OBJECTIVE

To determine the Lightship Particulars of the vessel, which include Lightship displacement and center of gravity (VCG, LCG) of the vessel.

PREPARATIONS

1.      Minor items to go onboard and off board shall be identified and minimized. Details like weight, LCG, VCG of those items to be collected. Trim of the vessel should be minimized.


2.      All tanks are preferred to keep empty. In case of practical difficulties with some of the tanks, they can be kept pressed full. Sounding table should be available for slack tanks. Number of slack tanks should be minimized to reduce the effect of FSM.


3.      The depth of water under the hull should be sufficient to ensure that the hull will be entirely free of bottom. Prior to the experiment the water depth is to be measured. The mooring ropes will be loose enough for easy movement of the vessel.


4.      Weight transfer: As a minimal prerequisite for acceptability, the following conditions would be required:


a.       Sand Bags or Iron weights with pre-weighted weights will be used for inclining experiment. The final weight will be confirmed before inclining experiment.


b.      The weights will be divided in to four (4) groups of equal weights in each group and to be kept on main deck at same transverse distance from longitudinal centerline of main deck.


c.       Mechanical shifting for the weights will be used.


d.      All inclining weights would be recorded before and after each shift and longitudinal and transverse centers should be calculated for each movement.


e.       The ship’s initial heel and trim angle should be established prior to the inclining. The draft marks (port and starboard) should be used when establishing the initial heel angle.


f.       The time to conduct the inclining would be evaluated.


5.      Two pendulums (one fwd and one Aft of the vessel) shall be used to measure the deflection of the vessel. The length of the pendulums should be long enough to get accurate results. Pendulum wire should be of piano wire or other monofilament material. One small tank filled with viscous oil to be arranged below each pendulum to dampen the movement of the pendulum bob. A calibrated batten or paper shift shall be arranged on both the tanks to measure the deflection of pendulum string.


6.      A small boat shall be available to aid in taking the draft mark reading.

ESTIMATION OF LIST ANGLE

Weight transfer details:

Transfer of weights:                             X kg. Required per shift
Total inclining weight required:          4X kg.

List measurement:

Two pendulums installed one Aft and one Fwd for measuring the list of the vessel. Piano wires should be used to hang the pendulum. Calibrated battens or paper shifts are fixed on the tanks filled with viscous oil for damping.

Estimated List Angle:

Estimated list angles during shift 1 and 2 are ____ and ___ degree respectively.

Pendulum details:

Aft pendulum: 

Location - Fr. ___
Length - _____ mm

Fwd pendulum:

 Location - Fr.___
 Length - ______ mm

Both the pendulums should be located at the Centerline of the vessel
Anticipated pendulum reading will be about ___ mm (each shift) and ___ mm (total shift) respectively for various shifts for above pendulum length.

PROCEDURE

Initial Walkthrough and Survey:

The person responsible for conducting the inclining test should arrive on board the ship well in advance of the scheduled time of the test to ensure that the ship is properly prepared for the test. If the ship to be inclined is large, a preliminary walk through may need to be done the day preceding the actual incline. To ensure the safety of personnel conducting the walk through, and to improve the documentation of surveyed weights and deficiencies, at least two persons should make the initial walk through. Things to check include: all compartments are open, clean, and dry, tanks are well ventilated and gas free, movable or suspended items are secured and their position documented, pendulums are in place, weights are on board and in place, a crane or other method for moving weights is available, and the necessary plans and equipment are available

Experiment Procedure:

Four groups of weights weighing X kg each will be kept on either sides of the vessel (port and starboard).
The width of the vessel at main deck is __ m. The center-to-center distance between the weights to be shifted transversely would be about __ m. The weights will be re arranged accordingly subject to availability of empty space and in same line in longitudinal direction. The centerline of the weights will be at about 1.5 m from longitudinal centerline of the vessel.
Once the vessel is ready for the experiment the draft readings of the vessel to be noted using the small boat arranged for the same. The specific gravity of the seawater is also measured using hydrometer. Now the initial reading can be marked on the paper strip fixed perpendicular to the fore and aft pendulum of the vessel [initial] reading should be marked on paper shift at the same time.

WEIGHT SHIFT SEQUENCE

Let each weight be X kg.

Reading 0

	Aft side	Fore side
Port  Side	X	X
Starboard Side	X	X

Reading 1

	Aft side	Fore side
Port  Side	X	0
Starboard Side	X	2X

Reading 2

	Aft side	Fore side
Port  Side	0	0
Starboard Side	2X	2X

Reading 3

	Aft side	Fore side
Port  Side	0	X
Starboard Side	2X	X

Reading 4

	Aft side	Fore side
Port  Side	X	X
Starboard Side	X	X

Reading 5

	Aft side	Fore side
Port  Side	X	2X
Starboard Side	X	0

Reading 6

	Aft side	Fore side
Port  Side	2X	2X
Starboard Side	0	0

Reading 7

	Aft side	Fore side
Port  Side	2X	X
Starboard Side	0	X

Reading 8

	Aft side	Fore side
Port  Side	X	X
Starboard Side	X	X

 

CALCULATION PROCEDURE

A shift of inclining weight or in other words an inclining moment due to such a shift may be related to a ships stabilizing moment as follows.

w x d = Δ x GM Tan θ (1)

Of all the quantities in the above equation, only GM is the unknown.

Thus,

GM = w x d /( Δ x Tan θ) (2)

This is the governing equation for an inclining experiment. Thus we relate the inclining moment w x d to GM and isolate GM from the exercise. Once GM is known, we may easily deduce KG (VCG) by relating the height of metacenter KM to GM as

KM = KG + GM (3)

KM can be obtained from the hydrostatic tables. GM is deduced as explained earlier. The KG can be determined as

KG = KM – GM (4)

This KG is the effective KG inclusive of the weights to go off board. This KG is corrected for,

i. Weights to go off board

ii. Weights to go onboard

iii. Weights to be re located

This will yield the actual lightship particulars of the ship. The weights to go off board shall consist of people onboard, any equipment to be offloaded, partially or fully filled tanks and the inclining weights (if a tank is partially filled its weight & FSM shall be deducted). The weights to go onboard shall contain those things that are part of the ship but currently off board.

DATA ACQUISITION

All data acquired during the test should be initialized by both the person conducting the test as well as the surveyor as an indication of their mutual agreement on the data. The following data are to be recorded and presented.

Before inclination experiment

1. Detail of inclining weights (LCG, VCG and TCG)
2. Distance of shift
3. Sequence of shift
4. Length, location of pendulums
5. Condition of tanks
6. Weights to go off board, onboard.
7. No. of persons present.
8. Draft mark location and distances from AP and FP.
9. Location of AP and FP.

Start of Inclination experiment

1. Draft readings
2. Condition of mooring ropes
3. Density of water
4. Temperature
5. Weather condition like wind etc.

During inclination experiment

1. Pendulum deflection for each shift duly numbered and identified with pendulum

After inclination experiment

1. Density of water.
2. Draft readings.

CALCULATION SHEET - I

Deflection readings:

Pendulum readings were marked on the batten boards. Initial positions were also marked. After each movement of the weights, pendulum deflection readings were recorded. The vessel was verified to be floating free before recording the pendulum readings.

Deflection of each of the three pendulums was measured. The measured deflections and the calculated tangents are shown below table below.

Based on final average Tangent value of all 8 readings, the GM and then KG values are calculated as mentioned earlier.

Shift	P	S	Weight(P)	Weight(S)	Dist	Def	tanθ	GMT
			t	t	M	M	-	m
0	2,4	1,3
1	4	1,2,3
2	-	1,2,3,4
3	1	2,3,4
4	1,3	2,4
5	1,2,3	4
6	1,2,3,4	-
7	2,3,4	1
8	2,4	1,3

 

CALCULATION SHEET - II

Lightship Calculation:

From the calculated vessel displacement and the calculated CG, corrections were made for the weights to deduct, weights to add and weights to relocate, to arrive at the lightship weight from the incline as per following table.

Description	Weight	LCG	TCG	VCG	Lmom	Tmom	Vmom
	t	m	m	m	t-m	t-m	t-m
Lightweight
Inclining Weight4
Inclining Weight3
Inclining Weight2
Inclining Weight1
Men on Board
Displacement

Hydrostatics at inclining condition:

Trim		m
LCB		m
MCT 1cm		tm/cm
Def		m
Heel (per shift)		deg
Length of Pend		m
GM		m
KMT		m
KG		m