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Sometimes even recreational boaters may feel like they have to become fluent in a new language of signals given the dizzying array of buoys and flags, all of which convey different messages. This is especially true for red and white flags, which have a variety of meanings depending on the design, and are also distinct from red and white buoys.
In North America, the most common red and white flag you’ll encounter is red field with a white diagonal stripe, starting on the upper hoist side and running down to the lower fly-side corner. This flag means that there’s a diver in the water nearby, either wholly or partially submerged. They may be scuba diving or snorkeling, but the message is to stay clear and keep your eyes peeled for people surfacing.
This flag is similar to the blue and white Alfa flag from the International Code of Signals (ICS), which also indicates there’s a diver down in the area and that other boaters should reduce speed and stay clear. The rules for either flag vary by jurisdiction; Florida, for instance, requires you to drop down to idle speed anytime you’re within 300 feet of a craft waving the diver-down flag out in open water. That drops to 100 feet in narrow channels like rivers or inlets.
Amongst the vast library of other marine flags (like the pirate flags some submarines fly), there are also two red and white ICS flags which mean either that a craft is headed into danger, or that the craft flying it requires assistance.
Other red and white flags to be aware of
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Red is a common indicator amongst marine signals and in many cases signals danger or caution, like the black and red buoy indicating that there’s some isolated hazard nearby. The same is true for the red and white ICS flags, which come in two design variations: Uniform and Victor.
The Uniform version is divided into four equal square or rectangular quadrants of alternating red and white. It’s intended to convey to that the observing vessel is running into dangerous waters, whether because of local conditions, hazards in the waterway, or some unseen navigation issue.
The Victor flag shows a white field crossed by a red X. The Victor flag is a direct request for assistance, though it doesn’t convey the same urgency as a Mayday. If you encounter a Victor distress flag, you should reduce speed and switch to VHF Channel 16. Only attempt to render assistance when it’s safe to do so, and keep your own vessel clear in case of hazards.
There are also red and white flags used primarily by Navy vessels. These include a flag divided in half vertically with white on the left and red on the white, the Hotel flag indicating that a vessel has a pilot on board. A white flag with a red diamond in the middle is the Foxtrot flag, and indicates that vessel is disabled, is requesting communication, or that flight operations are underway.