Mariner Studio

Bridge Clearance Planning with Tide Data

The tide gauge at the Charleston Bridge showed 4.2 feet above MLLW. The chart listed the bridge clearance as 65 feet at MHW. With our mast height of 62 feet, did we have clearance? The answer required understanding not just the numbers, but what they actually meant.

Bridge clearance planning is one of the most critical applications of tide data in navigation. Unlike most tide planning where being conservative costs you time, bridge clearance errors can cost you your mast—or worse. This guide explains how to calculate safe vertical clearances using tide predictions, understand tidal datum conversions, and plan bridge transits with appropriate safety margins.

Understanding bridge clearances and tidal datums

Bridge clearances listed on charts represent the vertical distance from the water surface to the lowest point of the bridge structure at a specific tidal datum. But here’s where it gets tricky: different regions use different datums, and understanding this distinction is essential for safe clearance calculations.

Chart datums vary by region: In the United States, charted bridge clearances are typically referenced to Mean High Water (MHW). This means the clearance listed is measured at average high tide conditions. In other regions, clearances might be referenced to Mean Higher High Water (MHHW) or other datums. Always verify which datum is used on your chart.

Your vessel’s air draft—the height from the waterline to the highest point of your vessel—is measured from the actual water surface at any given moment. This creates the fundamental challenge: your air draft is measured from today’s water level, while the bridge clearance is measured from a theoretical average. To compare them, you need to convert between datums using tidal datum relationships.

The relationship between datums varies by location. In areas with large tidal ranges, the difference between MHW and Mean Low Water (MLW) might be 8-10 feet or more. In areas with small tidal ranges, it might only be 2-3 feet. Mariner Studio displays tide predictions referenced to Mean Lower Low Water (MLLW), which is the average of the lower of the two low tides each tidal day. Understanding how to convert between MLLW, MLW, and MHW is crucial for accurate clearance calculations.

How to calculate bridge clearance in real-time

Calculating whether you have safe clearance requires combining three pieces of information: the charted bridge clearance, the current tide height, and the relationship between datums. Here’s the systematic approach that works every time.

Step 1: Gather your information

Before you can calculate clearance, you need four specific pieces of data:

  • Charted bridge clearance: Found on your nautical chart, typically listed in feet or meters with the datum specified (usually MHW in U.S. waters)
  • Current tide height: From Mariner Studio’s tide predictions, referenced to MLLW
  • Tidal datum differences: The height of MHW above MLLW at your location, found in tide tables or on NOAA’s website
  • Your vessel’s air draft: Measured from your waterline to your highest point (mast, antenna, arch)

Open Mariner Studio and search for the tide station nearest to the bridge. Add it to your tide favorites if you transit this bridge regularly. The app displays predictions referenced to MLLW, which is the standard for NOAA tide predictions in U.S. waters.

Step 2: Find the datum conversion

You need to know the height of Mean High Water above Mean Lower Low Water at your specific location. This relationship is unique to each tide station and doesn’t change—it’s a geographic constant based on long-term tidal patterns.

For Charleston Harbor, South Carolina, for example, MHW is 5.03 feet above MLLW. This means when the chart says “65 feet at MHW,” it’s measuring 65 feet above a water level that is 5.03 feet above MLLW. If you’re looking at Pensacola Bay, Florida, MHW might be 0.7 feet above MLLW because the tidal range is much smaller there.

NOAA provides these datum relationships on their tide station pages. Once you look it up for a particular location, record it—the relationship doesn’t change, so you only need to find it once per location.

Step 3: Calculate actual clearance

Now comes the math. The formula accounts for how much higher or lower the current tide is compared to the datum the bridge clearance is referenced to:

Actual Clearance = Charted Clearance + (MHW above MLLW) – (Current Tide Height)

Let’s work through our Charleston example with real numbers:

  • Charted bridge clearance: 65 feet (at MHW)
  • MHW above MLLW: 5.03 feet (datum relationship)
  • Current tide height: 4.2 feet above MLLW (from Mariner Studio)
  • Your air draft: 62 feet

Actual Clearance = 65 + 5.03 – 4.2 = 65.83 feet

With an air draft of 62 feet, you have 3.83 feet of clearance at the current tide height. That’s your working margin.

Understanding the calculation

Why does this formula work? When the tide is at MHW (5.03 feet above MLLW in Charleston), the clearance equals exactly what’s charted: 65 feet. If the tide is lower than MHW, you have more clearance because the water surface dropped. If the tide is higher than MHW, you have less clearance because the water surface rose.

At low tide in Charleston (near 0.0 feet MLLW), the calculation becomes: 65 + 5.03 – 0.0 = 70.03 feet. You’ve gained about 5 feet of clearance because the water dropped 5 feet from MHW. At an extreme high tide of 6.5 feet above MLLW, the calculation becomes: 65 + 5.03 – 6.5 = 63.53 feet. You’ve lost 1.5 feet of clearance because the water rose above average high water.

This is why many mariners prefer to transit bridges near low tide when possible—you maximize your vertical clearance. Planning your bridge transit timing using spring and neap tide cycles can help you identify the best windows for maximum clearance.

Using Mariner Studio for bridge transit planning

Mariner Studio transforms bridge clearance planning from a paper-and-pencil exercise into a real-time decision tool. Here’s how to use the app effectively for bridge transits.

Setting up your tide station

Search for the tide station closest to the bridge you’re planning to transit. For major bridges, there’s often a tide station within a few miles. Add this station to your favorites by tapping the star icon. This gives you one-tap access to current and predicted tide heights whenever you need them.

The app displays the tide curve graphically, making it easy to see the tide progression throughout the day. You can see at a glance when the tide will be at its lowest (maximum clearance) and when it will be at its highest (minimum clearance). This visual representation is particularly valuable when planning multi-day passages where you want to optimize transit timing.

Planning your transit timing

Open your saved tide station and look at the predictions for your planned transit day. The app shows you the time and height of each high and low tide. Identify the low tide closest to your planned transit time—that’s when you’ll have maximum clearance.

For a bridge with tight clearance margins, plan to transit between one hour before low tide and one hour after low tide. This gives you a two-hour window with near-maximum clearance while avoiding being locked into a very specific time. Remember that weather, current, and other factors might affect your actual arrival time at the bridge, so building in flexibility is wise.

If you’re planning a longer voyage that includes multiple bridge transits, add tide stations for each bridge to your favorites. Then you can review the entire day’s tide cycle at each location and identify the optimal departure time that gives you good clearances at all bridges along your route. This is where combining tide planning with multi-waypoint route planning becomes particularly valuable.

Real-time monitoring during transit

As you approach the bridge, pull up Mariner Studio and check the current tide height. The app interpolates between predicted high and low tides to give you the height at this exact moment. Run your clearance calculation one final time with the current height to verify you still have adequate margin.

Pay attention to the tide trend. Is the tide rising or falling? If it’s falling and you have adequate clearance now, you’re only getting more clearance as you approach. If it’s rising and your margin is tight, consider whether you should proceed or wait. The graphical tide curve makes this trend immediately obvious—a glance at the slope tells you which direction the water is moving.

Regional considerations and variations

Bridge clearance planning isn’t one-size-fits-all. Different regions have different challenges, and understanding these regional variations helps you plan more effectively.

Large tidal range areas

In places like the Pacific Northwest, Maine, or the Bay of Fundy, tidal ranges can exceed 10-15 feet. This creates both opportunities and risks for bridge clearances. The opportunity is that low tide gives you dramatically more clearance than high tide—sometimes 15 feet more. The risk is that if you miscalculate or get delayed, the rapidly rising tide can eliminate your safety margin quickly.

In these regions, timing is everything. A bridge that’s impassable at high tide might offer comfortable clearance at low tide. Many mariners in these areas plan their entire day’s voyage around tidal range extremes, departing at specific times to catch low tides at each bridge along their route.

Small tidal range areas

In the Gulf of Mexico, the Caribbean, or other micro-tidal areas, the range between high and low tide might only be 1-3 feet. This means timing matters less—your clearance doesn’t vary much throughout the day. However, it also means you have less flexibility. If a bridge is too low at high tide, waiting for low tide might only gain you a foot or two.

In these regions, non-tidal factors become more important. Wind can pile water up against a coast or blow it away, effectively raising or lowering the water level independent of astronomical tides. Storm surge, barometric pressure changes, and seasonal river flow variations all affect water levels more dramatically than tides. Be conservative with your margins in these areas.

River and estuary bridges

Bridges on tidal rivers and estuaries introduce an additional complexity: freshwater flow. Heavy rainfall upstream can raise the water level in the river, reducing clearance even if the tide table shows low tide. Conversely, drought conditions might lower the water level below predicted heights.

NOAA tide predictions account for average river flow conditions, but extreme events create deviations. If you’re transiting a river bridge after major rainfall, add several feet to your clearance calculations as a safety buffer. Check upstream river gauges if available—many USGS gauges provide real-time water level data that can help you assess whether the river is running high.

Safety margins and conservative planning

Here’s a truth learned from decades of commercial navigation: your clearance calculation is only as good as your least reliable input. Building appropriate safety margins protects you from the inevitable uncertainties.

Minimum recommended margins

For recreational vessels, maintain at least a 3-foot safety margin above your calculated air draft. This accounts for errors in measurements, unexpected water level changes, and vessel motion from wakes or waves. For vessels with expensive equipment at the top of the mast (like radar or antennas), consider a 5-foot minimum margin.

Commercial vessels typically use even larger margins—often 10% of the air draft or 5 feet, whichever is greater. There’s a good reason for this conservatism: the consequences of hitting a bridge are catastrophic. A destroyed mast is expensive and dangerous. Damage to the bridge itself can bring legal liability in the hundreds of thousands of dollars.

Sources of error

Several factors can introduce errors into your clearance calculations. Your air draft measurement might be off if you measured to the wrong high point or if your vessel is loaded differently than when you measured. Tide predictions, while accurate, are predictions—actual tides can vary from predicted by 6-12 inches due to weather effects. Charted bridge clearances might be measured to different points on the bridge structure than you’re actually passing under.

Wind and wave action can cause your vessel to pitch and roll, momentarily increasing your effective height above the waterline. A good following sea might lift your stern, raising your bow and mast several feet. This dynamic motion means your static clearance calculation doesn’t tell the whole story.

When to wait or find an alternate route

If your calculated clearance is less than your minimum safety margin, don’t push it. Waiting for a more favorable tide is inconvenient but safe. Most bridge transits can be delayed 6 hours to catch the next low tide cycle with dramatically better clearance.

Consider your experience level, conditions, and vessel honestly. In calm conditions with an experienced crew, you might accept a smaller margin. In rough conditions, at night, or with less experience, err on the side of caution. Some situations call for finding an alternate route entirely—many ICW bridges have associated land cuts or offshore passages that avoid the bridge.

Special cases and complications

Not all bridge clearance situations are straightforward. Here are some special cases that require additional consideration.

Opening bridges and timing

Many fixed bridges have nearby opening bridges as alternatives. If your clearance is marginal at a fixed bridge, consider requesting an opening from a nearby drawbridge instead. The drawbridge might have scheduled openings or open on demand, giving you unlimited vertical clearance when open.

Check the bridge’s opening schedule and VHF channel before your arrival. Many bridges open on a fixed schedule (every hour on the hour, for example) while others open on request but may have restricted hours during rush traffic. Plan your arrival timing to coincide with scheduled openings when possible.

Extreme spring tides

During extreme spring tides—particularly around new and full moons when the sun and moon align—water levels can exceed typical predictions by a foot or more. If you’re planning a bridge transit during a spring tide period, add extra margin to your calculations. The combination of spring tide, onshore winds, and low barometric pressure can create water levels well above predicted highs.

Mariner Studio displays the complete lunar cycle and tidal range predictions, helping you identify these extreme tide periods. If possible, plan challenging bridge transits during neap tide periods when tidal ranges are smaller and water levels are more predictable.

Current and speed under bridges

While planning vertical clearance, don’t forget about tidal currents flowing through the bridge. Many bridges are located at constricted passages where current flows strong, particularly during mid-tide periods when the volume of water flowing in or out peaks. Strong currents can affect your vessel control and approach angle to the bridge.

Check current predictions for the bridge area alongside your tide planning. If possible, time your transit to coincide with slack water or minimum current flow. This gives you better vessel control for precise navigation through the bridge opening while also typically occurring near times when clearance is better (slack often occurs near high or low tide).

Documentation and chart notes

Professional mariners maintain detailed records of bridge clearances and transit planning. Adopting this practice improves your safety and efficiency on repeat transits.

Recording bridge data

Create a simple reference sheet for bridges you transit regularly. Include the charted clearance, the datum it’s referenced to, the nearest tide station, the MHW-MLLW relationship for that station, and any notes about currents or approach considerations. Store this with your navigation materials.

In Mariner Studio, use the notes feature when saving tide station favorites. Record the bridge clearance and datum relationship directly in the station notes. Next time you need to calculate clearance, all the information is in one place. You can quickly run the calculation without hunting for the datum conversion or charted clearance.

Learning from each transit

After each bridge transit, note the actual clearance you observed. Did your calculation match reality? Was there more or less clearance than expected? Over time, you’ll develop confidence in your calculations and identify any systematic errors in your approach or measurements.

If you noticed a discrepancy, try to identify why. Was the tide higher than predicted due to weather? Did you measure your air draft incorrectly? Was the charted clearance measured to a different part of the bridge structure? These lessons improve your future planning.

Common mistakes to avoid

Bridge clearance planning seems straightforward, but several common errors catch even experienced mariners. Awareness prevents problems.

Confusing datums

The most common error is failing to convert between datums properly. Mariner Studio shows tide heights referenced to MLLW. Bridge clearances are typically referenced to MHW. If you compare these numbers directly without converting, you’ll underestimate your actual clearance by the height of MHW above MLLW—often 3-8 feet in most locations.

Always verify which datum the bridge clearance uses. International charts might use Mean Sea Level, Lowest Astronomical Tide, or other datums. Using the wrong datum in your calculations can create errors of several feet—more than enough to turn a safe transit into a collision.

Trusting old measurements

Your air draft measurement from three years ago might not be accurate today. If you’ve added new equipment (radar, antennas, solar panels, a davit), modified your mast, or even just accumulated more bottom paint and growth, your air draft has changed. Re-measure annually or after any modifications.

Similarly, vessel loading affects waterline and thus effective air draft. A heavily loaded boat sits lower in the water, increasing the distance from waterline to masthead. If you measured air draft when the boat was empty and you’re now fully loaded with fuel, water, and provisions, you might have several inches less clearance than expected.

Ignoring weather effects

Strong onshore winds can pile water up against a coast, raising water levels above predicted tides by a foot or more. Low barometric pressure allows water levels to rise—roughly one inch per millibar below normal pressure. Heavy rainfall can raise river levels rapidly. Storm surge can raise coastal water levels by several feet.

Check weather conditions before trusting your clearance calculations. If strong onshore winds have been blowing for 12-24 hours, if barometric pressure is unusually low, or if heavy rain has fallen upstream, add extra margin to your clearance calculations. Better yet, wait for more settled conditions.

Advanced planning techniques

Once you’ve mastered basic bridge clearance calculations, these advanced techniques help you plan more efficiently and safely.

Creating clearance tables

For bridges you transit frequently, create a simple clearance table showing actual clearance at different tide heights. List tide heights in one-foot increments from 0.0 to 7.0 feet (adjust range for your area), then calculate and record the clearance at each height. Laminate this table and keep it in your navigation station.

Now when you check Mariner Studio and see the current tide height, you simply look up the clearance on your table—no calculation needed. This is especially valuable when conditions are challenging and you want to make quick decisions without arithmetic errors.

Planning around tide gates

When your route includes multiple bridges with different clearances, you create what’s called a tide gate—your timing must work for all bridges. Start by identifying which bridge is most restrictive. Then work backward from that bridge’s optimal tide timing to determine your departure time from the starting point.

For example, if Bridge A (2 hours from your start) needs low tide at 1400 for safe clearance, and Bridge B (4 hours from your start) needs low tide at 1600, you can’t make both low tides. Instead, calculate whether Bridge A at 1400 gives adequate clearance even if not optimal, then plan around Bridge B’s more critical 1600 low tide. This might mean departing at 1200 to arrive at Bridge B at 1600.

Using tidal curves for precision

The graphical tide curve in Mariner Studio shows the complete tide cycle, not just high and low tide times. This curve helps you identify periods of slow tidal change where clearances remain relatively stable for longer periods.

Near high and low tide, the water level changes slowly—the tide “stands” for a period. This gives you a wider time window with similar clearances. During mid-tide, the water level changes rapidly—clearances can decrease by several inches per minute. Understanding this curve helps you plan not just for a single optimal time, but for a realistic window that accounts for inevitable delays or variations in your passage timing.

Emergency procedures and backup plans

Despite careful planning, situations sometimes develop that require quick decisions about bridge clearances. Having emergency procedures planned in advance keeps you safe when time is short.

What to do if clearance is uncertain

If you’re approaching a bridge and you’re uncertain whether you have adequate clearance, slow down or stop before reaching the point of no return. At many bridges, you can anchor or stand-off nearby while you recalculate and verify your clearance. Don’t commit to transiting until you’re confident.

Contact the bridge tender via VHF if there is one. Bridge tenders often have good local knowledge about clearances and can sometimes provide guidance. Some bridges have clearance gauges showing actual vertical clearance at the present time—if you can see one, use it to verify your calculation.

Alternative routes

Before departing, identify alternative routes that avoid problematic bridges. Many waterways have parallel routes with greater clearances, offshore passages that bypass bridge-limited areas, or circuitous but passable routes. Mark these on your charts and store them in Mariner Studio’s route library.

If you encounter an impassable bridge, having a pre-planned alternative saves you from making hasty decisions under pressure. You can simply activate your alternate route and continue safely, even if the detour adds hours to your voyage.

When to request a bridge opening

Some fixed bridges have adjacent opening bridges. If your clearance calculation is marginal, consider requesting a bridge opening instead. This eliminates all clearance concerns, though it may involve waiting for a scheduled opening or dealing with restricted operating hours.

Know the VHF channel and opening procedures for bridges along your route before departure. Many bridges open on request but have blackout periods during rush hours. Some require advance notice by phone or radio. Having this information readily available gives you more options when conditions change or calculations prove uncertain.

Related features and learning

Bridge clearance planning is just one application of tide data. Understanding the broader context of tides and their effects on navigation improves your overall planning capability:

Key takeaways

Bridge clearance planning with tide data protects your vessel and ensures safe transits through restricted waters. The fundamental calculation—charted clearance plus datum adjustment minus current tide height—gives you the actual clearance available at any moment. Combining this calculation with Mariner Studio’s real-time tide predictions, graphical tide curves, and favorites system transforms bridge planning from a paper exercise into an integrated navigation tool.

Remember these essentials: always verify which tidal datum your chart references, maintain conservative safety margins regardless of your calculations, account for weather effects on water levels, and plan alternative routes before departure. With practice, bridge clearance calculations become second nature—quick mental math you perform routinely as part of comprehensive passage planning.

The next time you approach a fixed bridge, check Mariner Studio’s tide predictions, run your clearance calculation, and transit with confidence knowing your mast will clear safely. That knowledge—earned through understanding tidal datums and proper calculation techniques—is one of the fundamental skills that separates cautious mariners from anxious ones.