Fishing Vessel Stability Training

Stability of Fishing Vessels
3-Day Training of Trainers Course

Introduction
Aim:
To improve the awareness of stability of vessels among fishermen and
safe working practices at sea to enhance safety standards of fishing
vessels.
Objectives:
i. Trainees should be able to demonstrate and explain all aspects
governing the stability of fishing vessels.
ii. Trainees should be able to conduct theoretical and practical
demonstration classes to fishermen on fishing vessel stability.

Topics to be covered: 1st day
1. Main dimensions and terminology
2. Definition of center of buoyancy
3. Definition of center of gravity
4. Hull form and lines plan
5. Simpson’s Rules and estimation of areas
6. Hull form coefficients
7. Definition of initial metacenter

Topics to be covered: 2nd day
8. Hydrostatic particulars and their estimation
9. Intact stability and three equilibrium conditions
10. Tender and stiff vessel
11. Effect of weight movement on G and initial stability
12. Effect of loading/unloading a weight on G and initial stability
13. Effect of free liquid

Topics to be covered: 3rd day
14. Stability at large angles of heel and GZ
15. Assessment of intact stability and stability criteria
16. Effect of vessel’s parameters on GZ
17. Effect of weight movement on GZ
18. Watertight and weathertight integrity
19. Some dangerous situations
20. Preparation and trainee of “Half-day Course for Fishermen”

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 moulded (Bm)
• The maximum breadth of the midship section
• For GRP boats with shell thickness

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
• Below the weather deck the hull is watertight
• 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

Inclination angle
Φ
*Measure the inclination angle:

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 (+)

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

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

Topics to be covered: 1st day
5. Simpson’s Rules and estimation of areas and volumes
6. Hull form coefficients

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.

The Center of Gravity:
• Moves up when a weight is loaded
above the center of gravity of the
light vessel
• Moves down when a weight is
loaded below the center of gravity of
the light vessel

4.1 Profile plan, waterline plan and body plan
Lines drawing are composed of three different sectional views:
• Profile Plan
• Waterlines Plan
• Body Plan
Each of them is developed by an intersection of parallel planes through
the hull
And the sectional curves are projected into the plane of figure

4.1 Profile plan, waterline plan and body plan
Waterlines Plan
(waterline)
Profile Plan
(buttock line)
Body Plan
(station)

4.2 Lines plan of a fishing boat

4.3 Use of lines plan in stability assessment
Offset tables:
• Used to calculate the particular of the underwater hull: displacement,
location of center of buoyancy, waterplane areas, etc.
• Shows the half breadth values at each intersection point of stations and
waterlines
Stations
Waterlines

4.3 Use of lines plan in stability assessment
Obtaining the half breadth values:

5.1 Simpson’s First Rule (1-4-1 Rule)
Any curve is approximated by a set of parabolas fitted through three adjacent,
equally spaced, data points of the function (odd number of points).
The area under the fitted parabola is estimated as:
 cba
h
Area  4
3

5.2 Simpson’s Second Rule (1-3-3-1 Rule)
Any curve is approximated by a set of parabolas fitted through four adjacent,
equally spaced, data points of the function (even number of points).
The area under the fitted parabola is estimated as:
 dcba
h
Area  33
8
3

5.3 Estimation of waterplane area (AWP)
 
 
 432121
4322
211
424
3
4
3
4
3
bbbbb
h
AA
bbb
h
A
bbb
h
A
T
T



 87654321 4242424
3
2 bbbbbbbbb
h
A TBWP 
Odd number of points  Simpson’s First Rule

6. Hull form coefficient
6.1 Block Coefficient (CB)
• It is the ratio of the volume of
displacement to a given waterline () to
the volume of a rectangular box (LPPBmT)
• CB can be used to estimate 
TBL
C
mPP
B


)( TBLC mPPB 

6.2 Prismatic Coefficient (CP)
• It is defined as the ratio between the
volume of displacement () and the
volume of the prism whose cross-section
is the same as the midship section (AMLPP)
• High speed boats have a lower CP
PPM
P
LA
C



6.3 Midship Section Coefficient (CM)
• It is the ratio of the immersed area (AM) of the midship section to the
area of the circumscribing rectangle (BmT)
TB
A
C
m
M
M 

6.4 Waterplane Coefficient (CW)
• It is the ratio of the waterplane area (Aw) to the area of the
circumscribing rectangle (LPPBm)
mPP
W
W
BL
A
C 

Transverse Stability:
• When a vessel is floating upright, B and G
will be vertically above K
• If the vessel is inclined by an external force,
B moves to B1
Metacenter:
• Vertical lines drawn from the B1, at small
angles of heel, will intersect at the
metacenter (M)
• The height of M is measured from K and is
called KM
• It is a fixed point (M0) until the angle of heel
reaches 5 or 6 degrees.

8.1 Main hydrostatic properties
T Draft
 Volume of displacement
Δ Displacement
zB (or KB) Vertical center of buoyancy
xB Longitudinal center of buoyancy
AW Area of waterplane
xF Longitudinal center of flotation
KM Transverse metacenter above base line
MTC Moment to change trim by 1cm
TPC Tones per cm of immersion

8.1 Main hydrostatic properties

8.2 Hydrostatic particulars in graphical form

8.2 Hydrostatic particulars in tabular form

9. Initial stability and three equilibrium conditions
9.1 Intact stability definition
• Means stability of intact (undamaged) vessels
• It is divided into:
• Initial Stability:
• Stability at upright condition or small angles of heel (up to 5 or 6 degrees)
• Defined by GM
• Large Angle Stability:
• Stability when a vessel is inclined to a large angle
• Defined by GZ

9.2 Equilibrium conditions
Stable equilibrium:
• If, when inclined, the vessel tends to return to the
upright
• M above G, positive GM
Unstable equilibrium:
• If, when inclined, the vessel tends to incline further
• M below G, negative GM
• A vessel in this state has loll, there is a danger to
capsize
Neutral equilibrium:
• If, when inclined, the vessel has no tendency either
to return to original position or further incline
• G and M coincide, zero GM

*Unstable equilibrium:
• Vessel can incline to any side and be in equilibrium at an angle
• The equilibrium angle is called loll angle
• The loll angle increases depending on how large the negative GM is

9.3 Estimation of GM
KGKMGM 
BMKBKM  Known
From:
• The hydrostatic particulars or;
• Calculated using the Simpson’s Rules 
 TI
BM
From:
• The hydrostatic particulars or;
• Calculated using the Simpson’s Rules
*IT: Second moment of area of the waterplane about the centerplane

9.4 How to improve GM: For a given hull shape
• Minimum recommended GM for a fishing vessel is 0.35m
• For a given hull shape, KB and BM are fixed at a particular draught
• GM can be improved by lowering G (reducing KG):
• 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
KGBMKBKGKMGM 

9.4 How to improve GM: Changing the beam
• Minimum recommended GM for a fishing vessel is 0.35m
• If the beam is changed (BWL), KB and KG remain practically constant
but BM is modified:
• By making the boat 10% more beamy, BM increase by 21%
• By making the boat 20% more beamy, BM will increase by 44%
• The effect of change of B on BM is:
KGBMKBKGKMGM 
2









original
new
change
B
B
BM

10.1 Relationship between GM and TΦ
The rolling period of the boat (TΦ) can be calculated empirically by:
For GMmin (0.35m): For GM = 0.64m:
GM
B
T
80.0

BT GM 35.1min, 
BT mGM 00.164.0, 
TΦ recommended

10.2 Definition of stiff and tender vessel
Stiffer vessel:
• High GM, small TΦ
• A stiff vessel tends to be comparatively difficult to heel and
will roll from side to side very quickly and perhaps violently
• A vessel with very high GM may be uncomfortable for crew
on board because it roll faster
Tender vessel:
• Low or zero GM, large TΦ
• A tender vessel will be much easier to incline and will not
tend to return quickly to the upright.
• The time period taken from side to side will be
comparatively long
• This condition is not desirable, can be corrected lowering G

11.1 Effect of vertical movement of a weight
• When a weight is moved up, G is also moved up and GM is reduced
(as M is fixed at small heel angles)
• The difference between G0 and G1 and the new GM is:
h
oG
1
o
G
B
K
W
W
K
B
G
o
o
oM Mo



hW
GG 10 100001 GGMGMG 

11.2 Effect of transverse movement of a weight
• When a weight is moved transversely, G moves also transversely.
• Then, the vessel incline at the same direction until the equilibrium is
reached.
• The vessel can capsize if the movement of G is high.
• The difference between G0 and G1 and the angle reached is:
0000
10
tan
MG
dW
MG
GG






dW
GG 10
o
o
G
B
K
W W
K
B
G
o
1Go
d
Initialy Upright
d
o
G
1
o
G
B
K
W
B1


oMMo
o
o
G
B
K
W
W
K
B
G
o
2
Go
1G G1
o
G 2
o
G
B
K
WMo
B1
h

Initialy Upright
• G moves vertically and transversely.
• The vertical movement of G will affect the GM
• The transverse movement of G will affect the inclination
11.3 Effect of general movement of a weight
hWMG
dW
GGMG
GG





001000
21
tan



dW
GG 21



hW
GG 10
100001 GGMGMG 

• 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
• Vessels can incline to large angle if the
vessel has low stability (low GM)
11.4 Effect of suspended weight

• The objective of the inclined test is obtaining the center of gravity of
the light ship
11.5 Inclined test
oM
W
K
G1GG
0
K
W
Mo
W
o
Pendulum
oB B1
Battern
W d
e
l
Bo
Before moving a weight After moving a weight
tan
tan 1





 
dW
GM
l
e
GMKMKG 

• G moves towards the added weight or moves
away towards the removed weight
• If the weights added are top weights, G will
rise and GM will be reduced creating a
dangerous situation.
12. Effect of loading/unloading a weight on G and initial
stability

• 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.
13.1 Effect of free liquid in tanks to GM

• The vessels center of gravity oscillates as the tank liquid moves,
and G1 can be the center of gravity at an instance of small
inclination of the vessel
• Then, the effective center of gravity of the vessel moves up to GF
and, hence, the effective GM reduces by G0GF
oM
K
Go 1GoG
K
Mo
g g1
F
G

• G0GF is called the “free surface correction” and is equal to:
Where:
• ρf the density of the tank liquid
• If the second moment of area of the free surface of the liquid about the centerline of
the tank. For a rectangular tank of length lf and breadth bf:
oM
K
Go 1GoG
K
Mo
g g1
F
G


ff
F
i
GG

0
12
3
ff
f
bl
i



• Significantly depends on if, and especially on bf, as ρf and Δ remain constant for a particular loading
condition
• Tanks are subdivided to reduce the free surface effect:
13.2 Reducing the effect of free liquid
12
3
ff
f
bl
i

 






 








 

124
1
12
)2/(
2
33
ffff
f
blbl
i
As a single tank of breadth bf
As a divided tank of breadth bf /2


ff
F
i
GG

0

• 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
13.3 Ingress of water on deck

14.1 Definition of righting lever (GZ)
• When heeled by an external force, the vessel’s weight acts vertically
downward through G
• B has moved to B’ and the buoyancy force acts vertically up through B’
• The horizontal distance from G to the vertical line from B’ is called the
righting lever or GZ
The force involved in returning the vessel to the
upright position is the weight of the vessel acting
down through G multiplied by GZ
(moment of statical stability)

14.1 Definition of righting lever (GZ)
• The lower G, the bigger is GZ
• If G is near the metacenter, the vessel will have only a small metacentric
height (GM) and the righting lever (GZ) will also be a small value.

14.2 Calculation of GZ and the cross-curves
• For a small angle of heel:
• M is constant, the following relation
between GM and GZ can be stated:
sin GMGZ
• For large angles (or in general):
• The metacenter no longer remain fixed. Therefore, the metacenter height is not
used to study the stability at large angle of heel
• GZ is calculated using the “cross-curves”

• The righting lever can be calculated from the
following equation:
• Where KN are the so called “cross-curves of
stability”
1B
B
K
o
G
B
Z
N
sin KGKNGZ

• KN (or LK) only depend on the hull geometry (not on G) and are
usually given on tabular or graphical form

14.3 Curve of statical stability
• GZ varies with the heel angle
• GZ initially increases and tend to decrease subsequent to deck
immersion
Stability curves
(GZ curves)

14.3 Curve of statical stability
• GZ curves show graphically the stability levers (GZ) as a function of heel
• General characteristics:
(a) GM
(b) GZmax
(c) Point of vanishing
stability
(d) Flooding angle
(e) Range

15.1 Estimation of Δ, KG and GM
• The steps are:
1. Calculate the sum of deadweight items and its center of gravity:
• Fuel
• Fresh water
• Fishing gear
• Crew, stores, etc.
2. Calculate the total displacement (Δ) and its center of gravity (KG) by adding:
• Light ship weight
• Deadweight
3. Calculate GM

15.1 Estimation of Δ, KG and GM
Item Mass (Kg) KG (m) Mkeel
(m)
Lightship 8345 1.70 14187
Ice 7200 1.05 7560
Fuel 4500 1.38 6210
F.W. 2200 1.52 3344
Fishing Gears 2300 2.30 5290
Stores 350 3.00 1050
Provisions 250 2.10 525
Crew 240 2.50 600
25385 1.53 38766
320.0527.1847.1
527.1
385.25





KGKMGM
m
M
KG
t
keel

15.2 Estimation of GZ
Angle of heel KN KG·sinΦ GZ
Φ (deg) (m) (m) (m)
0 0.0000 0.0000 0.0000
2 0.0669 0.0533 0.0145
5 0.1675 0.1309 0.0366
10 0.3362 0.2609 0.0754
15 0.5081 0.3888 0.1193
20 0.6795 0.5138 0.1657
25 0.8483 0.6348 0.2135
30 1.0081 0.7511 0.2570
40 1.2683 0.9656 0.3027
50 1.4327 1.1507 0.2820
60 1.5261 1.3009 0.2253
70 1.5601 1.4115 0.1486

15.3 IMO Criteria for fishing vessels

1. The Area A under the GZ curve from 0 to 30° must not be less than 0.055 m·rad
2. The Area A+B under the GZ curve from 0 to 40° or Φf (whichever is the smaller) must
not be less than 0.090 m·rad
3. The Area B under the GZ curve from 30 to 40° or Φf (whichever is the smaller) must
not be less than 0.030 m·rad

4. The minimum upright GM value must not be less than 0.35 m
5. The angle of heel, Φx, for the maximum GZ must be at least 25°, and preferably
in excess of 30°
6. The GZ should be at least 0.20 m at an angle of heel equal or greater than 30°

16.1 Increasing beam at constant draft and freeboard
• Higher values of GM and GZ
• The point of vanishing stability will be less, the vessel will capsize at a
smaller angle of heel

16.2 Increasing freeboard at constant draft and beam
• Higher values of GZ
• The point of vanishing stability will also be higher

17.1 Vertical movement of a weight
• Raising G causes a decrease in GM and, thereby, smaller values of GZ

• If G is above M, the vessel is in an unstable situation (negative GM)
• The vessel will capsize or float at an angle from the upright to one side
(loll angle)

• If a weight (W) is moved a distance h vertically upward, the new G1
will be:
• And the new GZ curve will be:



hW
GG 10
sin100011  GGZGZG

17.2 Transversal movement of a weight
• Moving a weight transversally creates an initial heel angle (angle of list) and
decreases the GZ values

17.2 Transversal movement of a weight
• If a weight (W) is moved a distance d transversally, G0 will move to G2:
• And the new GZ curve will be:



dW
GG 20
cos200022  GGZGZG

17.3 Changes in the stability curve during a voyage
A fishing vessel’s stability constantly changes during its voyages,
depending on how the vessel is loaded and operated

17.3 Changes in the stability curve during a voyage
Suitable stability information, to the satisfaction
of the competent authority, should be provided
to enable the skipper to easily assess the stability
of the vessel under various operating conditions

18.1 Watertight and weathertight definitions
Watertight:
• The structure is designed and constructed to withstand a static head of water
without leakage
• 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

18.2 Watertight and weathertight integrity
Precautions to be taken to maintain watertight and weathertight integrity:
• 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

18.3 Built-in buoyancy for undecked vessels
• Undecked vessels do not have a fixed watertight and will therefore not
have the watertight and weathertight integrity of decked vessels
• 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

The main items are:
• Listing
• Wrong loading
• Overweight on deck
• Free surface
• Fishing gear effects
• Open hatches and doors
• Following and quartering seas
• Crossing sand bars and beach landings
• Alteration to vessels

19.1 Listing
• Listing to one side can be due to:
• The center of gravity is not at centerline
• Shift weight transversally to higher side
• Add weight to higher side
• Remove weight from low side
• The metacentric height (GM) is near to zero
• Eliminate free surface
• Add low weight symmetrically about centerline
• Remove high weight symmetrically
• Shift weight down symmetrically

19.2 Wrong loading
• It is very important to:
• Load the boat evenly
• Secure the load fastened:
• Fishing gear on deck
• Ice in fish hold
• Fish in holds
• Shifting of heavy weight

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

19.4 Free surface
• 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

19.5 Fishing gear effects
• 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 if it is larger than the righting moment
• Factors that increase the heeling moment:
• 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

19.6 Open hatches and doors
• All hatches, doorways, side scuttles and port deadlights, ventilators
and other openings through which water can enter into should be
kept closed in adverse weather condition
• When the vessel is heeled by an external force, a substantial part of
its buoyancy comes from enclosed superstructures, which must be
fitted with appropriate closing appliances

19.7 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

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

19.9 Alteration 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!!

20. Preparation of trainee of
“Half-day Course for Fishermen”
20.1 Relevant notes and demonstrations
to be performed
20.2 Ways of imparting the knowledge of trainers
to fishermen

Fishing Vessel Stability Training

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