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Inland waterways are frequently spanned by bridges. You can’t transit these stretches very far before you’re going to have to pass under one of them. When my wife and I first started boating together years ago, she was quite willing to take the helm if I needed a break, but until she gained more experience, she would always ask, “You’re going to be back before we get to this bridge up here, aren’t you?”

Bridges can be a bit intimidating, and judging from the number of scrapes or damaged timbers we see on fender systems, there is good reason for the concern. U.S. Coast Guard statistics show that a significant number of boating accidents involve vessels striking bridges. So, how do you get comfortable passing under bridges safely? As with most aspects of boating, knowledge goes a long way in overcoming fear.

Bridges spanning navigable waterways fall into two categories: fixed and movable. All have published vertical and horizontal clearances. Movable bridges come in a variety of configurations, including swing, bascule, lift and floating.

When approaching a bridge, begin with the most obvious concern first: Is there enough vertical clearance for your boat to pass under?

The answer to this question isn’t always as simple as it may seem. Most boat manufacturers publish the air draft of a boat, describing the distance from the top of a boat’s highest point to the waterline. Regardless of having this figure from the manufacturer, measure your specific boat to verify the dimension. The consequences of miscalculating can be catastrophic.

The amount of vertical clearance under a bridge is measured at mean higher high water, meaning the worst-case scenario. On drawbridges, the clearance is also measured to the lowest point of the bridge structure spanning the channel. It is not unusual to have 3 feet to 4 feet of additional height near the center or at the point of high steel.

The vertical clearance under the bridge is indicated on gauges attached to the fender system. They mark the clearance, typically, in 1-foot increments; however, the lower portions of the gauges are frequently worn away or unreadable because of marine growth. Bridge operators generally will not tell you what the current clearance is, and they usually can’t see the gauges from their position anyway. You are solely responsible for determining the clearance and for safely passing beneath.

For many years, the governing height for new fixed bridges constructed over major U.S. navigable waterways has been 65 feet. You will, of course, find some much higher, such as the Chesapeake Bay Bridge, and you will still find numerous fixed bridges over lesser waterways lower than 65 feet.

The closed height for draw, lift and swing bridges varies. I know of swing bridges as low as 4 feet, such as the Centerville Turnpike bridge over the Intracoastal Waterway in Virginia, and some drawbridges as high as 83 feet, such as the Woodrow Wilson Bridge spanning the Potomac River in Alexandria, Va.

Movable bridges operate on request or on a set schedule, or a combination. Consult local cruising guides to know which it is for any bridge you’re approaching. Bridges in areas with busy vehicle traffic may have periods during morning and evening rush hours when they will not open at all.

Bridge operators may also carefully monitor the need for a bridge opening. According to federal law, it is illegal to request an unnecessary bridge opening. Fines can be as high as $25,000. If you can easily lower antennas or similar components, enabling you to pass under a bridge, you are required to do so.

Bridges have overhead lights that indicate the location of the navigable channel under the bridge. This is the safest place to pass beneath a bridge. There are a variation of red and green light arrangements indicating the center of a channel or whether it’s safe to pass through. The light arrangements vary with the type of bridge. A description of light arrangements can be found in the Bridge Lighting Manual here:

Fender systems are structures erected around the channel piers to guide vessels through the navigable channels. When waiting for a bridge to open, stay outside of the fender system until the bridge is in the fully raised position and the lights indicate it’s safe to pass through.

Swing bridges, by their nature, create two channels, one on either side of the bridge when it’s in the center open position. Some swing bridges allow opposing vessel traffic to pass on opposite sides of the bridge, while others require all traffic to pass on the same side. Know what the situation is at any swing bridge you are approaching before attempting to pass through.

In the next issue, we’ll discuss the right-of-way for vessel traffic passing under bridges, communication with the bridge operators and other considerations for navigating safely through bridges.

This article was originally published in the April 2022 issue.