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slack water
Slack water, which used to be known as 'the stand of the tide', is a short period in a body of tidal water when the water is completely unstressed, and therefore no movement either way in the tidal stream, and which occurs before the direction of the tidal stream reverses.〔(''The American Practical Navigator'', Chapter 9:Tides and Tidal Currents ), page 139. Accessed 3 September 2011.〕 Slack water can be estimated using a tidal atlas or the tidal diamond information on a nautical chart.〔Sport Diving, British Sub Aqua Club, ISBN 0-09-163831-3, page 167〕 It is a common misconception that the time of slack water occurs at high and low water, that is, at the greatest and lowest height of tide. In fact, the time of slack water, particularly in constricted waters, does not occur at high and low water,〔(''The American Practical Navigator'', Chapter 9:Tides and Tidal Currents ), pp.141-142. Accessed 19 December 2013.〕 and in certain areas, such as Primera Angostura, the ebb may run for up to three hours after the water level has started to rise, and the flood may run for three hours after the water has started to fall. Thornton Lecky, writing in 1884, illustrates the phenomenon with an inland basin of infinite size, connected to the sea by a narrow mouth. Since the level of the basin is always at mean sea level, the flood in the mouth starts at half tide, and its velocity is at its greatest at the time of high water, with the strongest ebb occurring conversely at low water.
==Implications for seafarers==
For scuba divers, the absence of a flow means that less effort is required to swim, and there is less likelihood of drifting away from a vessel or shore. Slack water can reduce underwater visibility, as there is no flow to remove debris such as sand or mud. In areas with potentially dangerous tides and currents, it is standard practice for divers to plan a dive at slack times.
For any vessel, a favourable flow will improve the vessel's speed over the bottom for a given speed in the water. Difficult channels are also more safely navigated during slack water, as any flow may set a vessel out of a channel and into danger.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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