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The Capsizing of the Ethan Allen Highlights a Mistake Many Recreational Boaters Make

by Wayne Spivak, National Press Corps, United States Coast Guard Auxiliary

Weight shift likely caused Ethan Allen to capsize

LAKE GEORGE, N.Y. — The passengers aboard a tour boat that capsized on Lake George, killing 20 people, were sitting on long benches and slid sharply to one side of the vessel just before it flipped over, authorities said Monday. [Oct 4, 2005 Times Argus - www.timesargus.com

Vessels capsizing are a major cause of both accidents and deaths. In 2004, 393 vessels were reported capsized (as the primary casualty event), with 229 injuries and 184 fatalities, according to the United States Coast Guard Office of Boating Safety's 2004 Boating Statistics. (See pdf/Boating_Statistics_2004.pdf.) Unfortunately, the trend of these statistics over the last five years has been less than encouraging.

Boaters can easily reduce their risk, though, so let's take a closer look at capsizing and how to prevent it.

Weight Distribution

Recreational boaters can reduce the possibility of capsizing by understanding some basic laws of physics.

The laws of balance discussed here have four components: the center of gravity, the center of buoyancy, the righting arm, and the righting moment.

When a vessel is riding evenly in the water, the center of gravity is directly over the center of buoyancy. As weight is placed on one side of the vessel, the center of gravity moves toward that side, and the boat lists. This listing causes the center of buoyancy to move, so that now the center of buoyancy is no longer directly under the center of gravity.

The distance between the centers of gravity and of buoyancy is called the righting arm. As the righting arm increases, the ability of the vessel to right itself (bring the center points back in line) decreases. This is intuitively obvious: the more unbalanced the vessel, the more unevenly it rides in the water, and the more likely it is to capsize.

The righting moment is related to the ability of a vessel to right itself. When the righting moment is positive, the vessel will return to a stable position in the water if it is rocked. When the boat is rocked too far, though, the righting moment becomes negative, and the boat will "lose its balance," keep going over, and capsize.

So what does all this physics mean to the typical recreational boater? Put simply, the more uneven the weight distribution on a boat, the more likely the vessel is to capsize.

Weight Distribution Is Not Always Obvious

What changes the weight distribution on your boat?

If we remember our science lessons from school, we recall that matter normally exists in one of three forms: solid, liquid, and gaseous. On our boats, we may find all three, but most of us can discount gaseous matter, since the weights are negligible. Let's look at solids and liquids. Solids are easy to think of: the fire extinguisher, the tackle box, or the cooler, for example. Liquids are also easy: the contents of the water tanks and fuel tanks.

Solids

Of the solids aboard your vessel, there are two types, static and dynamic. A static solid stays in one place. For example, if you securely mount your fire extinguisher to the starboard side of the boat, that solid is now fixed in place, and its weight will stay on that side of the boat. The same holds true of anything that is permanently attached to the superstructure of the vessel. It has a weight that is fixed in place.

But what about that cooler you placed on board, and put in the corner of the deck? It too has a weight, but unlike the fire extinguisher in our previous example, that cooler can move if the vessel rolls or pitches. The cooler can slide across the deck, forward or sideways or diagonally, so it counts as a dynamic (moving) solid.

As the cooler moves, your vessel's weight distribution changes. This change then moves the center of buoyancy and center of gravity apart, creating a righting arm. If we remember the physics we just went over, the larger the increase in the righting arm, the more difficult it is for the vessel to move back to the neutral position (where the centers of gravity and buoyancy are in line).

We'll get back to this concept shortly.

Liquids

If you fill a water bottle right up to the top, put its cap back on, and then shake it, do you feel any water move? No.

If you fill a water bottle up only half-way, put the cap back on, and then shake it, do you feel any water move? This time, definitely yes!

This concept applies to the water and gas tanks on your boat. As tanks gradually empty due to consumption, the liquids will slosh around more, even though the tanks probably have baffles. This sloshing also changes your centers of gravity and buoyancy.

But are there other weights on your vessel? Think about it!

Hidden Factors Aboard

Every boat has two other factors that affect its ability to right itself, but they are easy to overlook.

What are they? They are people and bilge water.

People tend to move around the vessel, both with the movements of the vessel and of their own accord. As they move, sometimes unpredictably, they make changes in the righting arm.

Bilge water moves more predictably than people, but unless you are constantly monitoring and manually operating your bilge pump, you're not quite sure how much bilge water is down there. Again, both these factors change your ability to right the vessel.

While we don't want to jump the gun on the investigation and findings of the National Transportation Safety Board (NTSB), the newspapers have reported that many of the passengers on the Ethan Allen were, just before it capsized, on one side of the vessel.

External Factors

As you read about weight distribution and the centers of gravity and buoyancy, I bet you assumed that the vessel was in flat water. Those of us who boat on most lakes assume that the water is essentially flat, with maybe ripples of a wake caused by the occasional passing boat. But waves, too, change the centers of gravity and buoyancy.

Did you also consider wind? If your boat is floating free in the water, not under power, and with a moderate wind, you'll find the boat moving. If your boat's superstructure is high enough, it acts as a sail, making your boat heel. This is one of the reasons for a keel in a sailboat, to change the righting moment equation, so that the sailboat can accept more list before it capsizes.

When you factor wind, waves, and weight placement together, the picture becomes more complicated. While no single item may have caused the Ethan Allen, or any other vessel for that matter, to capsize, the combination may have done so.

What You Can Do

We'll know better about the Ethan Allen after the NTSB investigation, but the Coast Guard and Coast Guard Auxiliary want you to be aware of these factors.

To boat more safely, please keep some tips in mind:

  • Load your boat evenly and, when possible, secure things so they do not shift.
  • Remind your passengers to keep themselves more or less evenly distributed on the vessel, and to avoid sudden movements.
  • Keep an eye out for bow wakes, wind shifts, and other things that may affect your boat's balance.

The Coast Guard Auxiliary as well as other organizations offer safe boating courses all over the country. Safety starts with education, and taking an approved course is a good way to start boating more safely.

For more information about safe boating courses, please contact the United States Coast Guard Auxiliary at www.cgaux.org or call 1-877-875-6296.

 
 
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