Slack water

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Slack water is the time during which no appreciable tidal current is flowing in a body of water.

Slack water usually occurs near high water and low water, and are occasions in between when the direction of the flow reverses. Slack water is easily determined using a current table and can be accurately calculated in most regions using a tide table combined with either a tidal atlas or the tidal diamond information on a nautical chart.

Slack water is of particular interest to sailors and scuba divers.

For divers, the absence of a current means that less effort is required to swim to and remain at a given site, and there is less likelihood of drifting away from the boat providing surface cover. Slack water can also provide better visibility, as there is no current to stir up sand and silt from the bottom, although in some situations the current can be beneficial in removing diver-induced low visibility from the dive site. Except when drift diving, it is standard practice for divers to plan to dive at slack water.

For sail boats with limited top speeds, a favorable current can substantially improve speed over the ground. Difficult channels are also more safely navigated during slack water, as current may set a vessel out of a channel and into dangerous shoal water.