Squat & UKC

Q. Is the passage planning mandatory?
Ans. Passage planning is mandatory vide chapter V of SOLAS. Nowadays, most companies expect that passage planning be done on their ships prior any voyage is undertaken. Larger sized ships than yesteryear are plying today through the restricted passages.  The number of ships transiting these areas, have also increased.

Q. In determining UKC, what precautions will you take?
Ans. In determining the under keel clearance, I will allow for the height of tide; inaccuracies caused due limitations of marine survey methods; and squat. Vessel may be adequately trimmed and transducer may be located in some intermediate part along the length. The tidal predictions may not be accurate. Thus, the combined effect may be adequate. Accordingly, the required safety margin must be allowed in determining the expected UKC.

Q. What is squat?
Ans. Squat is a hydrodynamic phenomenon whereby the reduction of under keel-clearance of a ship and change of trim is caused due to the relative movement of the ship’s body with respect to the medium, i.e. water.

Q. What happens to ship?
Ans. The ship’s body sinks deeper into the water as compared to her static posture. The sinkage is also associated with a slight change of trim. Thus, the squat, rather than just the increase of hydrostatic draft is usually considered as also causing the change of fore & aft drafts with reduced UKC.

Q. Why is trim change associated with squat?
Ans. The virtual vacuum below the ship’s bottom causes a bodily sinkage of the ship’s hull in the water. If the sinkage is bodily, then the buoyancy is added at the centre of floatation. Depending on the position of the centre of floatation, viz. forward or abaft the centre of buoyancy, she will trim aft or forward respectively. This however, is the squatting in a hypothetically static condition.

In the dynamic condition, sometimes, the squat effect is more at the bow & at other times, it is more in the stern area. It is further influenced by;

1. the slope of sea bed ; and

2. the distribution of underwater volume about amidships,. The bow wave formation would influence the ship’s behavior in terms of pitching and hence change of drafts at ends.

Q. Does the squat affect ship handling?
Ans. The buildup of water ahead of the ship opposes the advance of ship & shifts the pivot point. The ship will also show the signs of shallow water effects. The overall effect will be the reduction of steering efficiency.

Q. What are the governing factors for squat?
Ans. Squat depends on:

  • Speed: It varies directly as approximately, the square of speed.
  • Block Coefficient (Cb): Squat increases with block coefficient.
  • Blockage factor: Blockage factor is the fraction indicating the part by which transverse section of effective channel is blocked by the transverse under water section of the ship. Blockage factor is found from the ratio, \dfrac {bd}{BD}, where b & d are the breadth & draft of the vessel respectively. B & D are the breadth and depth of channel respectively.
  • The shape of water plane & the shape of underwater part.
  • Width of the narrow channel: A shallow channel may be either restricted or unrestricted width wise.

Q. What are the different ways in which the information regarding squat may be provided?
Ans. There are several ways in which the information relating to squat may be provided in the maneuvering characteristics booklet on ships. Thus, for various draft to depth ratios relevant to various shallow water situations, squat at bow and stern may be provided.

Squat curve for bow and stern may be provided for very shallow water situation \dfrac {D_{P}}{D_{R}} <1.1

Q. How can the squat, complicate the situation further?
Ans. Several cases have been reported, of large ships running aground in shallow waters subsequent to the bow sinkage of up to 2 metres. In addition to the possibility of grounding forward, there also exists the possibility of losing control on steering and the vessel sheering violently out of a channel. Counter helm may be sluggish, though, once the ship responds, she may sheer violently the other way. The ship may run out of the channel and go aground in just a few minutes.

Smelling the ground is a similar phenomenon. If the helmsman is found failing to respond to a large angle of helm, it is advisable to give the rudder hard over and a bold ‘kick ahead’ to regain control.

Q. What is the basic principle of physics behind these phenomena?
Ans. Increase in velocity is associated with constriction or restriction of passage. Also, increase in velocity of flowing water results in pressure drop (Bernoulli’s Theorem). The principle behind Bernoulli’s theorem is the law of conservation of energy. It states that energy can neither be created nor destroyed, but merely changed from one form to another.

To understand this, let us pass liquid from the left side (A) of our test instrument. The pipe is constricted at (B). If the cross-sectional area at (B) is half the cross-sectional area at (A): the liquid will have to move about twice as fast at B, in order to allow the same amount of water to pass in the same time. Liquid at B must have more kinetic energy. Increase in kinetic energy is associated with decrease in potential energy. This happens with reduced column of water and reduction of pressure. Thus, there will be a low-pressure area in the construction of the pipe at B. It can also be seen that the pressure and velocity at C are the same as at A.

Q. Is there any other equipment onboard, using this principle?
Ans. This principle is used in eductor system on board ships. Eductor is used for pumping out liquid from bilges or enclosed spaces.

Q. Do you know any simplified formulae to calculate squat?
Ans. The simplified formula commonly used for vessels in unrestricted waters (open water conditions) reads as follows:
Squat = 0.01 x Cb × V2 m
In restricted waters (confined water condition), increased value of squat is found as follows:
Squat = 0.02 x Cb × V2 m
V is the speed through water in knots. The squat found is in metres.

Q. What is the best way to control squat?
Ans. Squat is almost directly proportional to square of speed. Thus, if the speed is reduced from 16 to 8 kn, a 2 m squat would reduce to 2\times \dfrac {64}{256}=0.5 m.

Q. Is there a formula showing the influence of blockage factor?
Ans. A relatively more accurate formula to calculate squat is as follows:
Squat in metres = 0.05 × Cb × V2.08 BF 0.81……… [BF is the blockage factor]
Following values of block coefficient may be assumed for different ships:
Container ships → 0.55              
Passenger liners → 0.60
General cargo ship → 0.70              
Oil tankers / OBOs → 0.80
VLCCs → 0.83

Q. What happens if the channel is very wide?
Ans. For a wide shallow channel, the effective breadth is found by the following formula:
Effective breadth = [7.7 + 20 (1-Cb)2].b
Alternately, the artificial or effective breadth of canal may be taken as 8.35b for loaded tankers, 9.5b for a general cargo ships & 11.7b for container ships.

Q. Can you give some practical instances, where squat played an important role? Ans. The advantage of squat was taken by the world’s one of the largest cruise ships, ‘Oasis of the Seas, to pass under the Great Belt bridge, Denmark. The cruise ship was completed and given to Royal Caribbean on 28th October, 2009. Just two days later, she sailed from Finland for U.S. The bridge has a clearance of 65 m above the water; Oasis normally has an air-draft of 72 m. Transiting was possible due to retraction of the funnels which are telescopic. Even this did not suffice so an additional reduction of 30 cm was possible by the squat effect. The vessel traveled at a speed of 20 knots & passed under it with less than 60 cm of clearance.

Queen Elizabeth 2 (QE2) had run aground on 7th August, 1992 off Cuttyhunk Island. At the time of the QE2’s grounding, her speed was 24 knots and her draft was 9.8 m. The rock upon which she grounded was an uncharted shoal, later determined to be of 10.5 m. Probably, at slow speed, she would have cleared the rock but, the squat caused the hull to hit the rock. Probably, QE2’s officers underestimated the size of squat. The officers allowed for 0.61 m of squat in their calculations but, in the inquiry held it was concluded that her squat at that speed and depth would have been between 1.4 and 2.4 m.

Q. What are the factors, which may further increase the squat?
Ans. Once the ship has to bodily sink with the motion, the way she will trim will depend on the shift of pivot point due to longitudinal resistance, shape of underwater hull, change of seabed gradient etc. The squat may also be much more in the presence of another vessel in close vicinity, particularly, if going on reciprocal course. Best is to maintain a good margin of safety.

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