There are 2 main types of diver down flag in use currently. These are incredibly important flags and it is vital for you to recognise both if you plan on sailing.
Diver down flags are also known as scuba flags. They appear on the surface of the water to alert other vessels there is a human diver under the water. This allows other sailors to keep clear of the area where the diver is and alerts them to reduce their speed.
The meanings of the flags differ slightly, but as a rule of thumb we suggest flying them both to ensure all of your bases are covered.
Why Are They Important?
You may say that divers go deeper than boats, and so surely it's irrelevant whether waterway traffic drives over you or not.
If you are mid-dive or deep in the water then you are probably still going to hear the boats. They are likely to be able to simply drive over you in a safe method at this point. While it is loud, if you are deep enough this is completely safe.
If you are at a more shallow depth, are beginning a dive, or are returning to the surface after a dive, this is when you are most at risk. This is because you will be entering the same level of the water as larger boats. In a collision between the two, the diver is never going to come off better.
Dive Flag Types
Divers Flag
This flag is red with a white diagonal stripe running from the top left to the bottom right corners. It is often used to alert other boaters to scuba diving and snorkelling in the ocean.
This flag was invented by a US Naval officer in 1956 called Denzel James Dockery. It was widely used by the navy and has remained popular in the US.
It must be shown on a float if you are diving in state waters. The flag is designed primarily to protect the people in the water, rather than on the boat.
In open water, you must stay within 90 meters (300 feet) of the flag. In inlets, rivers, and navigation channels, you must stay within 30 meters (100 feet). As a diver, you should try to reascend within a 45 meter (150 feet) radius of the flag.
International Maritime Signal Flag Alpha (Alfa)
This flag is white on the left and blue on the right. The blue half has a triangular notch cut out of the edge.
It is used to alert other vessels to the presence of a diver, or divers, in the water. As a result, the boat cannot move very far and this is a warning to protect the boat from collisions. It is also used in other situations where the movement of a vessel is restricted.
This flag is also known as Coda Flag A. It must be hoisted at least 1 meter (3.3 feet) high when in use and it should be clearly visible from any direction.
The Alpha flag must be shown on boats and other vessels when they are in federally controlled water.
Surface Marker Buoy (SMB), or Surface Location Marker
A third, more specific use marker is a surface marker buoy (commonly referred to as SMB). While not technically a dive "flag" meant to communicate with external parties, it is a marker meant to help specifically pinpoint a divers location for pickup.
Actually, an SMB is a more accurate pinpoint of where exactly a given diver is, whereas a typical dive flag is a general marker for surrounding boat traffic.
Divers carry uninflated surface markers with them on their dive, and then inflate them to signal the dive boat that they are ready to be picked up and where they are. This type of marker is most useful during when drift diving, as you typically don't return to the initial drop in to get back on the boat.
SMBs are often bright neon yellow or orange, and are usually shaped in a long and narrow cylindrical shape.
Size
If you are hoisting a flag on your boat, the dimensions should be at minimum 20 x 24 inches. This flag should be attached to the highest point of your boat to fly, to maximize the visibility.
If you are attaching the flag to a buoy in the sea, the dimensions must be at least 12 x 12 inches.
What Does This Mean For Other Vessels?
If you are also on the water and you see either of these flags it means you are likely to need to alter your path. If you see a diver down flag you must pass the area with a wide berth, of at least 100 yards. You should also pass at a slow speed.
You should not put one of these flags in a busy area. If you are too near other divers, or in the way of water traffic, this will be a major inconvenience. Use your common sense and look for a safer, less chaos inducing place to dive.
How To Tow A Dive Flag
When a dive flag is in use, you will need to remain within 15 meters (50 feet) of the flag as the diver. If you need to adjust your position in the sea, you will need to tow the flag to your new location.
Using a reel, unspool the line as you dive. Ensure you keep the tension high, as loose flags can become tangled and caught. A good tip for this is to tie a small weight onto the bottom of the line to keep it tense.
Hold the line in your right hand. This allows you to check your SPG and buoyancy with your left hand and the inflator hose. We recommend holding the line out at an arm’s length so that it does not get caught in your equipment.
Never tie the line to yourself or any of your diving equipment. In unlikely cases, flags can get caught on boats and dragged along. It can also be caught by a current.
In either of these cases, to protect your personal safety, you should immediately let go of the line. This cannot be done if you have tied the line to yourself.
If you intend to stay in one area, you can use a rock or other very heavy object to tie the line to. Take care not to disturb any of the existing ecosystem. You can also use a double-ended clip to keep the length of the line constant. This prevents the line unreeling and becoming loose.
We suggest carrying a line cutting device when diving. This is to allow you to cut the line to the flag if it gets caught on something. This will protect your safety.
Another safety precaution is to carry an inflatable surface marker buoy in case you get lost.