Table 4. Figure 3.
(includes Table 5) Maximum squats for
Queen Victoria. In the United Kingdom, over the last 40
years the Department for Transport (DfT) has shown its concern by issuing eight ‘M’ notices concerning the problems of ship squat and accompanying problems in shallow water. Tese alert all mariners to the associated dangers. Te signs that indicate a ship has entered
depth/static draught) of 9.60m/8.00m. Again, the Cb is 0.600. For Figure 3, the river is assumed along
its length, to range in width from 100m up to and beyond 351m. See Table 5. For the Queen Victoria, a width greater than 351m constitutes a confined channel. Beyond 351m, water width is akin to an open water situation. See later, specific widths of difference for several ship types.
40 years of ship squat Ship squat has always existed on smaller and slower vessels. Tese squats have only been a matter of centimetres and thus have been considered inconsequential. However, from the mid-1960s and into
this new Millennium, ship size has grown steadily, until we have supertankers of the order of 450,000dwt and above. These
The Naval Architect February 2009
supertankers have almost outgrown the ports they visit, culminating in small static even-keel under-keel clearances of only 1.00m to 1.50m. Alongside this development in ship size has
been an increase in service speeds. Modern passenger liners and cruiseships can have service speeds of up to 25knots to 30knots. As the static under-keel clearances
have decreased and as the service speeds have increased, ship squats have gradually increased. Tey can now be of the order of 1.50m to 2.00m. Today, these squats are no longer inconsequential!
Recent incidents To help focus the mind on the dangers of excessive squat one only has to recall the grounding of the 14 vessels shown in Table 6 over recent years.
shallow water are: 1. Mean bodily sinkage increases. 2. For the Queen Victoria, generally there will be trim by the stern. 3. Te ship develops increases in squat. 4. Wave-making increases, especially at the forward end of the ship. 5. Ship becomes more sluggish to manoeuvre. A pilot’s quote, ‘almost like being in porridge’. 6. Draught indicators on the bridge or echo-sounders will indicate changes in the end draughts. 7. Propeller revs/min indicator will show a decrease. If the ship is in ‘open water’ conditions ie. without breadth restrictions, this decrease may be up to 15% of the service revs/min in deep water. If the ship is in a confined channel, this decrease in revs/ min can be up to 20% of the service revs/ min. 8. Tere 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%. 9. Te 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. 10. Any rolling, pitching and heaving motions will all be reduced as ship moves from deep water to shallow water conditions. Tis is because of the cushioning effects produced by the narrow layer of water under the bottom shell of the vessel.
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