163630982-Squat

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PORT REVELSHIPHANDLING

SHALLOW WATERS & SQUAT

DECEMBER 2011

Sogreah - Port Revel Shiphandling Shallow Waters & Squat 1

When the keel clearance is less than 50% of the draft, the ship is in shallow water.When a ship enters shallow waters, the movement of water is restricted by the barrierof the sea-bed. The same volume is needed to replace the space left by the passage ofthe hull at a constant speed, but because of the now restricted place in which to act,the water particles must move with a correspondingly greater velocity. Friction andturbulence are increased and the wave form changes, resulting in reduction in speed,increased sinkage, change of trim and reduced propeller and rudder efficiency.Due to the passage of a ship, the water is affected under the keel and off the sides untila certain distance, which varies with the shape of the hull and the speed of the shipover the ground. These distances are called the depth and the width of influence:

depth of influence below the keel is typically 50% of the draught,width of influence on each side of the ship is typically 3.5 beams (for ULCC) to 5beams (for cruise ships).When the bottom clearance is less than the depth of influence, the ship is said to benavigating in shallow waters.When the side clearance is limited by a slope or a bank at a distance less than thewidth of influence, the ship is said navigating in confined waters.


The effects of shallow water on the ship are:- More ship's power is absorbed by the water due to increased friction.- Ship's speed, therefore, decreases.- Larger waves and troughs are formed and the ship "sinks" closer to the bottom thanshe would do at the same speed over the ground in deep water.- At the same time the ship's trim changes: sinkage is greater forward than aft for shipshaving a large block coefficient.- Turbulence interferes with rudder and propeller effectiveness.- Diameter of turning circle increases.

Grounding will occur by head or stern, depending on trim.


As a hull moves, it pushes water away in all directions. Since water is fluid, but notcompressible, it flows around and under the ship to fill the space left behind, andgenerate what we call the "wave form".The wave form consists of:- a bow wave: water rising around the bow,- a trough along the main body of the ship with speed-up of water,- a stern wave due to the water rushing behind the ship,- a following wave.The shape of the wave form depends on the shape of the hull and the speed of the shipthrough the water.


The bridge pier in the flow has the same effect as a ship steaming ahead.

Increased flow velocity induces lowered water level, according to the law of Bernoulli.


a) A fall in water level around the entire ship, which drops towards the sea-bed, wesay that she "sinks.b) Secondly, a loss of buoyancy in the main body of the hull, buoyancy which isrecovered by the bow and the stern waves. Since the shape of the two ends of the shipis different, the ship sinks with a change of trim.

Ship squat thus is made up of mean bodily sinkage plus a trimming effect.(Barrass, 2009).

Or according to PIANC 2007:Squatis the reduction in Under Keel Clearance between a vessel at-rest andunderway due to the increased flow of water past the moving body. The forward motionof the ship pushes water ahead of it that must return around the sides and under thekeel. This water motion induces a relative velocity between the ship and thesurrounding water that causes a water level change along the ship that is similar to theBernoulli effect in that kinetic and potential energy must be in balance (Newman 1977).This phenomenon produces a downward vertical force (sinkage, positive downward)and a moment about the transverse axis (trim, positive bow up) that can result indifferent values at the bow and stern.


TheFrouderatio(1)indicateshowtheshipsquats.

This was also schematized by Barrass by stating simply that:if block coefficient CB > 0.70, then maximum squat occurs at bow,if block coefficient CB < 0.70, then maximum squat occurs at stern.


Note that confined water sinkage is twice that of open water sinkage.

The amount of squat depends on:- ship's speed V- hull shape (block coefficient CB)- restriction of water (Blockage Factor BF):

. under the keel: in shallow waters (H/D ratio)

. and on the sides: in confined waters, channel & canal

The complete Barrass formula (2009) is:


More than 2 m (7 ft) sinkage on shallow open waters is a realistic possibility at highspeed

In order to avoid grounding due to squat, a speed reduction can be computed from thefollowing equation:HS = D + U where H = water depth

S = squat according to BarrasssimplifiedformulaD = ship draught U = Under keel clearance

The resulting limit speed on shallow open water is:

V[knots] = 10 (H-D-U)/CB[meters]

e.g. H=18 m, D=14 m, U=2 m, CB=0.6yields: V= 18 knots.


For this particular test, the turning circle diameter on deep water is around 3 S.L. and on shallow water around 5 S.L.


1. Straight section of channel or canal- Stay in the middle of the canal or just off the centre line.- In case of yaw:

. overestimate the amount of rudder at the very beginning;

. when hard over on the rudder is not enough, give a kick ahead, as short aspossible, on the engine.2. Channel bendsThe bank cushion can be used to advantage in safely making bends in a narrowchannel or canal by favouring the right side when the canal bend is to the left and byfavouring the left side when the bend turns to the right. The ideal track round a bend isthe one that lets the ship follow the canal with the least amount of rudder.3. Channel or river bends with a current- When heading downstream, stay close to the middle of a channel bend.- When heading upstream, stay wide, keep in the bend.Two sound basic principles, but in rivers or channels with current and irregular banks,extreme caution is necessary when good local knowledge is not available.


Bow cushionAs the bow pushes water away, opening way for the passage of the hull, confined wateron one side will restrict the flow of that water. The result is a "cushion" effect betweenthe bow and the bank. The direction of this force is not only horizontally outwards fromthe bow, but in all directions between the bow and the bank. The bow wave becomeshigher on the side toward the bank and the increase in pressure, forces the bow awayfrom the bank.Propeller suction"Suction" in shallow water exists at the stern. This is particularly noticeable in the supplyflow to the propeller which drives the water away a little faster than the water can flow in.As a result, the difference is made up by a greater inflow from the sides. This depressesthe surface at the sides just forward of the propeller. When the sea bed is regular, this"suction" is equal on both sides of the stern. However, if there is a bank or the water isshallower on one side, the source of flow is restricted and the water surface between theobstruction and the propeller is depressed. The increased suction on the shallow sidebegins to act and the stern is drawn toward the bank.NB: with a twin-screw ship the suction is increased by the smaller distance to the bank.


In a canal or narrow channel, shallow water effects become very pronounced. Not onlyis there a restriction on the flow of water under the ship, but there are banks restrictingthe water on either side.There is a limit speeddepending on the blockage factor: when the speed of the shipincreases, it induces more sinkage and higher speed of water flow under and near theship, which in turn induce more sinkage, etc. For the higher blockage factors, the shipacts like a piston in the canal.Close to this limit speed, the ship becomes very difficult to control. The waves andtroughs about the hull are steeper and larger than before. The banks act as a cushionon both sides of the bow and cause suction on both sides of the stern. The ship is in abalance between these forces only when she is in the centre of the canal section. Assoon as the ship gets slightly away from the centre (due either to her steering or anirregularity in the canal), the bow cushion and stern suction between the ship and thenearer bank both increase.For usual Blockage Factors of 10 to 25% the Schijf Limit Speed is around half the translation wave speed of (gH).

Steaming in a canal at near limit speed is an unsafe practice : one should not exceed 75% of the limit speed.


Ship speed must be less than propagation speed of translation wave : V < Vwave

Deep water: V[knots] = 2.5 SL[meters] where SL = Ship Length in meters

Shallow water: V[knots] = 6.2 [meters] where H = water depth in metersbutcareforsquatduetoBernoullieffect:

Bernoulli equation: conservation of energy along a flow line yields a decrease of water level when flow speed is increased.Squat: combined sinkage (vertical) and change of trim:V[knots] = 10 (H-D-U)/CB[meters] where D = ship draught in meters

CB = Block coefficient of shipU = Under keel clearance in metersConfined water: Schijf limit speed is even smaller as squat adds up with piston effect

of ship pushing water in front of her in the canal:V[knots] < 50 % of V on open shallow water, for BF = 20%.


Each ship moves along with its own hydraulic flow field as follows:>> when the ship moves ahead in a water body, water has to flow from the bow to the stern. The narrower the canal, the higher the flow velocity (under the keel and on each side of the ship). Due to the Bernouilli effect, the ship sinks into the water. So the ship moves in a kind of trough which will attract the meeting ship.>> at the bow, the ship is pushing water ahead, creating the well known bow wave which will induce a positive pressure repelling the meeting ship.>> at the stern, the propeller sucks water down to below the ship, creating a negative pressure which will attract the meeting ship.

Now,letsseewhathappenswhenthoseflowfieldsmeetinacanal


First, the bows will repel each other due to the bow waves.Second, the bow of each ship will be attracted into the trough of the other ship, and then by the stern of the other ship.


Then each ship has to climb out of the trough of the other ship.Finally, the stern of each ship will be attracted by the stern of the other ship.


43 m beam for the Otello and 54 m for the Q-Max: 1.4 Q-Max beam was left on each side when passing in this narrow area.In this case, maximum mooring line forces ranged from 80 to more than 200 tons for speeds from 3.5 to 6.5 knots. Not to speak about the uncontrolled movements of the mooredship

A good reason to slow down when passing a moored ship.

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