By Joshua Bremmerer, CEO at Komodo Covers
In tidal waters, passage planning involves two separate questions: how much water will be available, and which way it will be moving. Tide heights affect access to shallow channels and anchorages. Tidal streams affect speed over the ground and can carry a boat off its intended track.
How do tides work?
Tides are driven mainly by the tide-generating forces of the Moon and Sun. Their timing and range also depend on the shape and depth of ocean basins and coastlines. A semidiurnal tide has two high waters and two low waters of roughly similar height during a lunar day, about 24 hours and 50 minutes. Other locations have diurnal tides, with one high and one low water, or mixed semidiurnal tides, with unequal successive highs and lows.
Around new and full moon, the lunar and solar tidal effects reinforce one another, producing spring tides with a larger tidal range. Around the first and third quarters, the solar effect partly offsets the lunar effect, producing neap tides with a smaller range. These patterns do not replace local predictions of tide heights and tidal streams.
Tidal streams and what to be aware of
Tidal streams vary in direction and speed from one location to another. Use current charts, tidal-stream atlases or current predictions for the waters you intend to sail. Do not assume that slack water coincides with local high or low water: the relationship varies by location.
Check tide-height predictions separately for shallow passages, harbour entrances and anchorages. Use up-to-date charts and sailing directions alongside the relevant local predictions, and check the time zone, daylight-saving convention and vertical datum stated by each source. NOAA Tides & Currents provides predictions for US waters; elsewhere, consult the relevant hydrographic service.
Check the marine forecast alongside the tidal-stream predictions. Wind blowing against the stream can produce steeper, rougher seas, particularly near headlands and in confined or shallow water. A favourable current is not, on its own, a reason to proceed.
Actions to take from your evidence gathering
Plan the departure around the conditions along the whole route, including the depth and tidal stream expected at the destination. The fastest passage is not always the most suitable one.
For a simplified example, assume a boat maintains 5 knots through the water over a straight 15-nautical-mile leg. A constant 2-knot stream directly along the course would give 7 knots over the ground and a passage time of about 2 hours 9 minutes. The same stream directly against the boat would reduce speed over the ground to 3 knots, extending the passage to 5 hours. Real streams change during a passage, so calculate each leg using the predicted direction and speed for the relevant times.
Before anchoring, check the minimum depth expected during your stay, allowing for the boat’s draught and an appropriate under-keel clearance. Account for the maximum depth when deciding how much rode to deploy, and leave enough swinging room for changes in wind and tidal stream. Recheck the boat’s position and holding as conditions change.
Time (and tide!) waits for no one, as the expression goes. They are both relentlessly adapting and changing, so this is why pre-planning is crucial. At the same time, be prepared to make adjustments to your route to have a safe and enjoyable voyage.
About Josh Bremmerer
Josh Bremmerer is an innate navigator, an experienced leader in the boating industry with over a decade of experience in management and company development and a lifelong passion for boating. As the manager of Glacier Ski Shop, Josh grew the small Shop into a nationally recognized business and increased profits by 400%. With that background, it’s no wonder Josh started his own business in the boating industry: Komodo Covers.