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.

   • Also, when loading to a particular draft, squat could result in under loading if the drafts are read when the tide is running. 

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] b, where ‘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.

 

• Other approximate formulae are:

• Applicable only for open water conditions where H/T is within 1.1 to 1.4

• 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.