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Fish Safe
Stability I: Know your center of gravity
Stability and watertight integrity are the two most important requirements of safe boating and fishing. Many capsizing tragedies are preventable. Understanding the vessel’s center of gravity and careful planning of stowage and added superstructures can reduce the risk of capsize.
In this, the first in a series of three columns on the concept of stability, we’ll discuss center of gravity. Upcoming subjects will deal with buoyancy and gravity and, finally, stability calculations.
Two factors or forces are at play when we talk about stability. The first is weight, which is pushing the vessel down. The second is buoyancy, which is pushing the vessel up.
The point on a vessel where the weight is pushing directly down is known as the center of gravity. All the weight of the vessel seems to go through this point.
Boats are designed to have a certain point as the center of gravity, and skilled designers take into account how the boat will be loaded and that the center of gravity will change based on the load.
If weight is added below the center of gravity, such as by stowing fish in the hold, the center shifts down to a lower position and stability can improve to a point.
If weight is added above the center of gravity, such as by having fish on the deck, ice on the rigging, or by adding an A-frame superstructure, stability decreases.
Try this
You can simulate the effect of a change in the center of gravity by doing a small experiment using simple materials. This is an especially good activity to help young fishermen understand stability and loading.
Just follow these steps.
1. Take a piece of cardboard 10”-12” square and make a hole in the exact middle of the square, just large enough for a straw to slip through easily.
2. Cut off the corners as indicated (see illustration) or as you like in order to make the square look like a boat.
3. Place the straw through the hole and hold it level. Watch the “boat.” Does it move?
4. Now place a rubber band around the “pilot house,” close to the center of gravity (the hole), and attach a metal ring such as from a key chain or anything small that has a little weight.
5. Attach one or more paper clips on each side of the ring. I put four on each side on my boat.
6. Hold the straw level and check the boat for any movement. Move the rubber band and its paper clip decorations slowly up the pilot house and check the boat for movement with each repositioning of the rubber band. Where is the rubber band when the boat heels? Where is it when the boat capsizes?
It is astonishing how much difference a small change in the position of the rubber band makes to the stability of the boat.
If the rubber band represents fish or water on the deck, it is clear to see that even a slight sloshing of the load to one side would be enough to cause a sudden capsize.
Maybe that shouldn’t come as a surprise since one comment commonly heard after a capsize is how fast it happened.
FISH SAFE
• Consider the center of gravity.
• Keep loads low.
• Be alert to vessel and environmental situations that may decrease stability.
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