Recreational Boating Safety – Stability of Towers and Poling Platforms

By Bob Currie, Vessel Examiner
United States Coast Guard Auxiliary Flotilla 081-06-08
I fish Galveston East Bay a lot, and I see a variety of modifications being done to fishing boats, and many of these modifications can affect the stability of the boat. One of the most popular modifications is to add a tower above the center console. Some towers are simple fishing platforms, and some are quite advanced and contain the same operating controls as the center console does. Another popular fishing boat modification is the addition of a poling platform. Poling platforms allow boats to be pushed through very shallow water using a long pole to push the boat. The issue here is stability.

Flotilla 081-06-08 is based at Coast Guard Station Galveston. The Coast Guard Auxiliary is the uniformed civilian component of the US Coast Guard and supports the Coast Guard in nearly all mission areas. The Auxiliary was created by Congress in 1939. For more information, please visit www.cgaux.org.

What is Stability?
Stability is the ability of a fishing vessel to return to its upright position after being heeled over by any combination of wind, waves, or forces from fishing operations due to a change in the center of gravity. A stable vessel is one which has sufficient stability to counter the forces which caused it to heel over and return to an upright position. An unstable vessel is one which does not have sufficient stability to return to an upright position after being heeled over.

Manipulating the Center of Gravity
It is important to understand that the center of gravity rises and falls when weights are added, moved and/or removed from the vessel. By placing weights low on the vessel, we lower the center of gravity. By moving the weight up high on the vessel we raise the center of gravity. The greater the weight or distance from the original center of gravity the greater the shift of the center of gravity.

Forces That Create a Vessel’s Stability
There are two primary forces, gravity and buoyancy, that act upon vessels to provide stability. Gravity is the force that acts to pull the boat down in the water, causing it to sink. For each vessel there is a center of gravity. Buoyancy is the force acting to push the boat up in the water, making the boat float provided that the force of buoyancy is greater than the force of gravity.

When the center of gravity is much higher than the center of buoyancy, the center of gravity acts as a fulcrum to cause a boat that has heeled over to continue to heel over and capsize. That is what we should be worried about when we occupy a tower or poling platform.

Why a Vessel Remains Upright
To understand how a vessel remains upright after being heeled over, imagine the rocking of a baby cradle as in the diagram. The slightest disturbance (waves, shifting of weight) causes the vessel to roll (heel over). As the vessel rolls, the center of buoyancy shifts outboard. To keep the vessel upright, the center of buoyancy must shift faster than the center of gravity.

Positive Stability
Positive stability occurs when the combination of the center of gravity pulling down coupled with the center of buoyancy pushing up creates a righting action that forces the vessel back up to its upright position.

Negative Stability
Negative stability is when the combination of the center of gravity pulling down coupled with the center of buoyancy pushing up creates a capsizing action that forces the vessel to continue rolling over. It is plain to see that by raising the center of gravity that the vessel is increasingly more likely to have negative stability.

Initial versus Overall Stability
Initial stability is that which is felt when seas are calm. As seas increase, the heeling forces on the side of the boat increase. The same thing happens when the side winds increase. A boat which has a sufficient righting capability in calm weather may not have a sufficient righting capability when the seas are no longer calm. Thus, you must take into consideration how your vessel handles during increasing seas before occupying a tower or poling platform. As Dirty Harry would say, “You have to know your boat’s limits.”

Summary
Adding a tower or poling platform to your boat decreases its stability when it is occupied. Your understanding of this concept and by operating prudently, you can decrease your chances of capsizing your boat due to a too high center of gravity.

For more information on boating safety, please visit the Official Website of the U.S. Coast Guard’s Boating Safety Division at www.uscgboating.org. Questions about the US Coast Guard Auxiliary or our free Vessel Safety Check program may be directed to me at rt.currie@gmail.com. I am available to perform free Vessel Safety Checks, and I will come to your location to perform them. SAFE BOATING!

[12-17-2018]

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