Tide-Dependent Departure Time Calculator

Enter your low tide time, tidal range, vessel draft, safety margin, channel length, and boat speed to find the earliest safe departure time and tidal transit window.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Low Tide Time
Input the time of low tide at your departure point in 24-hour format (HH:MM).
2
Specify Tidal Range
Enter the vertical difference between low and high tide at your location in meters.
3
Provide Vessel Draft
Input the depth your vessel draws below the waterline at full load in meters.
4
Enter Safety Margin
Add an extra depth buffer above your draft for safety, typically 0.3–0.5 meters.
5
Indicate Channel Length
Input the nautical mile distance of the tidal channel or shallow section you plan to transit.
6
Specify Boat Speed
Enter your expected speed through the channel in knots.
7
Review your results
The calculator will display your earliest safe departure time, required water depth, and estimated arrival time.

Example Calculation

A boater with a 1.8m draft and 0.5m safety margin needs to depart through an 8nm channel at 6 knots. Low tide is at 06:30, and the tidal range is 12m.

Low Tide Time (time)

06:30

Tidal Range (m)

Vessel Draft (m)

1.8

Safety Margin (m)

0.5

Channel Length (nm)

Boat Speed (kts)

Results

Tips

Prioritize Safety Margin

Always use a generous safety margin (at least 0.5m or 1.5ft) above your vessel's draft, especially in unfamiliar waters or areas with swell. This buffer accounts for unexpected shoaling or waves.

Verify Low Tide Time Accuracy

Ensure your low tide time is from a reliable, local source (e.g., official tide tables). Small inaccuracies can significantly alter safe departure windows, especially in areas with rapid tidal changes.

Consider Transit Time in Channel

If your channel transit is long, the tide will continue to change during your passage. Plan your departure so you have sufficient depth not only at the start but also throughout the entire shallow section.

Precision Planning: Calculating Tide-Dependent Departure Times

For mariners navigating shallow channels or tide-restricted harbors, precise timing is not just convenient—it's essential for safety and efficiency. The Tide-Dependent Departure Time Calculator empowers boaters to determine their earliest safe departure based on tidal range, vessel draft, and a critical safety margin. This tool integrates the Rule of Twelfths to predict water depths, ensuring safe transit through challenging waters and optimizing voyage planning for 2025.

Why Timely Departures are Critical in Tidal Waters

Timely departures are critical in tidal waters because they directly impact a vessel's ability to safely clear shallow areas and avoid grounding. In regions with significant tidal ranges, depths can fluctuate by several meters over a few hours, transforming a navigable channel into a dry bar. Misjudging the tide can lead to costly hull damage, extended delays, and potential environmental hazards. For a vessel with a 2-meter draft, departing just one hour too early in a 4-meter tidal range could mean the difference between safe passage and being stuck on a sandbar for six hours. This planning ensures uninterrupted and secure voyages.

The Rule of Twelfths for Tidal Height Progression

The Tide-Dependent Departure Time Calculator utilizes the Rule of Twelfths to model the progression of tidal height after low tide. This empirical rule provides a practical approximation for how the tide rises (or falls) over a 6-hour period, allowing mariners to estimate the water depth at specific intervals. The calculator uses this information to determine when the water depth will meet or exceed the combined requirement of the vessel's draft and the specified safety margin.

The cumulative rise in feet for each hour after low tide, as a fraction of the total tidal range, is:

Hour 1: 1/12
Hour 2: 3/12 (1/12 + 2/12)
Hour 3: 6/12 (3/12 + 3/12)
Hour 4: 9/12 (6/12 + 3/12)
Hour 5: 11/12 (9/12 + 2/12)
Hour 6: 12/12 (11/12 + 1/12)

The main logic then involves:

Calculate Minimum Required Depth: min_depth = draft + safety_margin
Determine Minutes After Low Tide: Find the time when the cumulative tide height from the Rule of Twelfths first exceeds min_depth.
Calculate Earliest Departure Time: low_tide_time + minutes_after_low_tide

💡 For managing the financial aspects of maritime operations, our Cost per Hour on the Water Calculator helps assess operational expenses.

Planning a Departure from a Tidal Berth

Imagine a boater with a 1.8-meter draft and a 0.5-meter safety margin, meaning they need a minimum of 2.3 meters of water depth. Low tide is at 06:30, and the tidal range is 12 meters. They need to transit an 8-nautical mile channel at 6 knots.

Here’s the calculation:

Minimum Required Depth: 1.8m (draft) + 0.5m (safety margin) = 2.3m.
Tide Rise per Hour (Rule of Twelfths for 12m range):
- Hour 1: 1m (cumulative 1m)
- Hour 2: 2m (cumulative 3m)
- Hour 3: 3m (cumulative 6m)
Time to Reach 2.3m: The depth is 1m at Hour 1 and 3m at Hour 2. The required 2.3m is 1.3m past the 1m mark, which is 1.3 / (3-1) = 0.65 of the second hour. So, 1 hour + 0.65 hours = 1.65 hours after low tide, or 99 minutes.
Earliest Departure Time: 06:30 (low tide) + 99 minutes = 08:09.
Channel Transit Time: 8 nm / 6 kts = 1.33 hours, or approximately 80 minutes.
Estimated Arrival Time: 08:09 + 80 minutes = 09:29.

The Earliest Departure Time is 08:09, ensuring the vessel has sufficient depth throughout the channel.

💡 To maintain an accurate path during transit, our Cross-Track Error Calculator can help you stay on your intended course.

Strategic Planning for Tidal Transits

Strategic planning for tidal transits is paramount for mariners, as it directly impacts both safety and operational efficiency, especially for vessels with deep drafts navigating shallow channels or bars. Maritime authorities and experienced pilots often recommend aiming for mid-tide to high tide for critical passages, as this period offers maximum water depth and a buffer against unforeseen conditions. For example, a commercial vessel with a 3-meter draft might wait an additional 90 minutes after low tide to ensure a 4-meter clearance over a known 1-meter shallow bar. Misjudging these tidal windows can lead to costly outcomes, including grounding that can result in thousands of dollars in hull repairs, engine damage, or even salvage fees, underscoring the necessity of precise calculations and adherence to tidal forecasts.

International Regulations for Underkeel Clearance

International maritime organizations (IMO) and national authorities, such as the US Coast Guard (USCG) and the UK's Maritime and Coastguard Agency (MCA), establish critical guidelines and regulations for minimum underkeel clearance (UKC) to ensure safe navigation. These regulations are not uniform but vary significantly based on factors like vessel type, the nature of the seabed, prevailing weather conditions (e.g., swell), and the specific characteristics of the waterway. For large commercial vessels in restricted waters, minimum UKC can range from 0.5 to 1.5 times the vessel's draft, while recreational vessels might aim for a more conservative 1-2 meters. Non-compliance with these guidelines can lead to severe penalties, including fines, operational restrictions, and even criminal charges in the event of an accident, highlighting the strict adherence required to these safety standards.

Frequently Asked Questions

How does the Rule of Twelfths help determine safe departure times?

The Rule of Twelfths is an empirical method that approximates the change in tidal height over a 6-hour cycle between low and high tide. It states that the tide rises or falls by 1/12, 2/12, 3/12, 3/12, 2/12, and 1/12 of its total range during each successive hour. By applying this rule, mariners can estimate the water depth at specific times after low tide and determine the earliest moment when sufficient depth (vessel draft + safety margin) is available for a safe departure.

Why is vessel draft and safety margin crucial for tide-dependent navigation?

Vessel draft and safety margin are crucial for tide-dependent navigation because they define the minimum water depth required for safe passage. The draft is the deepest point of the vessel below the waterline, while the safety margin is an additional buffer added to account for factors like waves, squat (increased draft when moving), and charted inaccuracies. Insufficient depth can lead to grounding, hull damage, or being stranded, making accurate calculation of this combined depth essential.

What are the risks of departing too early in a tide-dependent channel?

Departing too early in a tide-dependent channel, before sufficient water depth is available, carries significant risks. The most immediate danger is grounding, which can cause severe damage to the hull, keel, or propulsion system, leading to costly repairs and potential pollution. It can also result in being stranded until the next high tide, causing significant delays and inconvenience. In worst-case scenarios, a grounded vessel in a falling tide can capsize or break apart, posing a serious threat to life and property.