11. Te 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. 12. Turning circle diameter (TCD) increases. TCD in shallow water could increase 100%. 13. Stopping distances and stopping times increase, compared to when a vessel is in deep waters. 14. Effectiveness of the rudder helm decreases. 15. Width of the wake increases considerably.
Factors governing squat Te main factor governing ship squat is ship speed V. Squat actually varies as speed to the power of 2.08. However, we can say that squat varies approximately with the speed squared. In other words, we can take as an example, that if we halve the speed we approximately quarter the squat. Put another way, if we double the speed we quadruple the squat. In this context, speed V is the ship’s speed
relative to the water. Effect of current/ tide speed with or against the ship must therefore be taken into account. Squat will vary directly with V2. Another important factor is the block
coefficient Cb. Oil tankers will therefore have comparatively more squat than passenger liners and cruiseships. Squat varies directly with Cb. Blockage factor ‘S’ is another factor to
consider. Tis is the immersed cross-section of the ship’s midship section divided by the cross-section of water within the canal or river. Range of ‘S’ is from 0.100 to 0.250. squat will vary as S0.81. If a ship is in open water, the width of
influence of water can be calculated. Tis ranges from about 8.25b for supertankers, to about 9.50b for general cargo ships, to about 11.75 ship-breadths for containerShips. For the Queen Victoria, her width of influence in open water is about 10.87 times ship breadths, amounting to 351m. Water depth (H) / ship’s draught (T) also
affects ship squat. Approximately, we can say squat will vary as the reciprocal of H/T. Range of H/T is from 1.10 to 1.40. Squat will vary as T/H. The presence of another ship in a
narrow river (passing, overtaking or simply moored) will also affect squat, so much so,
The Naval Architect February 2009
Table 5
that squats can double in value as they pass/ cross the other vessel.
Reducing ship squat 1. Reduce the mean draught of the vessel if possible by discharge of water ballast. Tis causes two reductions in one: a. At the lower draught, the block co-efficient Cb will be slightly lower in value. For the Queen Victoria, it will not make for a significant reduction. Tis is because of the boot-topping depth being a lot less than for many other ship types.
b. At the lower draught, for a similar water depth, the H/T will be higher in value. Higher H/T values lead to smaller squat values.
2. Move the Queen Victoria into deeper water depths. For a similar mean ship draught, H/T will increase, leading again to a decrease in ship squat. 3. When in a river if possible, avoid interaction effects from nearby moving ships or with adjacent riverbanks. A greater width of water will lead to less ship squat. Tis will be so, unless the vessel is in water greater than her width of influence.
4. Te quickest and most effective way to reduce squat is to reduce the speed of the Queen Victoria. Halving ship speed will quarter the squat values.
Conclusion 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. Te officer on watch or ship pilot will
know which speed to reduce to in order to ensure the safety of his or her vessel. Tis could save the cost of a very large repair bill. In May 1997, the repairs to 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. 2. Ship-officers could load the ship up an extra few centimetres (except of course where load-line limits would be exceeded). If a 100,000dwt tanker is loaded by an extra 30cm or an SD14 general cargo ship is loaded by an extra 20cm, then the effect is an extra 3% onto their dwt. Tis gives these ships extra earning capacity.
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