Understanding the Mariners' 1-2-3 Rule

We are in the midst of the 2010 Atlantic hurricane season, which meteorologists and storm watchers alike have predicted to be active to extremely active. The first step for a yacht would be to avoid the storm path -- not an easy feat considering how erratic a hurricane can be.

The National Oceanic and Atmospheric Administration’s (NOAA) National Hurricane Center is a wealth of information and provides guidelines for avoiding hurricanes while at sea. One guideline is the 34-knot rule, which  says that mariners should avoid the outermost radius of a storm's 34-knot winds. NOAA says 34 knots was chosen because when wind speeds reach this strength, the ocean state will change, allowing for less mobility of the vessel. Once winds have hit 34 knots, the likelihood of a vessel finding its way out of the storm’s path decreases.

The Mariners’ 1-2-3 rule, also called the Danger Rule, utilizes the 34-knot rule and is one of the most important means used by mariners to keep out of a storm’s path. The system uses the past 10 years' forecast track errors (FTE) to formulate minimum distance mariners should be from any tropical storm or hurricane. NOAA reminds those on the water that this rule does not account for sudden changes to storms.

According to the Mariner’s Guide for Hurricane Awareness in the North Atlantic Basin, by Eric J. Holweg, NOAA, “The danger area to avoid is the area inscribed by the connecting tangent lines of the outermost radius of 34 KT wind plus a safety margin derived from the ten-year average Atlantic tropical cyclone errors at 24-, 48- and 72-hour forecast positions. Adding 100 NM at 24-hour forecast, 200 NM at 48-hour forecast and 300 NM at the 72-forecast positions.”

Once a hurricane has been forecasted, you can take the position and apply the 1-2-3 Rule, connect the lines and avoid anywhere within them as that is what is known as the “danger zone.”

Captains must decide well in advance of a storm if they plan to run or ride it out in port. Holweg says the most important factor in this decision is time versus distance.

Holweg also stresses that mariners should never cross the track of a storm because sudden accelerations cannot be predicted and although the storm may be days away from where you plan to cross, if the storm strengthens, that prediction could change completely, putting you in the middle of the danger zone.

This diagram excerpted from Mariner’s Guide for Hurricane Awareness in the North Atlantic Basin is an example of the 1-2-3 rule at work. For more information about this rule, refer to www.nhc.noaa.gov/marinersguide.pdf.