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SQUAT & SHALLOW WATER EFFECTS

SHIP SQUAT

When a ship proceeds through water, she pushes water ahead of her. In order not to leave a 'hole' in the water, this volume of water must return down the sides and under the bottom of the ship.  The streamlines of return flow are speeded up under the ship. This causes a drop in pressure, resulting in the ship dropping vertically in the water.

As well as dropping vertically, the ship generally trims for’d or aft. The overall decrease in the static under keel clearance, for’d or aft, is called Ship Squat.  It is not the difference between the draughts when stationary and the draughts when the ship is moving ahead.

If the ship moves forward at too great a speed when she is in shallow water, say where this static even-keel under keel clearance is 1.0 to 1.5 meters, then grounding due to excessive squat could occur at the Bow or at the Stern.

For full-form ships such as Supertankers or OBO vessels, grounding will occur generally at the BOW.  For fine-form vessels such as passenger liners or container ships the grounding will generally occur at the STERN.  This is assuming that they are on even keel when stationary.

Salient points

• The squat is the decrease in under-keel water, that is, the difference between her under-keel clearances when making way and when stopped over the water.

• It is not the increase in the draft as visually read or as shown on draft indicators.

• Bernoulli’s theorem states that in any moving fluid, the sum of the potential energy, the kinetic energy, and the pressure energy is a constant. As the water flows aft at greater speed, its kinetic energy increases. According to Bernoulli’s theorem, when the kinetic energy of the water increases, it's pressure energy must reduce. Since the ship is supported by the pressure energy of the water, as the pressure energy has reduced, the ship sinks to a longer draft.

• In addition to the bodily sinkage that occurs, the ship also trims by the head or by the stern.

• With a static even keel trim, full form vessels such as tankers and bulk carriers with Cb more than 0.7 trim by the head.

• Fine form vessels such as passenger ships and containers vessels with Cb less than 0.7 trim by the stern.

The overall decrease in under –keel clearance due to sinkage and trim is the squat forward or aft.

The factors that affect the amount of squat

1. The ships speed over the water

• The squat varies approximately directly as the speed over the water in knots squared.

• Squat occurs even when the ship is moored if a tide is running.

• Hence squat should be taken into account when conducting draft surveys.

2. The block coefficient, Cb

• The squat varies directly as the Cb. The Cb values generally vary from about 0.85 for very large tankers to about 0.75 for bulkers, about 0.7 for general cargo vessels to about 0.6 or less for passenger vessels and container ships.

3. The blockage factor, S

• The blockage factor, S, is the ratio between the immersed cross-sectional area of the vessel and the cross-sectional area of the water in the canal

• S = b x Static Draft / B x depth of Water

•  where ‘b’ is the breadth of the ship and ‘B’ is the width of the canal.

• Even in open waters, this factor is to be considered using the width of influence ‘B’ in place of the width of the canal B.

• The width of influence ‘B’ in open waters is obtained as ‘B’  = [ 7.7 + 20 (1-Cb)2] bwhere ‘b’ is the breadth of the ship.

• The ‘B’ value in open waters varies from about 8*b for large tankers to about 9.5*b for general cargo vessels to about 12*b for container and passenger ships. In open waters where the depth of water to a draft of the ship ratio is about 1.2, the value of the blockage factor S will be around 0.1.

4. The static under keel clearance

• The lesser the under-keel clearance, the more is the squat because of the streamlines of return flow aft of the water, past the vessel increases due to the reduced clearance under the vessel.

• This increases the kinetic energy and therefore further reduces the pressure energy of the water.

• Thus as the ratio of the depth of water to draft to ship reduces, the squat increases.

5. The at-rest trim of the vessel

• The squat at the bow increases to a greater extent if her at rest trim was by the head.

• The squat at the stern will increase to a greater extent if her at rest trim was by the stern. The calculated maximum squat should, therefore, be applied to the greater of the two end drafts to obtain the minimum under keel clearance.

6. Passing another ship in a river or canal

• When the ship is passing or overtaking another vessel in a river or canal, the squat can increase up to twice the normal value as the combined blockage factor, S becomes the sum of the blockage factor of each ship.

7. The squat increases if the ship is close to the bank of a river or canal.

8. Formulae

• From the analysis of many measured squat values on ships and results of ship model tests some empirical formulae have been developed for satisfactorily estimating the maximum squat is confined and open waters.

• Obviously the squat is greater in confined waters and lesser in open waters.

• For a vessel at an even keel static trim when the ratio of the depth of water to the draft of a ship is in the range of 1.1 to 1.4, the maximum squat in open or confined waters may be predicted fairly accurately by either of the expressions:-

1. Maximum squat  = (Cb x S^0.81   x V^2.08)/ 20

in the above expressions:

‘S’ is the blockage factor.

‘V’ is the ship’s speed over the water in knots.

• Maximum squat in open waters = (Cb x V^2 )/100

• Maximum squat in confined waters:

• Applicable only to confined channels where S = 0.1 to 0.265

• Where S is between 0.1 and 0.265 = (Cb x V^2)/50

Both the above approximate formulae slightly overestimate the maximum squat thereby erring on the safer side.

• At this point, a consideration may arise as to the depth of water, which can be considered shallow.

• This depends on the depth of influence of the ship, which is approximately 5/Cb x draft.

• In depths above the depth of influence, the ship may be considered in shallow waters.

• The depth of influence is more than 5 times the draft, though the ship’s squat may commence increasing slightly at such depths it is not of much consequence.

• The increase in squat is significant when the depth to draft ratio is less than 2.

• It is much more pronounced and of consequence when this ratio is less than 1.5

• The best course of action to reduce squat is to reduce the ship’s speed, because the squat varies directly as the ship’s speed squared.

• Halving the speed will reduce the squat to a quarter.

• However, the fact that manoeuvering which is already sluggish in shallow waters may deteriorate further should also be considered when reducing the speed.

Signs that a ship has entered shallow water

• Maximum Ship Squat increases.

• Mean bodily sinkage increases.

• The ship will generally develop extra trim by the bow or the stern.

• Wave-making increases, especially at the forward end of the ship.

• A ship becomes more sluggish to maneuver - To quote a pilot, “almost like being in porridge.”

• Draught indicators on the Bridge or echo-sounders will indicate changes in the end draughts.

• The propeller rpm indicator will show a decrease.  If the ship is in 'open water' conditions i.e. without breadth restrictions, this decrease may be up to 15% of the Service rpm in deep water. If the ship is in a confined channel, this decrease in rpm can be up to 20% of the Service rpm.

• There will be a drop in speed.  If the ship is in open water conditions this decrease may be up to 35%.  If the ship is in a confined channel such as a river or a canal then this decrease can be up to 75%.

• The ship may start to vibrate suddenly.  This is because of the entrained water effects causing the natural hull frequency to become resonant with another frequency associated with the vessel.

• Any Rolling, Pitching, and Heaving motions will all be reduced as the ship moves from deep water to shallow water conditions.  This is because of the cushioning effects produced by the narrow layer of water under the bottom shell of the vessel.

• The appearance of mud could suddenly show in the water around the ship’s hull say in the event of passing over a raised shelf or a submerged wreck.

• Turning Circle Diameter (TCD) increases. TCD in shallow water could increase by 100%.

• Stopping distances and stopping times increase, compared to when a vessel is in deep waters.

• The effectiveness of the rudder helm decreases.

• The width of the wake increases considerably.

Wall or bank effect

Bernoulli’s law and continuity law

In order to understand the effect of a solid bank or wall on the behavior of moving ships along it, it is necessary to study pressure distribution around ship's hull and relevant basic laws governing flow phenomena.

Continuity law: Velocity x cross section = const

V1 x S1 = V2 x S2 = const.

Consequence: if cross-section decreases, velocity increases and vice versa

Bernoulli’s law: static pressure + dynamic pressure = const.

Static pressure = atmospheric pressure + head of water

Dynamic pressure = C x velocity squared

Consequence: if velocity increases, dynamic pressure increases, and static pressure and head of water decreases and vice versa.

SUCTION FORCE

When the ship is moving close to a solid wall or bank suction force is created drawing the ship closer to the bank. This is because of reduced cross-section, accelerated flow, and reduced pressure in the space between the ship and the bank.

The suction force is proportional to the speed of the ship squared and inversely proportional to the distance from the bank.

Suction forces calculated for example ship are shown below:

RESTRICTED WATER EFFECT

Suction force together with bow cushion effect make stern to move closer to the bank. Rudder is to be used to counter this effect.

Because of the proximity of the bank ship takes a sheer and suction force moves close to the stern.

USING SUCTION FORCE TO THE ADVANTAGE

PASSING THROUGH NARROW PASSAGE

Entering the passage closer to the bank helps turning to starboard as needed. If the ship is entering closer to the island, suction is in the wrong quarters and opposes turning to starboard.

ENTERING OR LEAVING SHALLOW BANK

When the ship is entering a shallow bank then due to restricted cross-section and reduced pressure under bow portion of the ship trim to bow may occur and the ship may hit the bottom with the bow.

When the ship is leaving shallow bank and entering deep-water area, the opposite may occur and the ship may hit the bottom with the stem.

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