Tides and Currents

Why Tides and Currents Matter

Ignore tides and currents at your peril. They ground boats, sweep vessels off course, prevent marina access, and create dangerous sea conditions. Understanding tidal behavior transforms you from a beginner who fights nature into a competent mariner who works with it.

Tides affect water depth—critical for avoiding groundings and accessing shallow harbors. Currents affect your course and speed—turning a 2-hour trip into 4 hours or creating standing waves that can swamp small boats. Every successful cruise plan considers both.

Understanding Tides

Tides are the rise and fall of sea level caused by gravitational pull from the moon and sun. Most locations experience two high tides and two low tides every 24 hours and 50 minutes (the lunar day).

Spring Tides occur during new and full moons when the sun and moon align, creating higher highs and lower lows. Tidal range (difference between high and low) is greatest during springs.

Neap Tides occur during first and third quarter moons when sun and moon are at right angles. Tidal range is smallest during neaps—less extreme highs and lows.

Tidal range varies dramatically by location. The Bay of Fundy sees 50-foot tides. The Gulf of Mexico has 1-2 foot tides. The Mediterranean has almost no tide. Local geography—bays, harbors, channels—amplifies or dampens tidal effects.

Reading Tide Tables

Tide tables predict high and low tide times and heights for specific locations (reference stations). They show:

• Time: When high and low tides occur (always in local standard time, not daylight saving time—add one hour if DST is in effect)
• Height: Predicted water level relative to chart datum (Mean Lower Low Water in the U.S.)
• MLLW (Mean Lower Low Water): The average of the lowest low tides—the zero point for chart depths

If a chart shows 6 feet depth and tide tables predict a 3-foot tide, you have 9 feet of actual water. If the tide is -1.5 feet (below MLLW), you have only 4.5 feet. Always calculate actual depth before entering shallow areas.

Secondary Stations

Most locations aren’t reference stations. Tide tables provide time and height corrections for secondary stations. To find your local tide, look up the nearby secondary station, find the reference station it’s based on, get the reference prediction, and apply the corrections.

Smartphone apps (NOAA Tides, Navionics, etc.) do these calculations automatically—but understand the process so you can work it manually if needed.

Understanding Tidal Current

Tidal current is horizontal water movement caused by tides. As tides flood (rise), water flows into bays and harbors. As tides ebb (fall), water flows back out. Current speed and direction change constantly throughout the tidal cycle.

Slack Water is the brief period when current speed is near zero, occurring at the transition between flood and ebb. Slack doesn’t necessarily occur at high or low tide—in some locations, maximum current occurs 3 hours before or after tide changes.

Maximum Flood/Ebb is when current is strongest, typically midway between slack periods. In narrow channels or inlets, current can exceed 4-6 knots—overpowering small boats and creating dangerous sea conditions.

Reading Current Tables

Current tables (separate from tide tables) predict current speed and direction at reference stations. They show:

• Slack Water Times: When current speed is minimal
• Maximum Current Times: When current is strongest
• Speed: Current velocity in knots
• Direction: Set (the direction current flows—always stated as direction TO, not FROM)

Like tide tables, current tables have reference and secondary stations with correction factors.

Practical Current Navigation

Ferry Angle
When crossing current, you must angle upstream to make good your intended track. A 2-knot current pushing your 6-knot boat requires about a 20° ferry angle. Your bow points upstream, but you track straight across. GPS COG shows your actual movement; your heading compensates for current.

Timing Transits
Plan passages through inlets, channels, and narrow waterways for slack water or favorable current. Entering against 4 knots of ebb is dangerous and exhausting. Wait 2 hours for slack and save fuel, stress, and risk.

Current Effects on Sea State
Wind against current creates steep, dangerous waves. A 15-knot wind over 3 knots of opposing current generates conditions far worse than the same wind over calm water. This is how bars and inlets become deadly—ebb current meeting oncoming swells creates breaking waves that swamp boats. Time your passages to avoid wind-against-current conditions.

Rule of Twelfths

For locations with two high and two low tides daily, the Rule of Twelfths estimates tide height between high and low:

If you have 6 feet of tidal range (difference between high and low):

• 1st hour: 1/12 (0.5 ft change)
• 2nd hour: 2/12 (1 ft change)
• 3rd hour: 3/12 (1.5 ft change)
• 4th hour: 3/12 (1.5 ft change)
• 5th hour: 2/12 (1 ft change)
• 6th hour: 1/12 (0.5 ft change)

Water level changes slowly at the beginning and end of the cycle, rapidly in the middle hours. This rule is approximate but useful for quick calculations.

Planning Around Tides and Current

Before Departure:

1. Check tide tables for your departure point, destination, and critical waypoints
2. Check current tables for inlets, channels, and passages
3. Verify you’ll have adequate depth throughout your passage (consider lowest tide along your route)
4. Plan to arrive at destinations with rising tide (so you don’t run aground and get stranded)
5. Time inlet transits for slack water or favorable current
6. Check weather forecasts—avoid wind-against-current conditions

Common Mistakes:

• Using high tide depth at low tide (running aground)
• Forgetting daylight saving time correction (arriving an hour early/late)
• Confusing tide change time with slack water time (they’re often different)
• Not accounting for current on ETA calculations (arriving much later than planned)
• Entering inlets at max ebb with onshore wind (extremely dangerous)

Resources and Tools

The NOAA Tides & Currents website provides free, official predictions for U.S. waters. Mobile apps like Navionics, ActiveCaptain, and dedicated tide apps offer convenient access with location-based searching.

Paper tide and current tables are still published annually by NOAA and commercial sources. Keep current-year tables aboard as backup—you can’t access apps if your phone dies.

Many chartplotters display tidal current predictions overlaid on charts, showing current direction and speed graphically. This integration makes current planning visual and intuitive.

Local Knowledge

Tide and current tables are predictions, not guarantees. Sustained winds, barometric pressure changes, and river discharge alter actual conditions. In rivers and estuaries, heavy rain creates stronger ebb currents. Strong winds can hold back or advance tide timing.

Talk to local boaters, harbormaster, and marina staff. They know when predictions differ from reality and can warn you about specific hazards. Build your own local knowledge by keeping a log of actual tide heights and current speeds—over time, you’ll understand how your home waters behave.

Respect tides and currents. They’re more powerful than your boat, and fighting them is futile. Work with nature, time your transits intelligently, and you’ll boat safer, cheaper, and with far less stress.