To find slack tide, also known as slack water, you can estimate it primarily by consulting a tidal atlas or the tidal diamond information located on a nautical chart. This moment occurs just before the direction of the tidal stream changes.
What is Slack Tide?
Slack tide is the brief period when the tidal current (or stream) slows down and momentarily stops before reversing direction. It's distinct from high or low tide, which refers to the maximum and minimum water levels. Slack water is about the horizontal movement of water. Finding slack water is crucial for safe navigation, anchoring, or activities like diving, as there is minimal current.
Methods for Estimating Slack Tide
As indicated by tidal navigation resources, the most reliable ways to estimate when slack water will occur are through specialized charts and atlases.
Using a Tidal Atlas
A tidal atlas is a series of charts that depict the predicted direction and strength of tidal streams at different times relative to high water or low water at a standard port.
- How to Use:
- Identify the tidal stream area relevant to your location on the atlas.
- Find the reference port for that atlas or area.
- Calculate the time difference between your desired slack water time and high or low water at the reference port (using tide tables).
- Flip through the atlas pages, which show tidal stream conditions at different intervals (e.g., -6 hours HW, -5 hours HW, etc.) relative to the reference port's high water or low water.
- Look for the chart that shows minimal or no tidal stream activity in your specific location. This time, relative to the reference port, indicates when slack water is predicted.
Using Tidal Diamonds on a Nautical Chart
Nautical charts often feature "tidal diamonds" – symbols (usually diamonds enclosing a letter) plotted on the chart at specific locations. These symbols correspond to tables printed on the chart's margin or in a separate tidal stream atlas.
- How to Use:
- Locate the tidal diamond nearest to your position on the nautical chart. Note the letter inside the diamond.
- Find the corresponding table, often titled "Tidal Streams" or similar, on the chart's margin or in a linked atlas.
- The table provides predicted tidal stream direction and rate (speed) for that specific diamond location at different times relative to high water or low water at a designated standard port (similar to using a tidal atlas).
- Examine the table column for the diamond's letter. Look for the time entry where the tidal stream rate is listed as zero or the lowest value. This indicates the predicted time of slack water at that location relative to the standard port's tide.
Example Tidal Diamond Table Snippet:
| Diamond | Time from HW (Standard Port) | Direction (°) | Rate (Knots) |
|---|---|---|---|
| A | HW - 3h | 090 | 2.5 |
| A | HW - 2h | 110 | 1.8 |
| A | HW - 1h | Slack | 0.0 |
| A | HW + 0h (HW) | 270 | 0.5 |
| A | HW + 1h | 290 | 1.2 |
In this example, slack water at Diamond A is predicted to occur approximately 1 hour before High Water at the standard port.
Understanding the Timing
It's important to note that slack water does not always coincide precisely with the exact time of high or low water. Slack water occurs before the direction of the tidal stream reverses. The period of slack water is typically short, often lasting only a few minutes. The exact timing and duration can be influenced by local geography, weather, and other factors.
By utilizing tidal atlases and the tidal diamond information on nautical charts, navigators and water enthusiasts can effectively estimate the times of slack water, ensuring safer and more efficient operations on the water.
