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Frequently Asked Questions for SMM Products

Ship's Squat and Tide Calculations Frequently Asked Questions
Which Factors Are Taken Into Consideration In The Calculation Of Under Keel Clearance?

  • Predicted Tidal Height
  • Changes in the predicted tidal height, which are caused by wind speed and direction and high or low barometric pressure
  • Nature and stability of the bottom – i.e. sand waves, siltation etc..
  • Accuracy of hydrographic data, (References to reliability is often included on charts).
  • The reliability of draft observations and calculations, including estimates of hogging and sagging correction.
  • Reduced depths over pipelines and other obstructions. 
  • Swell Height.

How the Ship's Squat/UKC/Speed Loss And Tide Calculations Software integrates UKHO Digital Tide Tables?

The clients, who are already registered with United Kingdom Hydrographic Office concerning the Admiralty Digital Publications SDK, can make use of the Harmonic Data ((Digital Tide Tables) for a particular location (Port) as an add-on “(Communication Protocol)” to Ship's Squat/UKC/Speed Loss And Tide Calculations software in order to avoid the manual input of Harmonic Constants.

The User, that may wish to utilize that feature, shall have installed the UKHO Software on the PC/Server, on which the Ship's Squat/UKC/Speed Loss And Tide Calculations software is running.



How do I predict Ship Squat?

As the static underkeel clearances have decreased and as the Service Speeds have increased, ship squats have gradually increased. They can now be of the order of 1.50 to 1.75 metres, which are, of course, by no means inconsequential.

The main factor is ship speed Vk. Squat varies approximately with the speed squared.


Another important factor is the block coefficient Cb. Squat varies directly with Cb. Oil Tankers will therefore have comparatively more squat than Passenger Liners.


The Blockage Factor "S" is another factor to consider.


Formulae that were utilized in the software are satisfactory for estimating maximum ships squats for vessels operating in confined channels and in open water conditions. These formulae have been derived from analysing about 600 results. some measured on ships and some on ship-models.







Why predict Ship Squat?

It can be stated that if we can predict the maximum ship squat for a given situation then the following advantages can be gained:


1. The ship-operator will know which speed to reduce to in order to ensure the safety of his/her vessel. This could save the cost of a very large repair bill. It has been reported in the technical press that the repair bill for the QE2 was $13 million plus an estimation for lost Passenger bookings of $50 million!!


In Lloyds Lists, the repair bill for the "Sea Empress" had been estimated to be in the region of $28 million. In May 1997, the repairs to the "Sea Empress" were completed at Harland & Wolff Ltd of Belfast, for a reported cost of £20 million. Rate of exchange in May 1997 was of the order of £1 = $1.55. She was then renamed the "Sea Spirit."


2. The ship-officers could load the ship up an extra few centimetres (except of course where load-line limits would be exceeded). If a 100,000 tonne dwt Tanker is loaded by an extra 30 cms or an SD14 General Cargo ship is loaded by an extra 20 cms, the effect is an extra 3% onto their dwt. This gives these ships extra earning capacity.


3. If the ship grounds due to excessive squatting in shallow water, then apart from the large repair bill, there is the time the ship is "out of service". Being "out of service" is indeed very costly because loss of earnings can be as high as £300,000 per day.


4. When a vessel goes aground there is always a possibility of leakage of oil resulting in compensation claims for oil pollution and fees for clean-up operations following the incident. These costs eventually may have to be paid for by the Shipowner.


5. A ship going aground in a river can seriously curtail traffic flow of vessels in a Port and, of course, revenue for the Port Authority. By knowing the relationship between speed and maximum squat, Harbourmasters can greatly reduce the possibility of such an incident occurring.


These five points illustrate very clearly that not knowing about Ship Squat can prove to be very costly indeed. Remember, in a Marine Court hearing, ignorance is not acceptable as a legitimate excuse!!


Summarising, it can be stated that because maximum ship squat can now be predicted, it has removed the "grey area" surrounding the phenomenon. In the past ship-pilots have used "trial and error", "rule of thumb" and years of experience to bring their vessels safely in and out of Port.


Empirical formulae quoted in this study, modified and refined over a period of 35+ years research on the topic give firm guidelines. By maintaining the ship's trading availability a Shipowner's profit margins are not decreased. More important still, this report can help prevent loss of life as occurred with the "Herald of Free Enterprise" grounding. 193 lives were lost.


It should always be remembered that the quickest method for reducing the danger of grounding due to Ship Squat, is to reduce the ship's speed.

Why is important the derivation of Static Condition from Dynamic Drafts at Restricted Waters?

When for instance, there is a draft survey with strong current, it is of utmost importance to obtain the static drafts for proper estimation of the Cargo Quantities.


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