Slack Window Finder

What it does

• Slack water is a period, not an instant, and its length changes with the range.
• The finder returns an earliest and latest usable time from four tide-table numbers.
• A spring-range day can cut the window to a third of what a neap day allows.

How to use it

1. Feed the finder the range, not the height: range drives the length of the window.
2. Where the tide table gives a rounded time, assume the error sits at the ends of the window.
3. Reduce the result by a third when wind opposes the current.
4. Keep the printed card with the day plan so the window can be re-checked without a phone.

Safety notes

• A slack window is an estimate, not a clearance to enter a bar or a channel.
• Never use the window as a substitute for a visual check of the actual water.