# Stability What is a moment The attempt of a force to turn a body

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Stability

What is a moment
The attempt of a force to turn a body. It is usually measured by the product of the
force and the length of lever

Weight x distance = moment          Wt x KG

Total moments = New KG
Total weight

Metacentric Ht      GM
A measure of the ships stability this is the distance between the center of gravity and
the metacenter of the vessel

Metacenter.
The point at which the vertical line through the center of buoyancy at a small angle
of heel cuts the ships center-line. It is only considered to exist for angles of up to 15°.

Inclining Experiment
This performed to find the ships light “ GM” and her light”KG” It consists of
shifting weights transversely across the deck of a ship when the latter is free to heel
The angle of heel is measured by the shift of a plumb bob along a batten.
The formula for the light “GM”
GM = w x d x CF        w= weight shifted d= distance CF = length of plumb bob
W       FL     w= vessel displacement FL = distance moved out from center

Dynamical Stability
It is the measure of work that is required to heel a ship it is a factor in determining
how a ship will roll. Thus the waves do the work Simpson’s Rules are used to find
the value of dynamical stability.

Statical Stability
This the moment, which will try to return the ship to, its upright positions when it is
heeled.
W x GZ = moment of statical stability

Free Surface.
When a liquid is run into or removed from a tank so as to cause free surface it will
affect the ship stability in two ways 1. The solid KG and displacement will be
changed by the weight added or removed. 2 there will be a rise of G caused by the
free surface this will have to be added to the new solid KG to give the fluid KG

Formula      see Purseys text.

What happens to your ship during a voyage as fuel and water are consumed
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Weight is being removed therefore G is rising towards the metacenter thus the GM
is getting smaller so you add water ballast to restore the GM.
G moves toward weight added and away from weight removed

If you had the same drafts for 2 voyages would the GZ curve be good for the second
voyage .No because although the drafts are the same the ship would be loaded
differently thus a different original GM.

What is the stowage factor?
The stowage factor of any commodity is the figure which expresses the number of
cubic feet which a ton will occupy in stowage, not the actual cubic feet measurement
of a ton and should include allowance for broken stowage and dunnage.
Vol. = amount in tons
S.F.,
You can find a list of stowage factors in Thomas Stowage.

What are cross curves?

These are another common method of drawing graphs of righting levers they are
different from the ordinary curves of statical stability however because each curve
shows the righting levers for one angle of heel only but at different displacements

What is Synchronism of wave action on your vessel?

(Purseys page 151 & 163)
Occurs when the ships period of roll is the same as that of the waves .the waves give
your ship a push each time the ship rolls in the direction in which the ship is rolling
thus the ship rolls more and more heavily.

What are racking stresses?

The stress occurs on the corners of the ship, tank side brackets and beam knees.
Transverse bulkheads and web frames or cantilever frames provide resistance to
this stress.

Principle Structural Stresses
Hogging and sagging rocking effect of water pressure and dry-docking

Principle Local Stresses
Panting pounding effect of local weights and vibration

Effect of water pressure
Water pressure tends to push in the sides of the vessel and the bottom of the ship
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Panting
Is an in and out motion of the plating in the bows of a ship and is caused by unequal
water pressure as the bow passes through successive waves It is the greatest in fine
bowed ships

Pounding when a ship is pitching her bows often lift clear of the water and then
come down heavily most in full bowed ships.

Tackle Regulations

Explain in detail annual and quadrennial inspections and who may perform them
All lifting machinery must be inspected annually by a competent person.

Competent person: a tackle inspector a steamship inspector or a surveyor employed
by a classification society acceptable to the board . A responsible person having the
necessary technical qualifications any person having a Certificate of Competency as
master or mate.

Quadrennial Inspection: of all lifting machinery shall be given a thorough
examination at least once every four years by a competent person.
It shall include a hammer test, drilling, opening of blocks shackles or machinery the
lifting of goosenecks and other such tests as are necessary to determine the condition
of the lifting equipment.

Competent person for quadrennial inspections are tackle inspectors a steamship
inspector or a surveyor acceptable to the board a responsible person having the
necessary technical qualifications employed by a testing lab.or a firm engaged in the
manufacturing or repair of the gear concerned
Any approved person firm in association.

Safe working loads for various cargo gear components and the proof loads

Schedule 1
Every winch with the whole of gear accessory thereto including derricks goosenecks
eye plates eyebolts or other attachments shall be tested with a proof load that shall
exceed the safe working load as follows.

Up to 20 tons                            25 % in excess
20 to 50                                5 tons     “
over 50                                 10 % in excess

Every crane and other hoisting machine with its accessory gear shall be tested with
a proof load that shall exceed the safe working load as follows

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Up to 20 tons                              25 % in excess
20 to 50                                  5 tons    “
over 50                                   10 % in excess

Chains, rings, hooks, shackles, and other loose gear whether accessory to a machine
or not shall be tested with a proof load equal to that shown against the article in the
following table

A) chain, ring, hook, shackle or swivel              100% in excess of the SWL

B) pulley blocksingle sheaves                        300 %            “
c) multiple sheaves block with SWL 20 tons           100 %           “
d) multiple sheave block with SWL 20 to 40            20%             “
e)             “                “ > 40                50%             “
f) pitched chains used with hand operated pulley
blocks and rings hooks shackles or swivels
permantly attached thereto                         50% in excess of the SWL
g) hand operated pulley blocks used with \
pitched chains and rings hooks shackles or
swivels permantly attached thereto                         “

Safety factors for all metal structural parts used in lifting machinery
SWL          10 tons or less                   5
SWL          over 10 tons                      4
For wooden structural parts used
In lifting machinery                          8
Chains                                        4½
Wire rope                                     5
Fibre rope                                    7

Every derrick or crane shall have the SWL plainly marked on it
If one SWL is marked on a derrick or crane it shall be the SWL with single
purchase only and if two SWL are marked the first shall be the figure with single
purchase and the second shall be for a double purchase.

Where a derrick has been certified and tested for operations in union purchase the
SWL shall be marked SWL (u) to indicate that this is the SWL in union purchase.

In each case referred to in subsections the SWL shall be the load at the lowest
inclination of the boom.
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Doubling a whip, this converts a single whip to a double whip and creates a
mechanical advantage. Sufficient wire must be available to reach the farthest part of
the worked compartment. If the second spider band shown in the drawing is not
available the end of the whip is turned around the derrick just below the head block
and shackled to the existing spider band. The derrick should be parceled with
canvas in way of the whip to prevent slipping and chafe. The method is used when it
is desired to lift a weight greater than the SWL of the fall.

Precautions when handling heavy lifts

All gear must have valid inspection certificates in good order and of adequate
strength to comply with the SWL and tackle regulations.
The stability of the ship must be checked so as to make sure that the rise in G will
not produce a negative GM. When lifting the critical moment occurs is when the
load is lifted off the deck the center of gravity of the load is at the head of the
derrick. If the ship had a small GM the GM may become negative and the vessel
may settle into an angle of loll. Either empty or fill as many tanks as possible, add
ballast or shift weights in order to maintain a positive GM. The vessel must be
upright and on an even keel. The mooring lines must be attended to and kept tight
so the vessel does not range along the dock, alert all crew of what is happening.
Have competent crew attending the winches, lines etc. If the lift is coming aboard
the deck might have to be shored up requiring dunnage proper lashing and secured
tight.

What do the tackle regs say about rigging a gangway between 2 ships.or the ship
and the shore?
When ships are alongside one another safe means shall be provided for the safe
passage of workers between the ships unless conditions are such that the workers
would not be exposed to undue risk if no special appliance were provided, the safe
means shall be provided by the vessel with the highest freeboard. Safe means shall
be provided for passage of workers to and from a ship and the wharf at which the
ship lies safe means required where practicable a ships accommodation ladder or
gangway similar construction not less than 560 mm wide properly secured and
securely fenced to a clear height of not less than 3 ft by means of an upper and lower
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railing rope or chain. If the accommodation is against the ships side then railing
need only be on one side.

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