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Fish Safe
Stability IV: Flooding not the only cause of capsize
In the third installment of our series on stability, we discussed heeling and righting moments. One reader pointed out that in a capsize event the center of gravity (COG) would move to the side of the vessel because of a shifting load. This situation is true in the case of flooding or the shifting of unsecured loads in the hold, such as fish, ice, or fuel.
However, fishing vessels develop stability problems for reasons other than flooding, and a vessel can capsize without flooding or shifting loads. Here are several examples.
• Fishermen often add superstructures, such as A-frames in the case of scallopers, a modification that can impair a vessel’s original intact stability.
• Ice accumulating on the rigging will also impair stability.
• Weight added above the original COG will raise the COG and make the vessel less stable and more vulnerable to capsize. And
• In the course of fishing, vessels can get hung down or caused to heel drastically by a fouled trap, an overloaded scallop drag, a codend that flips off the reel, severe icing on one side, an improperly executed turn, and more.
These situations can be further exacerbated by wind and/or waves acting on the high side. During these events, the COG will remain in the same place as it was when the boat’s stability was intact.
Three stability states
Stability, then, can be described in three different ways: impaired if weight has been added above the original center of gravity; damaged, if flooding has occurred; or intact, if stability is sufficient to withstand impairment or damage.
Taking the non-flooding, hung-down situation, the COG will remain in the original position while the vessel is heeling but the Center of Buoyancy (COB) will move in the direction of the heel and create a positive righting arm as the heel begins.
The positive righting arm will counteract the heeling moment and the vessel will remain stable.
As the heeling increases, the COB will move back toward and past the COG. This creates a negative righting arm that promotes the capsize.
FISH SAFE
• Be alert for gear placement and handling situations that increase the risk of capsize.
• A non-flooding capsize situation is more likely to take the crew by surprise than one that results from flooding.
• Be aware of the stability limitations of your vessel.
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