Stability of Fishing Vessels

Stability of Fishing Vessels
Half-day Awareness-raising Course

Course Aim
To improve the awareness of stability of vessels
among fishermen and safe working practices at
sea to enhance safety standards of fishing
vessels

Course contents:
1. Definition of vessel’s dimensions and terminology
2. Definition of metacenter
3. Definition of GM
4. Definition of three equilibrium conditions
5. How to improve GM
6. Tender and stiff vessels
7. Effect of weight movements and suspended weight
8. Effect of free liquid and how to reduce its effect
9. Watertight and weathertight integrity
10. Dangerous situations

1. Definition of vessel’s dimension and terminology
1.1 Main dimensions
Length over all (LOA)
• Length between two extreme ends (stem to stern)
Length between perpendiculars (Lpp)
• Distance between after and fore perpendiculars
• After perpendicular (AP), often through the rudder shaft
• Fore perpendicular (FP), often through the intersection of the design
waterline and the fore side of stem
Beam (B)
• The maximum breadth of the midship section

1. Main dimensions and terminology
1.1 Main dimensions
Length over all
(LOA)
Length between perpendiculars
(Lpp)
Design Waterline

1.1 Main dimensions
Depth to main deck (Dm)
• The height from base line to deck at side
Draught (T)
• Depth of water from the keel to the waterline
Freeboard:
• Distance from the waterline to the weather deck
• Provides reserve of buoyancy in case of flooding the vessel

1.1 Main dimensions
Depth to main deck
(Dm) Base line

1.2 Definitions and terminology
Volume of Displacement ()
• Volume of water the vessel displaces
• Measured in m3
Displacement (Δ)
• The total weight of the vessel
• The weight of the volume of water displaced by the vessel
• Measured in tones
Δ =  x ρseawater

Light ship weight (LSW):
• The actual weight of a vessel when complete and
ready for service but empty
Deadweight (DWT):
• The actual weight that a vessel can carry when
loaded to the maximum permissible draught
• Includes fuel, fresh water, gear supplies, catch
and crew
Displacement (Δ):
• The total weight of the vessel
Δ = LSW + DWT

List:
• The inclination of the vessel in transverse direction
by forces within the vessel (e.g. movement of
weight)
Heel:
• The inclination of the vessel in transverse direction
by an external force (e.g. from waves or the wind)
Loll:
• The state of a vessel which is unstable when
upright and which floats at an angle from the
upright to one side or the other

Trim:
• The inclination of the vessel in longitudinal direction
• Measured as the draught difference between forward (TFP) and aft
perpendiculars (TAP)
Lpp
TAP TFP
Trim = TAP - TFP (+ by stern)
If:
TAP = TFP  Even keel
TAP < TFP  Trim by bow (-)
TAP > TFP  Trim by stern (+)

1.3 Center of Gravity
Gravity:
• Response to the earth’s gravitational pull
• “What goes up must go down”
Center of Gravity (G):
• The point at which the whole weight of a body
can be said to act vertically downward
• It can be approximately estimated at the design
stage
• It can be accurately determined by the inclining
test, once the vessel is launched
• The position of G is measured from the keel (K).
This distance is called KG.

1.4 Center of Buoyancy
Buoyancy:
• It is a force pushing upwards
• It is equal to the displacement
Center of Buoyancy (B):
• The point to which the force of
buoyancy is considered to act vertically
upwards
• It is located at the volumetric center of
the underwater hull

1.4 Center of Buoyancy
The Center of Buoyancy (B):
• Changes for the various combinations of displacement (or draft), trim and
heel
• Oscillates when a vessel rolls
• Moves away from the centerline when a vessel’s inclination progresses to a
side

2. Definition of metacenter
Metacenter (M):
Vertical lines drawn from the center of buoyancy at consecutive
angles of heel will intersect at a point called the metacenter

3. Definition of GM
Metacentric Height or GM:
The distance between G and M

4. Definition of three equilibrium conditions

5. How to improve GM
• GM can be improved by:
• Lowering G
• Strength the keel by a heavy material
• Wheel house and above not very large and heavy
• Light vessel center of gravity should be as low as
possible
• There should not be heavy load on the deck or on the
wheel house
• Load heavy things to the bottom
• Increasing the vessel’s beam

6. Tender and stiff vessels
Stiffer vessel = Small TΦ
• A stiff vessel tends to be comparatively difficult to
heel and will roll from side to side very quickly and
perhaps violently
Tender vessel = Large TΦ
• A tender vessel will be much easier to incline and will
not tend to return quickly to the upright.
• Dangerous situation
BT drecommende 00.1, 

7. Effect of weights movement and suspended weight
The Center of Gravity:
• Moves up when a weight is moved up or is
loaded above the center of gravity of the
light vessel (= less stability)
• Moves down when a weight is moved
down or is loaded below the center of
gravity of the light vessel (= more stability)
• Moves transversally when a weight is
moved horizontally, creating an initial heel
angle or list (= less stability)

• The center of gravity of a suspended weight
can be considered to be acting at the point
of suspension (at the head of the boom)
• If not at the centerline, the vessel center of
gravity moves vertically upward and
transversely (= less stability)
• Vessels can incline to large angle if the
vessel has low stability
7. Effect of weights movement and suspended weight

• The liquid of a full tank acts like a solid
mass and does not cause any change
in G or GM
• The liquid of a partially-filled tank
oscillate with the vessel and inclines to
either site, changing its center of
gravity and affecting the vessel’s
center of gravity.

• Free surface effects are also caused by water on deck
• Collection of water on deck can be very severe because:
• It raises up the center of gravity, due to the weight of water on deck
• It further reduces the center of gravity, as it creates a large free surface
• The water on deck must be able to flow easily to the freeing ports, which must
always be clear

• How the catch is stowage may
produce an effect similar to a
free liquid

• The free surface effect is reduced by ¼ if the breath tank is divided by two
• Correct use of the freeing ports to ensure the quick release of water trapped
on deck
• Portable divisions in the holds to prevent a movement of the fish load
carried in bulk

Watertight:
• Water is not able to pass through the structure into or out of any of the
watertight compartments
• The vessel’s hull, working deck and bulkheads between compartments must
be watertight
Weathertight:
• In any sea condition, water will not penetrate into the vessel
• Hatches, side scuttles, windows and doors (or other openings on enclosed
superstructures) must be equipped with weathertight closing devices

Precautions to be taken:
• The vessel’s hull must be tight to prevent water from entering
• Closing devices to openings, through which water can enter the hull and deckhouses,
should be kept closed in adverse weather
• Any device such as doors, hatches, ventilators, air pipes, etc. should be maintained in
good and efficient conditions
• Discharge piping through bulkhead should be fitted with positively closing valves

• Undecked vessels do not have a fixed watertight
• The safety of undecked vessels is improved by fitting them with sealed
buoyancy compartments, which are filled with solid buoyancy material
• The vessel should stay afloat and on an even keel without listing even if the
vessel is fully swamped

10. Some dangerous situations
• Enclosed superstructure and means of closing
• Securing of heavy material
• Stowage of the catch
• Overweight on deck
• Effect of fishing gear on the stability
• Free surface effects
• Freeboard and loading conditions
• Following and quartering seas
• Crossing sand bars and beach landings
• Alterations to vessels

• Enclosed superstructure and means
of closing
• Securing of heavy material
• Stowage of the catch

• Overweight on deck or roof:
• Items on deck or roofs will move the G upwards
• A risk of weight shift
• Items on deck can stop water freeing from decks

• Effect of fishing gear on the stability
• Particular care should be taken when pull from fishing gear might have a
negative effect on stability
• The heeling moment caused by the pull from the fishing gear will cause the
vessel to capsize
• Risky factors:
• Heavy fishing gear, powerful winches and
other deck equipment
• High point of pull of the fishing gear
• Increased propulsion power (trawlers)
• Adverse weather conditions
• Vessels hanging fast by its fishing gear

• Free surface effect reduces the vessel’s
stability:
• Minimize the number of tanks which are not fully
• Fish in hold can have free surface effect if it is not
in compartments
• Minimize the water on deck, ensuring the quick
release of water trapped on deck by the freeing
ports

• Freeboard and loading conditions:
• Maintain adequate freeboard in all
loading conditions as it is used as a
reserve of buoyancy
• Use the Stability Notice, provided to the
satisfaction of the competent authority

• Following and quartering seas:
• Stability can be considerably reduced when the
vessel is traveling at a similar speed and direction
as the waves
• If excessive heeling or yawing (change of heading)
occurs, the speed should be reduced and/or the
course changed

• Crossing sand bars and beach landings
• Operation of vessels from unprotected beaches requires special skills and
special care should be taken in surf zones

• Alterations to vessels:
• Conversion to new fishing methods
• Changes in the main dimensions
• Changes in the size of the superstructures
• Changes in the location of bulkheads
• Change in the closing appliances of openings
through which water can enter into the hull
or deckhouses, forecastle, etc.
• Removal or shifting, either partially or fully,
of the permanent ballast
• Change of the main engine
To be approved by the
competent authority!!

Stability of Fishing Vessels

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