FACTORS AFFECTING STABILITY
(Ranger Hope © 2008, contains edits of material courtesy of A.N.T.A. publications.)
Practical Aspects of Stability
When a weight is lifted by a crane or derrick, the centre of gravity of the weight will be immediately transferred to the point the weight is suspended from (the head of the crane or the end of the derrick or boom). This occurs the instant the weight is lifted and from that point on the centre of gravity will not change further no matter how high the weight is lifted.
Figure 1
Consider the sequence (shown below) of events that occur when a vessel lying port side to a wharf discharges a heavy weight from the starboard lower hold by means of the vessel’s crane.
Figure 2
Figure 3
Lifting
As soon as the weight is clear of the deck and is being borne by the crane head, the centre of gravity of the weight appears to move from its original position, to the crane head (g to g1). In Fig 3 G the original position of the vessel’s centre of gravity, moves upward to G1 parallel to gg1. The centres of gravity will remain at G1 and g1 during the whole of the time the weight is being raised.
Figure 4
As the crane begins to swing the centre of gravity of the weight will remain at the head of the crane (g1). The vessel’s centre of gravity (G1) will begin to move out towards G2, parallel to the movement of weight and the vessel will begin to list. (Fig 4)
Lowering
Figure 5
The crane has now swung over to plumb the wharf and the boom is lowered. The crane head has moved from g1 to g2 and since the weight is suspended from the crane head, its centre of gravity will have also moved from g1 to g2. The vessel’s centre of gravity has also moved parallel to the weight, from G1 to G2. Maximum list will be experienced at this point. (Fig 5)
Landing
Figure 6
The wire is now lowered and the weight is landed on the wharf. It is in effect being discharged from the crane head and the vessel’s centre of gravity will move from G2 to G3 in a direction directly away from g2. G3 is therefore the final position of the vessel’s centre of gravity. The net effect of discharging the weight is a shift of the vessel’s centre of gravity from G to G3, directly away from the centre of gravity of the weight finally discharged (g1). (Fig 6)
It should be clear now that a vessel must have adequate stability before suspending weights from its derrick or crane. If the shift in the CG of the vessel is large enough to make it unstable, the vessel will take up an angle of loll. The angle of loll will be increased further due to the list caused by the suspended weight. In extreme cases, the vessel may even capsize.
When towing trawls or other fishing gear, the force exerted by the tow will be felt at the point of suspension, as shown in diagram Fig 7. This is the equivalent of a weight acting at the point of suspension. If the point is high above the deck, such as occurs when towing from a boom end, then the movement of G1 towards the point of suspension may be large. This can have a detrimental effect on stability. The same situation applies when gear is being lowered or lifted on board, using booms or powerblocks. If a vessel has good stability these operations should present no problems. If stability is poor, then steps should be made to improve stability.
Figure 7
If gear becomes foul when towing, there will be two effects:
1. Dynamic effect - the vessel will heel over because it will be still trying to move ahead.
2. Static effect - as long as there is any strain on the gear, the circumstances will be the same as described above, i.e. the vessel will heel. The angle of heel will be less than that caused by the dynamic effect.
All strain should be taken off the gear as quickly as possible by stopping the engines and if possible, slacking away on the trawl winches. If necessary, stability should be improved before action is taken to free the gear.
Further information about Loads on Fishing Gear can be obtained from the Trim and Stability Booklet.
All liquids in partially filled tanks have a free surface, which is free to slop backwards and forward with the motion of the ship. This free surface effect can cause a serious stability problem if the movement of the liquid is not contained. You might like to conduct a simple, practical experiment to demonstrate F.S.E. for yourself:
(a) Take a flat tray with raised sides and partially fill it with water. (A flat baking pan will work.)
(b) Now hold it level, supported by the palms of your hands, held horizontal at arms length and at shoulder height.
(c) Now gently raise your right hand a few centimetres.
As the water runs to the left of the tray/pan you will feel a marked increase in weight, tending to push your left hand down further and so aggravate the condition.
This is Free Surface Effect (F.S.E.). A ship reacts in the same way. It first rolls slightly to a small angle of heel as a result of the wave forces. The internal forces of the shifting water in slack tanks then increase the list further as the liquid flows to the low side. If this F.S.E. causes the vessel to list so that its deck edge is immersed below the waterline, it could well capsize. Fig 4.8 shows a vessel with a partially filled tank. Free surface effect reduces the size of GM. Therefore the size of GZ is reduced, and consequently the ability of the vessel to return to the upright position is reduced.
Figure 8
Free surface effect is at a maximum in tanks which extend right across the breadth of the vessel. By partitioning the tank longitudinally, the flow of liquids to the low side when the ship is heeled can be restricted. It is not removed completely, but the F.S.E. can be reduced to acceptable limits. Obviously, correct loading and ballasting of the ship is also important, but this is an operational consideration and not a design one. Practically all tanks, with the exception of the fore peak ballast tank, are longitudinally subdivided for this reason.
Tank subdivision is effected by a continuous watertight divider extending in a fore and aft direction to each end of the tank and vertically from the inner bottom of the tank to the underside of the tank top.
Fore peak tanks are usually narrow and do not present a very large free surface problem. For this reason, it is unusual to find any longitudinal subdivision in them.
Where tanks are not longitudinally divided by a watertight divider, there are usually longitudinal wash bulkheads which act as baffle plates. While these do not stop the sideways motion of fluids in the tank, they are designed to retard the flow so that the heeling force created by the free surface effect is out of phase with the rolling of the vessel. This tends to damp the vessel's rolling instead of aggravating it, which can be quite beneficial.
The depth or quality of the liquid in the tank does not affect the free surface to any great degree. Free surface area is the main factor. Only a completely empty or completely full tank will have zero free surface.
Practical Aspects of Stability
If water is shipped on board, then the effect is three fold. Firstly a weight is added high up in the vessel, thus reducing stability. Secondly, that water has a free surface effect, which will further reduce stability. Thirdly, the added weight causes the vessel to sink further in the water, thereby reducing freeboard, and reducing seaworthiness. Freeing ports are provided on deck, so that the water shipped on board can be cleared rapidly. These freeing ports should never be blocked.
Reserve buoyancy is the volume of watertight hull areas above the waterline. As weight is added to a vessel and it sinks in the water, the volume of space above the waterline decreases. When this space (reserve buoyancy), is gone the vessel will sink.
If part of the engine room or the vessel’s hold is above the waterline, then providing that they are enclosed they will contribute to the vessel’s reserve buoyancy. Hence, the reason that all watertight doors are to be kept closed (except for access), at all times.
Figure 9
It is necessary to have a certain reserve buoyancy as, when in a seaway with the ends or middle unsupported, the vessel will sink down to displace the same volume as it does in smooth water. This could result in the vessel foundering.
If a vessel is damaged, and water can enter a compartment which was previously watertight, the compartment is said to have been bilged. When a compartment is bilged the buoyancy provided by the underwater volume of that compartment is lost, as is the reserve buoyancy of the enclosed volume above it. Before bilging, the reserve buoyancy was the entire enclosed volume above the original waterline. After bilging it is the enclosed volume above the new intact water plane area.
If this compartment is to one side of the centre line then the vessel will take up an angle of list. Depending upon the location of the compartment, the vessel may also trim by the bow or stern. In any case, draught will increase, freeboard and therefore reserve buoyancy will decrease and the effect is always to reduce stability.
In case of flooding, the biggest danger is the loss of watertight integrity and the subsequent loss of internal buoyancy from the damaged areas. Your immediate action in this case should always be to close all watertight doors through the vessel to prevent further loss of buoyancy. It may be possible in some cases to bring the damaged area out of water deballasting the vessel or providing a list on the opposite side to the damage.
If a vessel is changed structurally, for example if a new wheelhouse is added or if an extra mast or winch is installed, the effect on stability is exactly the same as though these items were added weights. Because structural changes are usually complex and old material is often taken off the vessel as well as adding new material it is a survey requirement that all of the vessel’s stability is reworked after structural changes have taken place.
The term loll describes the state of a vessel which is unstable when in an upright position and therefore floats at an angle to one side or the other. If disturbed by some external force, caused by wind or waves, the vessel may lurch to the same angle of loll on the opposite side. Loll is quite different from list, being caused by different circumstances and requiring different counter measures to correct it and it is therefore most important that the mariners should be able to distinguish between the two.
Fig 10 shows how an unstable vessel takes up an angle of loll. Note that M is not on the centre line when the vessel is in the lolled position.
To correct for loll the following procedure should be observed.
First verify that it is loll and not list. Lists are caused by shifting of cargo or uneven distribution of fuel, water or cargo. If none of your cargo has shifted and your fuel and water tanks are more or less even on both sides, then you should suspect that your vessel has loll. You must lower the centre of gravity. There are two options open to you
Figure 10
(i) You can take ballast. If you do so, (and if your vessel has ballast tanks that you can fill) then you should begin by pressing up tanks on the low side first. This will initially make your angle of loll worse because you are adding weight on the side to which the vessel is leaning and you are introducing a free surface (if you are ballasting on an empty tank). This is still safer than ballasting the high side first, because that could cause the vessel to flop-over to the other side, and possibly capsize. By introducing ballast you lower the centre of gravity. If you are pressing up half-filled tanks, you are still lowering the CG and removing the free surface. The only negative effect of adding ballast is that it will increase your draft, reduce your freeboard and reserve buoyancy wit the result that your vessel will ship water at a much smaller angle of heel.
(ii) The second option open to you is to remove the cause. A loll does not suddenly occur. It is a result of decreasing stability which is caused by the progressive raising of the centre of gravity of the vessel. This can only occur if you are loading weights on deck, and using fuel or water from low down in the hull (where most tanks are located anyway). You would have felt the vessel becoming progressively more tender and the roll period, and angle of roll steadily increasing. You may have been catching a load of fish - your brine tanks full and a large load of fish on deck. Too much weight high up. In these circumstances you may have to jettison cargo. This may be a painful decision, but the cargo is no use if it capsizes the vessel.