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
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 -
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
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
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
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
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
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.
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
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.
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
- 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
or call 1-877-875-6296.