11. Shiphandler's Guide 100% Flashcards
<p>The --------shiphandler has to be aware of wind and \teather, crrrent and
<br></br>tldal change;. In most b€rth approaches, shallow water and interaction effects will
<br></br>also have to be taken into account.</p>
<p>Competent</p>
<p>There ls th€ range. type, availabillty and efficlency of ----- to be considered Also.
<br></br>the availabiltty and-aptitude of the personnel on each ship has to be ass€ssed for
<br></br>their ability to handle to$r lines and mooring lines All these factors effect ship
<br></br>manoeuvres \{hich in a tidal regime have to be completed within limited time windowsl
<br></br>so adding to the sense of anxiety should anything go wrong.</p>
<p>tugs</p>
<p>Thes€ principles are based upon a number of moving ------- some int€rnal
<br></br>like thrust and the positlon of the Pivot point, some external like wlnd and interaction.
<br></br>It is these moving variable forces that have to be mastered and the flrst plac€ to start
<br></br>ls the Divot Doint.</p>
<p>Influences</p>
<p>Sip Stopped</p>
<p>Unless stated otherwise, each example assumes a ship
<br></br>on even keel, ln calm conditions and still water. ln thls
<br></br>sltuation no forces are lnvolved and the ship has a pivot
<br></br>point colncldlng with its centre of gravlty, approximately
<br></br>amidships.</p>
<p>Making Headway</p>
<p>Two forces now come into play. Firstly, the forward
<br></br>momentum of the ship and secondly, l o n g i t u d i n a l
<br></br>reslstance to the forward momentum. cr€ated by the water
<br></br>ahead of the ship. These two forces must ultimately strike
<br></br>a balance and the pivot polnt moves forward. As a rough
<br></br>guide lt can be assumed that at a steady speed the plvot
<br></br>point will be approxl'nately 25 ora|/a ofthe ship's length
<br></br>from forward.</p>
<p>Making Sternway</p>
<p>The situation is now totally reversed. The momentum
<br></br>of sternway must balance longitudinal resistance, this time
<br></br>created by the wat€r astern of the ship. The plvot polnt
<br></br>now moves aft and establishes itself approximately 25%
<br></br>or a t/4 of the shlp's length from the stern.
<br></br>Although not intend€d. some publications may give the
<br></br>impression that the pivot point moves right aft with
<br></br>sternway. This ls clearly not correct and can sometimes
<br></br>be misleading. It should also be stressed that other factors
<br></br>such as acceleration, shape ofhull and speed may all affect
<br></br>the position of the pivot point. The arbitrary figures quoted
<br></br>here. however, are perfectly adequate for a simpl€ and
<br></br>practical worklng knowledge of th€ subj€ct.</p>
<p>Turning Levers</p>
<p>More important. perhaps. than the position of the plvot
<br></br>polnt. ls the eff€ct its shifting nature has upon the many
<br></br>turning forces that.can influence a ship. These are -
<br></br>rudder force, transverse thrust, bow thrust. tug Iorce,
<br></br>interactlve forces and the forces of wind and tide,</p>
<p>Vessel Stopped</p>
<p>lf we look at the ship used in our €xample, we can see
<br></br>that it has a length overall of 160 metres. It is stopped in
<br></br>the water and tv/o tugs are secured fore and aft, on long
<br></br>Iines, through centre leads. If the tugs apply the same
<br></br>bollard pull of. say, 15 tonnes (t) each, it is to a position
<br></br>80m fore and aft of the pivot polnt. Thus two equal turnlng
<br></br>Ievers and moments of 80m x l5t (l200tm) are created
<br></br>resulting ln even lateral motion and no rate of turn,</p>
<p>Making Headway</p>
<p>With the ship making steady headway, however, the
<br></br>pivot point has shifted to a position 40m from the bow.
<br></br>The forward tug is now working on a very poor turning
<br></br>lever of 4Om x f5t t600tm). whilst the after tug ls worklng
<br></br>on an €xtremely good turnlng lever of I20m x l5t {t80otmj.
<br></br>This results in a swing of the stern to port.</p>
<p>Making Sternway</p>
<p>The efficiency of the tugs wlll change totally when, by
<br></br>contrast. the ship makes sternway. Now the plvot point
<br></br>has moved aft to a posltlon 4Om from the stern. The forward
<br></br>tug ls worklng on an excellent turning lever of 120m x l5t
<br></br>(1800tm1 whilst the after tug has lost lts efflciency to a
<br></br>reduc€d turning lever of 4Om x l5t (60otm). This now
<br></br>results in a swing of the bow to porl</p>
It is therefore desirable to balance a —– and ——
speed of approach against a realist time scale.
safe and effective
It is usually obvious when the —– of a ship is too
——, and can easily be overcome with a small increase in
revolutions, lt is not always obvious. though. when the
speed is too high. The speed of a large ship during an
approach to a berth, particularly without tugs can increase
in in insidious manner. It is invariably difficult to reduce
that speed ln a short distance and keep control of the ship’
speed, slow
The duration of a kick ahead should be as short as
possible. Prolonged use of the power’ after the initial steering effect, has ceased will only result in a violent sheer and unwanted build up of speed This will result in the
need for yet another kick ahead to rectify the situation’
As soon as the revolutions reach the required maximum’
the power must be taken off
Kick Ahead duration
It is difficult to quantify the amount of power to apply.
for a kick ahead, as It very much depends on the size of the ship and the needs of the shiphandler at the time.
kick ahead power.
Ahead Movement
of the Propeller
THE EFFECT OF TRANSVERSE THRUST whilst making an ahead
movement is arguably less worrying than that of an astern
movement. perhaps because the result is less noticeable.
Propeller design is a complex subject area. but it is worth
looking at the main factors, which are evident with an
ahead-movement of a right handed propeller.
Astern Movement
of the Propeller
The importance of transverse thrust when using an
astern movement. is of much greater significance to the
ship handler. The helical discharge, or flow. from a right
handed propeller working astern splits and passes forward
towards either side of the hull. In doing so it behaves quite
differently. On the port quarter it is inclined down and
away from the hull whilst on the starboard quarter it is
directed up and on to the hull. This flow of water striking
the starboard quarter can be a substantial force in tonnes
that is capable of swinging the stern to port giving the
classic kick round’ or ‘cut of the bow io starboard
Pivot Point and
Transverse Thrust:
Vessel Making Headway
The forward speed of the ship must be considered.
because at higher speeds the full force of propeller wash
will not be striking the quarter. As the ship progressively
comes down to lower speeds and ]with the pivot point stil1
forward. the magnitude of transverse thrust will slowly
increase reaching lts peak just prior to the ship being completely stopped…
applied, transverse thrust is likely to be at its maximum
Vessel Making Sternway
With the same ship making sternway the€ pivot point
will now move to a new position somewhere aft of amidships
(see fiEure 6bl. With the propeller working astern the flow
of water on to the starboard quarter is still maintaining
its magnitude as a force of 6 tonnes but is now applied to
a reduced turning lever of 40 metres.
Unlike the situation with headway we now have a
reduced turning moment of 240 tonne-metres with
stern\r’av. In the first instance this may not seem strikingly
important. It must be remembered however’ t h a t
transverse thrust may be a poor force in comparison to
other forces such as wind and tide. With the example of
sternway, a wind acting forward of the Pivot point may be
strong enough to overcome that of transverse thrust This
will be investigated more thoroughly in later chapters concerning the effects of wind and tide.
It ls sometimes
power in the close
shallow water will
possible causes for
knowledge is likely
apparent that a ship when using stern
proximity of solid jetties, banks or
‘cut the wrong way. There are two
this occurrence and only a pilot’s local
io pinpoint them.
CPP
The controllable piich propeller rotates constantly in
the same directlon no matter what movement is demand€d
of it. Viewed from astern, a clockwise rotating propeller is
still rotating clockwls€ wlth stern power. only the pitch
angle of the blades has changed. This gives the same effect
as a conventional fixed pitch left handed propeller, which
is also rotating clockwise when going astern, the bow will
swing to port. Similarly if a variable pitch prop€ller
constantly rotates counter clockwise when viewed from
astern, this will be the same as a fixed pitch right handed
p r o p c l l e ru h r . h i < a l \ o r o l a r r n gc o u n l e r . l o c k \ i i s ed u r i n g
an astern movement, the bow will thus swing to starboard
Effect of Shallow water
The second possible cause of a cut the wrong way
may b€ attributed to the vicinity of shallow water.
Lateral Resistance
As a ship commences to turn and thereafter for the
duration of a turn, the ship is sliding sideways through
the water. This results in a large build up of water
resistance. all the way down the shlp’s side. which
continually opposes the rudder force and which we can
refer to as the ‘2. Lateral Resistance. The balance
between the rudder force and the lateral resistance. Plays
a crucial part in shaping all turning circles.
Speed During a turn
The speed of a ship during a normal turn is interesting,
ln so much that it suffers a marked reduction. As the ship
is sliding sideivays and ahead. the exposed side experiences
a substantial increase in water resistance. which in turn
acts as a brake.
Shallow water effect
As a rough guide lt can be assumed that a ship
may experience shallow water effect when the depth of
water is less than twice the draft
Draft in a turn
The amount of sinkage, in this case I meter can
be Surprising enough should not be forgotten when turning
at speed in shallow water.
Effect of wind with headway
turn into wind
Effect of wind with sternway
back into wind
Bank Effect
As a result of these two forces which have developed.
the stern of the ship is likely to be sucked into the bank.
It can be very difficult to break out of its hold, the ship
requiring constant corrective rudder and power, sometimes
hard over. in order to control heading. Excessive speed must be avoided.
Squat
Excessive speed is the main factor.
Working in a Tide
Improve slow speed control.
. create lateral motion.
Working the thruster differences
. thrusting when stopped.
. the thruster and headway.
. creating lateral motion.
. working the thruster with sternway.
The effect of torque on twin screws.
The term torque is used in this instance to describe
the natural turning effect that is created by one engine pushing ahead and one engine pulling astern or. to a lesser
extent. just one of those engines being used This
contributes towards good twin screw turning ability, but
occasionally lt ls possible to come across a ship where the
effect will be extremely disappointing. because the ship is
designed with the propellers too close together€
To gain a broad insight into the use of tugs, it is first important to be aware of the types of tug commonly available and in particular their strengths or weaknesses. To do this it is helpful, to group the many different types of tugs according to their working methods and this provides the following categories
- conventional tugs.
- tractor tugs.
- azimuth stern drive tugs (ASD).
The conventional or traditional tug, for years the work horse of the maritime industry and still widely employed, has two inherent design features which
- the traditional propulsion unit.
* the position of the towing hook.
A sudden and catastrophic loss of —— is the most likely cause of a capsize and this can occur even with a very slight collision. Tugs, it should be noted, roll over and flood extremely quickly, thus affording little time for the crew to escape!
stability
The resultant interaction forces may be too strong, causing a —– around the after body of the ship (see figure 60b). This is compounded by the more obvious and widely recognized risk that is associated with working under the stern, in the close proximity of the ship’s propeller.
vigorous suction, or low pressure area,
The conventional tug is clearly at its best when it can utilise its ——- and apply full bollard pull to the tow line or ship when so required. Naturally, there is a tendency for this to be at its best when the ship is fairly static i.e. when swinging, positioning, breasting or lifting off, etc. To the experienced mariner these movements are by and large straight forward and do not require elaborating upon.
maximum power
The key to the true tractor tug lies in the use of two multi-directional propulsion units, of which some are rather like large rotating outboard motors with others consisting of rotating vertical blades. They enable the thrust units of the tug to be placed side by side, more or less under the bridge, thereby facilitating —— in the right hands (see figure 64). Interestingly, these units have also enjoyed considerable success for many years installed on some ferries operating in inshore waters.
spectacular manoeuvrability
Advantages of tractor tugs
- full thrust over 360 degrees.
- rapid power-on response time.
- outstanding manoeuvrability.
- able to reposition quickly if so required by the pilot.
- simple control systems.
- very low risk of girting.
- can more effectively overcome interaction forces close in on a ship.
- improved operational capability in a restricted area such as a lock or an enclosed dock.
- may decrease the ‘turn round time’ of port movements that normally use tug assistance.
- reliable, robust propulsion units.
Disadvantages of tractor tugs
- high capital investment costs.
- less bollard pull per kilowatt power.
- repair and maintenance of complex underwater units may be expensive.
- handling in an open seaway might be poor with the short distance between the pivot point and thrust, creating a short steering lever.
- heeling angle with full side thrust may be up to 21 ° with some tugs. Therefore, risk of damage can exist when laying alongside a ship.
- sophisticated underwater units may be damaged if grounded.
- draft may be up to 5m. which is large in comparison to conventional tugs.
- the re-training of conventional tug masters is essential in order fully to maximise a tractor tug’s potential.
Azimuth Stern Drive (ASD) Tugs
The ASD tug can therefore be used in the same manner as a conventional tug. using the amidships towing position but with considerably improved handling.
Advantages of an ASD Tug
- better directional stability at speed.
- more suitable hull form for open waters and working in a seaway.
- improved bollard pull per kilowatt power.
- azimuth units easy to withdraw for maintenance and repair.
- maximum heel with side thrust less than 15° .
- shallower average draft of 3m.
Limitations of an ASD Tug
- side stepping ability-not as good.
- squat at the stern and flooding of the aft deck has been known to occur with certain design types when backing with full power.
- still susceptible to girting when using the after towing position.
- slightly more at risk from the effects of interaction.
- it is not uncommon for 99% of all towing to be limited to the forward position.
- complex control systems.
- more susceptible to rubbish damage in propellers.
