Cruising the Intracoastal Waterway

Prerequisites

US buoyage and "red right returning" (see the US Buoyage lesson).
Reading a chart, including charted depths and the tidal datum.
Basic VHF radio procedure.

Learning Objectives

This lesson covers inland cruising on the US Intracoastal Waterway (ICW), a common route for ASA 106 graduates moving a boat along the East or Gulf coast. By the end of it you will be able to:

Explain what the Atlantic and Gulf Intracoastal Waterways are and how they are numbered.
Read the yellow ICW symbols on top of normal lateral marks.
Check your mast height against fixed bridge clearances.
Request an opening from a drawbridge and plan around restricted schedules.
Pass through a lock safely.
Manage shoaling, and slow down for manatees.

What the ICW Is

The Intracoastal Waterway is a chain of rivers, bays, sounds and dredged canals running behind the barrier islands of the US coast. The Atlantic ICW runs from Norfolk, Virginia, to Key West, Florida, and the Gulf ICW runs along the Gulf of Mexico coast to Texas. It lets boats travel long distances without going offshore, which matters when the ocean is rough or the boat is small.

Most of the ICW is maintained by the US Army Corps of Engineers, and drawbridges are regulated by the US Coast Guard under 33 CFR Part 117. Positions along the route are given in statute miles from a starting point, and charts and guides refer to bridges, marinas and hazards by their mile number. On the Atlantic ICW, mile 0 is at Norfolk, and the numbers rise as you go south. The Bridge of Lions in St. Augustine, for example, sits at mile 777.9.

ICW Marks: Yellow Triangles and Squares

The ICW uses ordinary red and green lateral marks, but adds a yellow symbol to show how to treat a mark when you are following the waterway:

A yellow triangle means treat the mark as a red (starboard-hand) mark.
A yellow square means treat it as a green (port-hand) mark.

The ICW's own direction of buoyage runs clockwise around the US coast: southbound on the Atlantic side and westbound along the Gulf. Heading that way, keep yellow triangles to starboard and yellow squares to port.

The yellow symbol matters most where the ICW crosses an inlet or river channel. There, the red and green marks belong to the inlet channel, whose direction runs in from the sea, which may be across your course. The yellow symbol tells you how an ICW boat should pass a mark, even if that is the opposite of what its colour suggests. When red/green and yellow seem to disagree, follow the yellow while you are on the ICW.

Fixed Bridges and Mast Height

Fixed bridges set the hard limit for sailboats. The design standard for much of the Atlantic ICW is 65 feet of vertical clearance, and many cruising boats are built with masts that just fit under it. Some bridges are lower. The Julia Tuttle Causeway in Miami (around mile 1087) has about 56 feet, so taller boats go outside along that stretch.

Clearances on charts are measured above mean high water. At a higher-than-normal tide, or after wind has piled water up, you can have less room than the chart shows. Many bridges carry clearance boards (tide boards) at the fenders that show the actual clearance right now.

To pass safely:

Know your exact air draft: the height from the waterline to the top of the mast, including the VHF antenna, wind instrument and anchor light.
Compare it with the charted clearance and with the clearance board.
If the margin is small, wait for a lower tide.
Approach slowly and in the centre of the span, where the clearance is greatest on an arched bridge.

Opening Bridges

Many ICW bridges open: bascule (lifting leaves), swing and vertical-lift spans. 33 CFR Part 117 sets the general rules in Subpart A and a schedule for each bridge in Subpart B. Where the two conflict, the bridge-specific rule wins.

Many bridges do not open on demand. Examples from 33 CFR 117.261 (Florida's Atlantic ICW):

Bridge of Lions, St. Augustine (mile 777.9): on signal, except that from 7 a.m. to 6 p.m. on weekdays it opens only on the hour and half hour, and not at 8 a.m., noon or 5 p.m.
Parker Bridge, Riviera Beach (mile 1013.7): opens on the quarter and three-quarter hour.
West Venetian Causeway, Miami (mile 1088.6): on signal, except from 7 a.m. to 7 p.m., when it opens on the hour and half hour.

Schedules change, so check the current Coast Guard Local Notice to Mariners and a cruising guide before each day's run.

Calling the Bridge Tender

Call the tender on VHF: channel 13 is traditional in Virginia and North Carolina, and channel 9 further south. Check the cruising guide for each bridge.
Give the bridge's name, your boat's name and type, and your direction of travel, and ask for an opening.
The sound signal to request an opening is one prolonged blast followed by one short blast.
Five short blasts from the bridge mean it cannot open, or that you must wait.

While you wait, keep clear of the bridge fenders and hold position against the current. Do not start through until the span is fully open and the tender has cleared you. At a double bridge, make sure both spans are open before committing. More than one boat has hit a second leaf it could not see behind a bend.

Locks

Some ICW routes pass through locks:

The Great Dismal Swamp Canal in Virginia and North Carolina has locks at Deep Creek and South Mills, which open at set times of day. Overhead clearance on that route is much lower than 65 feet in places, so check before choosing it.
The Okeechobee Waterway crosses Florida through a chain of Corps of Engineers locks. In droughts some locks run on restricted hours to save lake water.

Lock basics:

Call the lock tender ahead and ask for the next opening.
Rig fenders on both sides and have bow and stern lines ready, with the crew in place.
Enter slowly when the light or the lockmaster says so.
Hold the boat against the wall with lines handed round bollards or held by the lock staff. Never cleat lines off solid. As the water rises or falls, the lines must be taken in or eased.
Stay alert to turbulence when the water starts to move, and leave only when told.

Shoaling and Depth

The Corps of Engineers aims to keep a controlling depth of about 6 feet on much of the Atlantic ICW. In practice, shoaling, especially where the waterway crosses an inlet, can leave much less. Inlet crossings are where most boats touch bottom.

Charted depths are referred to mean lower low water (MLLW). The actual depth is the charted depth plus the current height of tide. At a negative tide, you have less than charted.
Check the survey date on the chart. Old surveys deserve less trust.
Read the latest Corps survey reports, Local Notices to Mariners and cruising-guide updates for known trouble spots.
Pass through shallow sections on a rising tide if you can. If you ground, the tide will lift you off.
Watch the depth sounder constantly, and stay in the middle of the channel unless the marks show otherwise.

Manatee and Slow-Speed Zones

In Florida, the West Indian manatee is protected under the Endangered Species Act and the Marine Mammal Protection Act. Boat strikes are one of the main human-caused threats to manatees. Manatees surface to breathe every few minutes and rest just below the surface, where they are hard to see.

Florida waterways carry many posted slow-speed and idle-speed zones that are local rules, often seasonal. Obey the signs, and slow down further in shallow, clear, grassy water, where manatees feed. A wake also damages docks and boats along the waterway, and you are responsible for your wake.

Worked Example: A Day Through Florida's Bridges

You are southbound on the Atlantic ICW near St. Augustine, with a mast that is 62 feet including the antenna. You want to be through the Bridge of Lions before lunch.

The fixed bridges ahead are charted at 65 feet. With a 62-foot air draft you have 3 feet to spare at mean high water. A spring tide is due, so you check the clearance boards at each bridge before going under.
The Bridge of Lions opens on the hour and half hour on weekdays, but not at noon. You time your speed to arrive at 11:25 and call the tender on channel 9 for the 11:30 opening.
While you wait, you hold station up-current of the bridge, well clear of the fenders.
Once through, you cross the St. Augustine Inlet, where the chart shows shoaling. You check the latest survey and pass on the rising tide, following the yellow triangles and squares rather than the inlet's red and green marks.

Common Mistakes

Forgetting the antenna and masthead instruments when working out air draft.
Taking charted bridge clearance as guaranteed during a high tide.
Arriving at a restricted bridge just after an opening and drifting in the current for 30 minutes.
Following the inlet's red and green marks instead of the yellow ICW symbols at an inlet crossing.
Cutting corners at bends and inlets, where shoaling is worst.
Ignoring slow-speed zones and leaving a damaging wake.

Summary

The ICW runs behind the barrier islands from Norfolk to Key West, then along the Gulf. Positions are in statute miles.
A yellow triangle means treat the mark as red; a yellow square means treat it as green.
Fixed bridges are typically 65 feet at mean high water, but some are lower. Know your full air draft.
Drawbridges follow 33 CFR Part 117, and many open only on a schedule. Call on VHF or signal one prolonged and one short blast.
Expect shoaling at inlets. Depths are referred to MLLW.
Respect Florida's manatee and slow-speed zones.

Check Your Understanding

While following the ICW, you see a green can with a yellow triangle. How do you treat it? As a red mark: the yellow triangle governs on the ICW, so keep it to starboard when heading south on the Atlantic ICW.
Your mast is 63 feet including the antenna, and a fixed bridge is charted at 65 feet. Is it safe? Probably, but clearance is measured at mean high water. Check the clearance board and the tide before passing.
What sound signal requests a drawbridge opening? One prolonged blast followed by one short blast.
Where on the ICW is shoaling usually worst? Where the waterway crosses an ocean inlet.
Which regulation sets drawbridge opening schedules? 33 CFR Part 117.

Sources

https://www.law.cornell.edu/cfr/text/33/part-117
https://www.law.cornell.edu/cfr/text/33/117.1
https://www.law.cornell.edu/cfr/text/33/117.261
https://www.fws.gov/species/west-indian-manatee-trichechus-manatus
https://www.waterwayguide.com/knowledge-center/news-post/12673/bridges-what-every-icw-cruiser-needs-to-know
https://nauticalcharts.noaa.gov/publications/coast-pilot/