Tugboat

A tugboat, or tug, is a boat used to maneuver, primarily by towing or pushing,

other vessels (see shipping) in harbors, over the open sea or through rivers and
canals. Tugboats are also used to tow barges, disabled ships, or other
equipment like oil platforms.

Tugboats are quite strong for their size. Early tugboats had steam engines (see
steamboat); today diesel engines are used. Tugboat engines typically produce
500 to 2,500 kW (~ 680 to 3,400 hp), but larger boats (used in deep waters) can
have power ratings up to 20,000 kW (~ 27,200 hp) and usually have an extreme
power:tonnage-ratio (normal cargo and passenger ships have a P:T-ratio (in
kW:GRT) of 0.35 to 1.20, whereas large tugs typically are 2.20 to 4.50 and
small harbour-tugs 4.0 to 9.5). The engines are often the same as those used in
railroad locomotives, but typically drive the propeller mechanically instead of
converting the engine output to power electric motors, as is common for railroad
engines. For safety, tugboats' engines often feature two of each critical part for
redundancy.

A tugboat's power is typically stated by its engine's horsepower and its overall
Bollard pull.

Bollard pull is a value that allows the comparison of the pulling power of
watercraft, particularly tugboats.

Background

Unlike in ground vehicles, the statement of installed horsepower is not sufficient

to understand how strong a tug is - this is because other factors, like
transmission losses, propulsion type, propulsion system efficiency, have an
influence as well.

Bollard pull values are stated in tons. They are an indication of the maximum
pulling force that a ship can exert on another ship or an object.

How to determine

Values for bollard pull can be determined in two ways:

Practical trial

This method is useful for one-off ship designs and smaller shipyards. It is limited
in precision - a number of boundary conditions need to be observed to obtain
relatively reliable results. Summarizing the below requirements, practical bollard
pull trials need to be conducted in a deep water seaport, ideally not at the
mouth of a river, on a calm day with hardly any traffic.

 The ship needs to be in undisturbed water. Currents or strong winds

would falsify the measurement.
  The static force that intends to move the ship forward must only be
generated by the friction between the propeller discharge race and the
surrounding water. If the ship were too close to a wall, the measurement
would be falsified.
 The ship must be in deep water. If there were any ground effect, the
measurement would be falsified. The same holds true for Propeller walk.
 Water salinity must have a well-defined value, as it influences the specific
weight of the water and thereby the mass moved by the propeller per
time.
 The geometry of the towing line must have a well-defined value. Ideally,
one would expect it to be exactly horizontal and straight. This is
impossible in reality, because
o the line falls into a catenary due to its weight;
o the two fixed points of the line, being the bollard on shore and the
ship's towing hook or cleat, will hardly have the same height
above water.
 Conditions must be static. The engine power, the heading of the ship, the
conditions of the propeller discharge race and the tension in the towing
line must have settled to a constant or near-constant value for a reliable
measurement.
 One condition to watch out for is the formation of a short circuit in
propeller discharge race. If part of the discharge race is sucked back into
the propeller, efficiency decreases sharply. This could occur due to a trial
that is performed in too shallow water, too close to a wall.

Figure 1: bollard pull trial under ideal (imaginary) conditions

See Figure 2 for an illustration of error influences in a practical bollard pull trial.
Note the difference in elevation of the ends of the line (the port bollard is higher
than the ship's towing hook). Furthermore, there is the partial short circuit in
propeller discharge current, the uneven trim of the ship and the short length of
the tow line. All of these factors contribute to measurement error.
 Figure 2: bollard pull trial under real conditions

Tugboats are highly maneuverable, and various propulsion systems have been
developed to increase maneuverability and increase safety. The earliest tugs
were fitted with paddle wheels, but these were soon replaced by propeller-
driven tugs. Kort nozzles have been added to increase thrust per kW/hp. This
was followed by the nozzle-rudder, which omitted the need for a conventional
rudder. The cycloidal propeller was developed prior to World War II and was
occasionally used in tugs because of its maneuverability. After World War II it
was also linked to safety due to the development of the Voith Water Tractor, a
tugboat configuration which could not be pulled over by its tow. In the late
1950s, the Z-drive or (azimuth thruster) was developed. Although sometimes
referred to as the Schottel system, many brands exist: Schottel, Z-Peller,
Duckpeller, Thrustmaster, Ulstein, Wärtsilä, etc. The propulsion systems are
used on tugboats designed for tasks such as ship docking and marine
construction. Conventional propeller/rudder configurations are more efficient for
port-to-port towing.

The Kort nozzle is a sturdy cylindrical structure around a special propeller

having minimum clearance between the propeller blades and the inner wall of
the Kort nozzle. The thrust:power ratio is enhanced because the water
approaches the propeller in a linear configuration and exits the nozzle the same
way. The Kort nozzle is named after its inventor, but many brands exist.

A recent Dutch innovation is the Carousel Tug, winner of the Maritime

Innovation Award at the Dutch Maritime Innovation Awards Gala in 2006 [1]. The
Carousel Tug adds a pair of interlocking rings to the body of the tug, the inner
ring attached to the boat, with the outer ring attached to the towed ship by winch
or towing hook. Since the towing point rotates freely, the tug is very difficult to
capsize

Types of tugboats

There are two groups of tugboats, either Inland or Oceangoing.

Inland tugboats come in two categories:

Harbor tugs are the most typical of the tugboats that people recognize. They
are used worldwide to move ships in and out of berth and to move industrial
barges around waterfront business complexes. Their job has remained the
same, but their design and engineering has changed much over the decades.
Harbor tugs have evolved from paddle wheelers to the conventional tug known
by all, and now to the Ship Docking Moduals and tractor tugs in the modern
industry. In some cases this type has been used on estuarine rivers, cable
towing barges, while using a side tow with a springline for docking. In another
application, ocean-going tugs have been applied to railcar barge movement.
using specialized loading facilities and side towing. [3]

River tugs are also referred to as towboats or pushboats. They are designed
as large squared-off vessels with flat bows for connecting with the rectangular
stern of the barges. They are large and powerful, most commonly seen on the
big rivers of the world. They are capable of pushing huge fleets of barges that
are lashed together into "tows". Some tows can be up to 1,000 feet long and
205 feet wide. Smaller push boats are often seen handling only a few barges on
inland waters. Despite their size, they are designed to push their tow rather than
tow from the stern.

Oceangoing tugboats come in four categories:

The conventional tug is the standard seagoing tugboat with a model bow that
tows its payload on a hawser; hawser is the nautical term for a long steel cable
or large synthetic fiber rope. It operates independently and is used to tow
various loads, e.g., cargo barges, ships, oil rigs, etc. This is the most versatile
method of towing since the conventional tugboat is able to move its load three
ways: Pushing from behind, secured to the side of the towed vessel, or by
towing astern, all achieved by the use of various lines and cables in various
configurations. They are importantly recognized as the design of choice for
salvage and assistance of wrecked ships and in the rescue and safe return of
disabled ships from the high seas.

The notch tug is a conventional tug which is assigned to tow and push a
specific barge, usually built to the shape and specifications of that tugboat. A
notch tug has a large towing winch on its stern, but it gets its name from the
deep notch built into the stern of the barge. This notch is built in the exact shape
of the tug's forward hull and can be quite deep, up to 90 feet, sometimes more.
The tugboat fits snugly into the notch of the barge, and with the use of various
lines can be secured firmly enough to push the barge at much higher speeds
than it would if it were towing. The towing hawser remains rigged during
pushing. In the event that the seas get too rough to push safely, the tug merely
releases any securing lines and backs out of the notch while extending its
towing hawser. Once in calmer waters, the tug can maneuver back into the
notch and resume pushing.

The articulated tug and barge, or ATB, is a specially designed vessel,

composed of a tugboat and a barge which are coupled using specially designed
machinery. The tug is connected to the barge inside a notch, similar to the notch
boat, using a system of heavy pins, clamps, and/or side pads. ATBs remain
coupled all the time; the tug pushes its barge in all but the roughest seas.

The advantages of this system are speed, safety, and cost efficiency. As a unit,
the ATB can push much faster than a tug can tow from astern, and the use of a
coupling system eliminates many of the hazards associated with towing
winches and cables. The unit is considered by authorities to be coupled in a
"semi-rigid" manner and, thus, regulated by laws governing tugs and barges,
rather than ships. This makes the ATB a less expensive vessel to operate. To be
considered articulated, the two vessels may roll simultaneously but must pitch
independently. There are three popular systems to achieve this, each having a
method to lock the tug onto the barge and secure its side to side movement,
while allowing the tug to pitch freely.

Note: While ATB's can be considered integrated, the designation of ITB is not
widely used nowadays, due to industry changes in design and practice.

The "Intercontinental (Intercon) System" uses two pins on the tug that can fit
into specially designed grooves built vertically into the walls of the notch on the
barge. The grooves are built with a row of zig-zag "teeth" on each edge, forward
and rear. Two pins on each side of the tug's bow are equipped with the same
shaped teeth on their forward and rear that, when extended into the grooves,
will mesh with those on the grooves. The pins then press in tightly using great
mechanical pressure. The meshed teeth prevent the tug from floating up and
down or fore and aft in the notch, and the pins hold the tug evenly between both
sides of the notch, securing it from shifting side to side. The tug is allowed to
pitch inside the notch as it pivots on the pins' giant shafts as on axles.
The "Bludworth System" utilizes a large hydraulic clamp on the very bow of
the tug that fits onto a large steel bar in the deepest end of the barge's notch.
The clamp uses massive hydraulic pressure to squeeze two metal discs onto
either side of the bar, like a disc brake caliper on a car. The tug is also equipped
with two sets of large pads on each side near the stern. One side of these pads
is also fitted with hydraulic presses, and extend outward to secure the tug from
side to side. The large teflon pads are firmly in contact with each side of the
notch, so they are frequently lubricated to reduce friction during underway
movement. The clamp grips the bar tightly preventing the tug from floating up
and down or fore and aft in the notch. The side pads press out with equal
pressure, holding the tug evenly in the notch, securing it from shifting side to
side. The tug is allowed to pitch inside the notch as the pads are allowed to
slide up and down while the clamps buttons pivot inside the clamp housing like
axles.

The JAK System is now being used. It is similar in operation to the Intercon
System but uses different means of coupling. Instead of a vertical groove with
teeth, it uses a vertical row of evenly spaced holes (sockets) along each side of
the notch. Aboard the tug, round, solid pins without teeth are mounted in the
sides of the bow. The tug pulls into the notch and extends the pins, which fit into
the sockets. Great pneumatic pressure is used to press them firmly into place,
holding the tug in the notch. The pins cannot move around in the tight fitting
sockets and prevent the tug from floating up and down or fore and aft in the
notch. The pins hold the tug evenly between both sides of the notch, securing it
from shifting side to side. The tug is allowed to pitch inside the notch as it pivots
on the pins as on axles.

There may be other ATB coupling systems in use but these three are the most
widely used.

The integrated tug and barge, or ITB, is a rigidly connected tug and barge.
This means that it fits so tightly into the stern of its barge that it will roll and pitch
in the same manner with the barge. The systems used to couple the two
vessels are varied, but they are similar in that the connection point is virtually
seamless, and for all practical purpose, they appear to be a ship. These units
stay coupled under any sea conditions, and the tugs usually have poor designs
for sea keeping and navigation without their barges attached. Vessels in this
category cannot pitch independently from the barge and so are legally
considered to be ships rather than tugboats and barges. As a result of this
classification, they are regulated by authorities as ships.

Nozzles

Kort nozzle

The Kort nozzle is a shrouded, ducted propeller assembly for marine

propulsion. The hydrodynamic design of the shroud, which is shaped like a foil,
offers advantages for certain conditions over bare propellers.
Kort nozzles or ducted propellers can be significantly more efficient than
unducted propellers at low speeds, producing greater thrust in a smaller
package. Tugboats are the most common application for Kort nozzles as highly
loaded propellers on slow moving vessels benefit the most.

The additional shrouding adds drag, however, and Kort nozzles lose their
advantage over propellers at about ten knots (18.5 km/h).

Kort nozzles may be fixed, with directional control coming from a rudder set in
the water flow, or pivoting, where their flow controls the vessel's steering.

Shrouding of this type is also beneficial to navigation in ice fields since it

protects the propeller tips to some extent.

Origins

Luigi Stipa and later Ludwig Kort (1934) demonstrated that an increase in
propulsive efficiency could be achieved by surrounding the propeller with a foil-
shaped shroud in the case of heavily loaded propellers. A "Kort Nozzle" is
referred to as an accelerating nozzle and is generally a MARIN 19A profile or a
MARIN 37 profile.

Physics

In a Kort nozzle, the inflow velocity is increased, reducing pressure. This lowers
thrust and torque of the propeller. At the same time, a circulation occurs,
resulting in an inward aimed force, that has a forward component. The duct
therefore has a positive thrust. This is normally larger than the thrust reduction
of the propeller. The small clearance between the propeller and duct reduces tip
vortex, increasing efficiency.

As drag increases with increasing speed, eventually this will become larger then
the added thrust. Vessels that normally operate above this speed are therefore
normally not fitted with ducts. When towing, tugboats sail with low speed and
heavily loaded propellers, and are often fitted with ducts. Bollard pull can
increase up to 30% with ducts.
Azipod

Azipod is the registered brand name of the ABB Group for their azimuth
thruster. Originally developed in Finland jointly by Kvaerner Masa-Yards
dockyards and ABB, these are marine propulsion units consisting of electrically
driven propellers mounted on a steerable pod.

The pod's propeller usually faces forward, as in this puller (or tractor)
configuration, the propeller is more efficient. In addition, because it can rotate
around its mount axis, the pod can apply its thrust force in any direction.
Azimuth thrusters allow ships to be more maneuverable and enable them to
travel backward nearly as easily as they can travel forward. The Azipod concept
is not practical for use on warships because of damage control difficulties;
integrating propulsion with rudder makes both easier to damage or destroy.

The new CRP (Contra Rotating Propellers) Azipod places a counter rotating
azipod propeller behind a fixed propeller achieving improved fuel efficiency.
In the traditional azimuth propulsion system the (electric) motor is located inside
the ship's hull and rotation is transferred to the propeller through a gearbox. In
the Azipod system the electric motor is installed inside the pod. The propeller is
connected directly to the motor shaft. No gearbox is required, thus providing
greater efficiency.

Electric power for the Azipod motor is conducted through slip rings that allow
the Azipod to turn through 360 degrees. Because fixed pitch propellers are used
in Azipods, power for Azipod is always fed through a variable-frequency drive
that allows speed control of the propulsion motor.

Azimuth thruster

An azimuth thruster is a configuration of ship propellers placed in pods that

can be rotated in any horizontal direction, making a rudder unnecessary. These
give ships better maneuverability than a fixed propeller and rudder system.
Primary advantages are electrical efficiency, better use of ship space, and lower
maintenance costs. Ships with azimuth thrusters do not need tugs to dock,
though they still require tugs to maneuver in difficult places.

There are two major variants, based on the location of the motor:

1. Mechanical transmission, where a motor inside the ship is connected to

the pod by gearing. The motor may be diesel or diesel-electric.
Depending on the shaft arrangement the mechanical azimuth thruster
are divided into L-drive and Z-drive. An L-drive thruster has a vertical
input shaft and a horizontal output shaft with one right-angle gear. A Z-
drive thruster has an horizontal input shaft, vertical shaft in the rotating
column and a horizontal output shaft with two right-angle gears.
2. Electrical transmission, where an electric motor is in the pod itself,
connected directly to the propeller without gears. The electricity is
 produced by an onboard engine, usually diesel or gas turbine. Invented in
1955 by Mr. F.W. Pleuger and Mr. F. Busmann (Pleuger Unterwasserpumpen
GmbH), ABB Azipod was the first product using this technology.

Types of mechanical azimuth thrusters

Mechanical azimuth thrusters are available as fixed installed, retractable and

underwater-mountable. Mechanical azimuth thrusters are available with fixed
pitch propellers (FPP) and controllable pitch propellers (CPP).

1. Fixed installed thrusters are used for tugs, ferries and supply-boats.
2. Retractable thrusters are used as auxiliary propulsion for DP-vessels and
take-home propulsion for military vessels.
3. Underwater-mountable thrusters are used as DP-propulsion for very
large vessels such as semi-submersible drill rigs.

Controllable pitch propellers (CPP)

Controllable pitch propellers (CPP) for marine propulsion systems have been
designed to give the highest propulsive efficiency for any speed and load
condition. When the vessel is fully loaded with cargo the propulsion required at
a given ship speed is much higher than when the vessel is empty. By adjusting
the blade pitch, the optimum efficiency can be obtained and fuel can be saved.
Also, the controllable pitch propeller has a "vane"-stance, which is useful with
combined sailing / motor vessels as this stance gives the least water resistance
when not using the propeller (eg when the sails are used instead).
While it is true that a fixed pitch propeller (FPP) can be more efficient than a
controllable pitch propeller, it can only be so at one rotational speed and the
designed load condition. At that one rotational speed and load, it is able to
absorb all the power that the engine can produce. At any other rotational speed,
or any other vessel loading, the FPP cannot, either being over pitched or under
pitched. A correctly sized controllable pitch propeller can be efficient for a wide
range of rotational speeds, since pitch can be adjusted to absorb all the power
that the engine is capable of producing at nearly any rotational speed.

The CPP also improves maneuverability of a vessel. When maneuvering the

vessel the advantage of the CPP is the fast change of propulsion direction. The
direction of thrust can be changed without slowing down the propeller and
depending on the size of the CPP can be changed in approximately 15 to 40
seconds. The increased maneuverability can eliminate the need for docking
tugs while berthing.

A reversing gear or a reversible engine is not necessary anymore, saving

money to install and service these components. Depending on the main engine
rotational speed and the size of the CPP, a reduction gear may still be required.
A CPP does require a hydraulic system to control the position of the blades. A
CPP does not produce more or less wear or stress on the propeller shaft or
propulsion engine than an FPP. Therefore this will not be an argument to
choose between an FPP or a CPP.

Most ships that wouldn't take a CPP are large vessels that make long trips at a
constant service speed, for example crude oil tankers or the largest container
ships which have so much power that a CPP is not yet designed for them. A
CPP can mostly be found on harbor or ocean-going tugs, dredgers, cruise
ships, ferries and cargo vessels that sail to ports with limited or no tug
assistance.

At the moment the range of CPP goes up to 44000 kW (60,000 hp).

Kitchen rudder

The Kitchen Rudder is the familiar name for "Kitchen's Patent Reversing
Rudders", a combination rudder and directional propulsion delivery system for
relatively slow speed displacement boats which was invented in the early 20th
century by Admiral Jack Kitchen of the British Royal Navy. It turns the rudder
into a directional thruster, and allows the engine to maintain constant
revolutions and direction of drive shaft rotation while altering thrust by use of a
control which directs thrust forward or aft. Only the rudder pivots; the propeller
itself is on a fixed shaft and does not.

"Kitchener gear" or "Kitchener rudder" have been common misnomers for the
Kitchen rudder.

It is held under British Patent 3249/1914 and US Patent 1186210 (1916) and
has been improved with the design in US Patent 4895093 (1990)
The rudder consists of a pair of slightly conical (usually but not always - designs
vary), semi-cones mounted on a pivot either side of the propeller with the long
axis of the cone running fore and aft when the helm is midships. They are
pivoted about a vertical axis such that the cone may close off the propeller
thrust aft of the propeller, directing the thrust forwards and thus creating motion
astern.

In addition to the "jaws" of the cone being controlled the direction of thrust is
also controlled by rudder direction (compare this with an outdrive or an outboard
motor for direction of thrust of an unenclosed propeller where the propeller itself
pivots).

Modern equivalent include certain types of pump jets or the jet drive.
While not strictly Kitchen rudder technology, the "clamshell" thrust reverser on
some aircraft jet engines is an aeronautical derivative of the device. The picture
of the aircraft shows the clamshells deployed directing thrust forwards. This is
equivalent to the Kitchen rudder in the "full astern" position.

The operation of the Kitchen Rudder is performed with the propellor engaged,
even when the boat is stationary.[1] The rudder is controlled by a small wheel on
the tiller.

The engine is brought up to speed with the drive to the propeller engaged and
with the Kitchen rudder in the "neutral" position. This is a position where an
equal quantity of thrust is aimed forward and aft. [2] Each vessel will have a
unique "neutral" position.

Moving ahead

The Kitchen gear is opened up to direct an increasing proportion of thrust aft. As

the balance changes the vessel will move ahead.

Moving astern

The Kitchen gear is closed to direct an increasing proportion of thrust forward.

As the balance changes the vessel will move astern