the position of the shore bollard or positioning of the tug
assisting the ship. The external angle is likely to have a
horizontal and vertical component, and, as it cannot be
determined, a worst-case scenario should be assumed
(ie, ≈90 degrees).
Figure 5a: Line force is doubled due to the angle
around the fairlead
The load on the fairlead will depend on the load
on and angle of the line passing through it (the wrap
angle). The worst-case scenario is where the wrap
angle is 180 degrees, whereby the maximum load on
the fairlead and its foundation structures will be twice
the load on a line passing through the lead.
Fairleads are an attachment to the vessel and are
also often provided ‘off the shelf’ and made to a Typetested design (Figure 4). It is common to see the
marked SWL of the fairleads being the same as the
Figure 5b: Line force is doubled due to the angle
around the fairlead
3
Towage fittings on ships
The ship deck fittings will also routinely be used to
secure a towline for harbour towage, and in some ports
for escort towage, either in the ship’s bow or through its
centre lead aft (CLA). Unlike the calculation for mooring
line strength, the rules do not specify the forces that
may be required on the towline to safely manoeuvre the
vessel. IACS requires the fittings to meet the “intended
maximum towing load (eg, tug’s ‘static’ bollard pull)
as indicated on the towing and mooring arrangements
plan” (Table 4).
Strength requirements: towline
IACS
Intended maximum towing load
OCIMF
Not specified
Table 4: Towline strength requirements
Figure 6: TOW SWL designed for tug towline forces
The ship has no control over the power of tugs, or
the breaking strain of the towline provided. The power
applied by the tug may have to be limited if the bollard
pull/load on the towline is likely to exceed the load
limits of the ship’s fittings. In calculating towing loads,
it is assumed that the towline is secured to the bitts by
passing an eye over a single post of the ship’s bitts.
towline can be controlled by limiting the power applied
by the tug, in some circumstances (dynamic loads,
transverse arrest, indirect towing) the load on the line
can be excessive, comfortably surpassing the rated
bollard pull of the tug. In these cases, the basic IACS
design principle requiring the line to be the weakest link
in the system may be negated, resulting in overloading
of ship fittings or underpinning structure before the
towline MBL is reached.
It is generally accepted that the strength of the ship’s
fittings and foundation structures required for mooring
purposes will provide sufficient strength for towage
purposes. A problem arises as the loads applied on
some parts of the system during towage are different
to the loads applied for mooring. This is primarily due
to the different methods of attaching a mooring line
(figure-of-eight) and a towline (single eye) to the bitts,
plus the various towline angles, both in the horizontal
and vertical planes, that a tug in a dynamic environment
can generate. This is explained in more detail below.
Furthermore, it also must be acknowledged that
many ships were built years ago when no one had
envisaged harbour tugs of 70 or 80 tonnes BP, or
high-performance escort tugs with hydrodynamic keels/
skegs that can produce twice the tug’s rated bollard pull
(commonly ≈120 tonnes BP).
For towing purposes, it is assumed that the towline
is attached to the bitts by a single eye placed over one
of the posts. The load imposed on the bitts by a line
attached in this manner is equal to the load on the line.
As the design load of the bitts is 2 x SWL, the load on a
line attached by a single eye can also be 2 x SWL (see
Figure 7).
Ship fittings can be marked with a safe towing load
(TOW) that pertains to the forces relating to a tug. This is
the maximum load that can be applied to a line attached
to or passing around the fitting for towage purposes.
Note that this marking is rarely seen in practice, and
frankly is too simplistic to be useful (Figure 6).
Furthermore, there is a lack of awareness among
many tug masters and pilots of how the tug’s towline
forces on the ship’s deck fittings can be multiplied many
times over by a combination of towline angles and
the operating mode the tug is using (direct, indirect,
combination arrest, transverse arrest, etc).
As an example, in a worst-case scenario it is possible
that there can be a 600 per cent multiple factor applied
to the tug’s rated (published) bollard pull, meaning a
60-tonne BP tug can produced a 360-tonne force on to
the ship’s deck fittings. (Note: the above is not including
spike (shock) loadings caused by rough tug driving or
sea state.)
Consequently, the bollard pull of a tug and the MBL of
a towline supplied from a tug may significantly exceed
the SWL of the ship’s fittings. While the load on the
Figure 7: Force on post of bitts
4
Foundation structure of deck
fittings on ships
Basic understanding for fairleads
Any force on the fairlead is transferred to the
attachment and foundation structure. The foundation
structure must be able to withstand the loads imposed
on the fairleads at various wrap angles. Strengthening
required in way of the fairleads will normally be
determined by calculation and incorporated in the
vessel construction at time of building.
Basic understanding for bitts
The loads imparted on to the bitts by the line are
withstood by the structure of the bitts, and are not
transferred to the foundation structure. The bitts can be
considered as a single box mounted to the deck, with an
external force acting on the box equal to the load on the
line. The external force is transferred through the box to
the foundation structure. The load that the foundation
structure must withstand is equal to the load on the line.
The foundation structure is an integral part of the ship’s
design. Strengthening required in way of the bitts will
normally be determined by calculation, and incorporated
in the vessel construction at time of building (see
Figure 8 and Table 5).
Summary of system loads and
strength requirements
In the above reference, the strength of a ship’s deck
fittings is relevant to the MBL of the mooring in use.
Where commonly a ship’s mooring line may be in the
order of 150 tonnes MBL, a modern high-performance
tug can have towlines with an MBL greater than 300
tonnes. Hence the IACS and OCIMF reference data
above is not appropriate for towage operations (see
Tables 6 and 7).
Investigation results
Key points
•
Figure 8: Force on the bitts’ foundation
We found the rules for design and construction
for ship’s deck fittings from one industry entity to
another to be not only confusing, but in some cases
contradictory. Even industry experts we consulted
were at times confused by them.
Strength requirements: bitt foundation structure
IACS
MBL
= SWL bitts
Sufficient to meet mooring load
OCIMF
2 x MBL
Sufficient strength to accommodate line attached to bitt by single eye over one
post at a load of 2 x SWL
Table 5: Bitt foundation strength requirements
Loads on system
Figure-of-eight
(mooring)
Single eye
(towage)
Doubling load on single eye
T
T
2T
Fitting and
foundation
Up to 2 x T (depending on
wrap angle)
Up to 2 x T (depending
on wrap angle)
Up to 4 x T (depending on
wrap angle)
Fitting
2xT
T
2xT
Foundation
T
T
2xT
Line
Fairlead
Bitts
Table 6: Summary of system loads
Strength requirements
Line
IACS
OCIMF
MBL
MBL
Fairlead
Fitting and
foundation
Up to 2 x MBL dependent on 2 x MBL
wrap angle
(worst case wrap angle
(Marked SWL = MBL)
180o)
Bitts
Fitting
2 x MBL
(Marked SWL = MBL)
2 x MBL
Foundation
MBL
2 x MBL
Table 7: Summary of strength requirements
5
Towing
To meet expected
maximum towing load
•
Regarding the foundation structures and attachment
to the vessel of the fittings, it should be understood
these are not integral parts of the ship’s structure,
and must be attached to the ship. The strength
of the attachment and underlying structure are
an important part of the system but this is often
underestimated during the design and construction
of the ship.
Age matters
•
•
For ships constructed prior to 2007, there is no
guarantee that class rules adequately cover the
design of the ship’s deck fittings.
For ships constructed prior to 2012, there is no
guarantee that class rules adequately cover the
underpinning deck structure the ship’s deck fittings
are connected to.
Figure 9: Force on CLA
Findings pertaining to ship’s bitts
and leads
failure, and deck fitting failure before hull or foundation
failure. During towage operations this is usually far from
the reality (see Figure 10, opposite).
While it is all well and good to focus on the SWL of the
ship’s bitts the tug’s towline is connected to, what must
also be taken into account is the ship’s fairleads that
the tug’s towline runs through. It is important for a tug
master to know when to focus on the SWL of one over
the other.
Towline angles
To further complicate things, the angle of the tug’s
towline from the horizontal applies a significant
multiplication factor to the forces the tug is creating
into its towline and on to the ship’s deck fittings. As an
example, with the tug’s towline angled up to the ship at
60 degrees from the horizonal, there is a multiplication
factor of x2 (200 per cent) into the towline and a factor
of x1.8 (180 per cent) on to the ship’s fittings.
As pointed out above, we can double the SWL of a
set of ship’s bitts by placing the eye of the tug’s towline
at the base of one leg of the ship’s bitts, but this does
not apply to the ship’s fairleads, be they roller, cotton
reel or panama types.
At a 75 degree towline angle, this multiplication factor
increases to x3.8 force into the towline and a x3.3
factor on to the ship’s deck fittings. For example, a tug
producing 85 tonnes BP into the water with a vertical
towline angle of 70 degrees can have 248 tonnes force
in its towline and 223 tonnes force on to the ship’s deck
fittings (see Table 8).
Generally for tugs working in a push/pull mode on
the ship’s sides or directly astern when centre lead aft
(CLA), there is not a significant force being brought to
bear on the ship’s fairleads. Often what force is created
is in fact a downward force that tends to press the
ship’s fairlead into the deck, rather than trying to pull it
off its connection to the ship’s deck foundation.
The situation changes significantly when the tug’s
towline forces are at an obtuse angle to the fairleads,
which in turn can create a tearing/ripping sideways
force on to the fairleads (in this case, the CLA).
The worst case scenario is when the towline is near
horizonal to the water and at an angle of near 90
degrees to the ship’s fairlead, and the angle from
where the towline enters the fairlead (in this case CLA)
to where it leads to the ship’s bitts is greater than 90
degrees. This combination can create a destructive
force on to the ship’s fairlead (CLA) of up to ≈2x the
towline force the tug is creating (Figure 9).
Table 8: Force multiples due to towline angles
General design strength principle of
deck fittings
A possible worst case
As a general principle, the ship’s mooring fittings and
foundations should be designed to carry the force
imposed by an attached mooring line. The requirements
concerning the strength of the ship’s mooring fittings are
based on the principle of rope failure before deck fitting
A tug out square to the ship is producing (say) 65
tonnes BP into the water with a vertical towline angle of
75 degrees, hence producing 251 tonnes towline force
and 217 tonnes force on to the ship’s fittings. Now lead
the towline through the ship’s fairlead and back to the
6
Figure 10: Towline force on
ship’s fittings
ship’s bitts at an angle of near 180 degrees and there is
another multiple factor of x2, hence the 217 tonnes now
becomes a 434 tonne force on to the ship’s fittings!
While we are giving a worst case scenario here to
make the point, it is fair to say that this scenario is
theoretically possible, and should the planets ever
align in this way then something in the system will fail,
whether it be the tug’s winch brake slipping, the towline
parting, or (more likely) the ship’s deck fittings being
ripped from the deck.
If the 85-tonne BP tug is a high performance escort
tug (say, a RAstar85) with a keel/skeg that creates
hydrodynamic lift, it can produce up to x2 its rated
bollard pull: 170 tonnes BP.
Needless to say, this is not the real cost. Generally,
tugs only operate at maximum power due to the ship
being in a situation that demands it. When there is
an equipment failure, there is little to no time for the
pilot to save the ship from being involved in a serious
incident, due to not having the tug generating the
required forces.
Hence, in broad terms:
•
85 tonnes BP x 2 = 170 tonne towline force, created
due to indirect type towage assist
•
170 tonnes x 2.92 = 496 tonnes into the towline due
to a towline angle up to the ship of 70 degrees
•
496 tonnes x 2 = 992 tonnes on to the fairlead, due
to the towline angle around the fairlead being ≈180
degrees
Active escort operations
While the issue of ships’ deck fittings being adequate
for general harbour towage is serious and indeed real,
the issue goes to another level when it comes to active
escort towage operations. Modern high performance
(Note that the actual steering created by the tug on
to the ship is still only 170 tonnes.)
7
Table 9:
Force created
by rudder
angles
Basically, the rudder design is best described as an
aeroplane wing that, instead of being horizontal in the
air, is vertical in the water. As with a plane accelerating
down the runway, the faster the ship goes, the more lift
(or steering force) the rudder creates. This is required
because the faster the ship goes the more steering
force is required to control and steer it. These forces are
further exaggerated by environmental influences such
as narrow waterways and low underkeel clearances.
escort tugs are sophisticated designs with rated
bollard pulls commonly of 80-100 tonnes. They are
able to generate huge towline forces via their specially
designed keels or skegs.
There are good reasons to require escort tugs to be
able to produce these high tonnages. For example:
•
•
•
When a ship of, say, 100,000dwt is steaming ahead
at 8 knots, its rudder can produce steering forces in
the order of ≈70 tonnes steering force.
Increase the same ship’s speed to 10 knots, and its
rudder can produce ≈100 tonnes steering force.
Now consider a CapeSize or VLCC-type ship of
200,000dwt. At 8 knots, its rudder produces ≈100
tonnes steering force, and at 10 knots this increases
to ≈150 tonnes (see Table 9).
Modern high performance escort tugs (ASD, ATD and
VSP designs) have either keels or skegs of a similar
design to the ship’s rudder, so also create hydrodynamic
lift. Consequently, the faster the ship steams ahead, the
greater towline forces the escort tug can create. This is
needed in order to overpower the ship’s steering forces
in the event of a rudder failure (Figure 11).
When a ship is in a narrow waterway such as a
shipping channel, and/or there is low under keel
The reason for this is that a ship’s rudder is designed
to create hydrodynamic lift. What does this mean?
Figure 11:
Hydrodynamic
lift rudders
8
Given that a ship of ≈200,000dwt can produce >100
tonnes steering force via its rudder, it can be seen
there is an immediate issue, given the escort tug must
produce considerably more counter force via its towline
running through the ship’s fairleads to its bitts (see
Figure 12).
clearance, the towline forces required from the tug
increase quite dramatically. Furthermore, in the event of
a failure of the ship’s rudder these forces from the tug
must be applied very quickly to avoid the ship sheering
off course. In this type of scenario we are talking about
a counter steering force from the escort tug being
applied within ≈30 seconds. With modern tug designs,
high performance synthetic towlines and elite training of
the tug’s crew, this is now all possible. But the Achilles
Heel we have only recently come to understand fully is
the ship’s deck fittings.
Reality check
The reality, when holistically reviewing this topic, is
that most of these types of ships when transiting a
narrow waterway cannot in the event of a rudder failure
likely be saved by their escort tugs, given the steering
forces required being accentuated by the physics of
towline angles, regardless of the question marks over
the quality of the ship’s deck fittings and underpinning
support structure.
Inadequency of ship’s deck fittings
Earlier in this paper we alluded to there being potential
issues with ship’s deck fittings on ships older than
2007, due to them not necessarily being covered
by class rules, and ships older than 2012 due to the
underpinning deck structure not necessarily being
covered by class.
What to do?
Having gained an understanding of the issues,
SeaWays has been involved in extensive live escort
trials with one of its clients, their port authority and the
port’s pilot service. The results of the first stage of these
trials supported the findings of our research.
In addition to this, in many cases the actual
SWL rating of ship’s deck fittings utilised in towage
operations has proven to be of serious concern. As
an example, our research into CapeSize bulkers has
shown more than half of these ships have deck fittings
that are required to be used in escort towage of 50-80
tonnes SWL. This is assuming that their SWL ratings
are actuals, taking into account the points we have
raised within this paper.
Of course, the next question is: what to do? The
whole point of investing in high performance escort
tugs, as well as an extensive training programme for
pilots and tug masters, is to to ensure the operational
Figure 12: Rudder forces on a CapeSize or VLCC-type ship
9
Figure 13: Dual
escort towage
integrity of the shipping channel. A channel blockage
due to a ship running aground in the port where we
conducted our trials would cause an estimated cost
running into billions of dollars, due to lost trade over the
likely period before the channel could be cleared.
each tug could easily produce, say, 80 tonnes towline
force (80 tonnes x 2 = 160 tonnes required steering
force) spread over two sets of ship’s fittings.
Dual escort towage also allows the pilots to have
numerous initial response scenarios and ongoing
control over the ship, as well as redundancy in the case
of mechanical failure of a tug or its towline (Figure 13).
Interim fix – dual towage (T2)
The second stage of our trials involved undertaking dual
escort towage operations. This concept, also known as
T2, was originally developed by Capt Greg Brooks from
Towage Solutions. It involves using two escort tugs
connected either side of a ship’s transom.
Naturally, this type of activity comes at a significant
cost, though when considering the cost of a channel
blockage it can be argued that it is a small investment.
Note that it is important to understand that while there
can be towline forces in the hundreds of tonnes, this is
not related to the actual steering force the tug is creating
on to the ship. The actual steering force is the same
as the force the tug is generating into the water (its
bollard pull), not into its towline. There are other towage
techniques that can also be utilised, depending of the
circumstance and attending tug, but this is beyond the
scope of this presentation, and will have to wait for when
we have another opportunity to present to industry.
Using dual escort towage in this manner can have the
benefit of doubling the overall steering forces the tugs
can create, but this was not our motivation. Instead,
we wanted to be able to generate the required steering
forces without the risk of the ship’s deck fittings failing.
Hence, while (in this case) one of the Robert Allan Ltd
RAstar85 escort tugs we were using in our trials could
produce upwards of 160 tonnes towline force, using two
of these tugs in a dual escort configuration meant that
10
Long-term fix
Pragmatic information guide
The client involved in our escort and dual escort
trials has placed its charterers on notice that they have
two years to ensure their ships have adequately rated
fairleads and bitts, so that in an emergency the escort
tug can produce the required towline steering force to
overpower the ship’s rudder lock.
The diagram (see Figure 14, overleaf) is a free gift
to the industry from SeaWays. It has been produced
in an A3 landscape format ready for printing, and can
be obtained either here at the ITS 2018 conference
from our SeaWays desk in the exhibition hall or by
going to our websites www.seaways.net.au or www.
seawaysglobal.com and downloading it.
In our opinion, now that the research group has
undertaken this work, there is an onus, indeed
a responsibility, on ship owners, designers and
classification societies to ensure that all newbuilds have
deck fittings, particularly in the bow and stern of ships,
that are of a rating to be able to sustain the forces an
escort tug has to generate in order to save the ship in
a rudder steering failure-type emergency. These deck
fitting ratings must also acknowledge the physical
forces in play due to towline angles.
To assist industry, particularly tug masters and pilots,
SeaWays has developed a diagram that provides
insight into the issues identified in this paper and some
practical ‘rule of thumb’ operational guidelines. The
challenge was to condense a significant amount of
research material into a one-page diagram that can
be understood by mariners whose first language is
not necessarily English. Hopefully we have had some
success with this.
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Figure 14: Operational guidelines
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