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Conservation of angular momentum
I'm pretty sure I read it in Perry's Yacht Design book but I haven't been able to find the reference so I'm going by memory.
They were doing tank tests on hull stability when someone had the bright idea of putting a stick on the models to more accurately approximate real life.
They assumed that with the introduction of weight above the models would logically be more susceptible to capsize. What they found was the opposite.
In a purely static situation the boat without the mast would have the full lever arm of the keel to keep it upright.
In a real dynamic situation however the mass of the mast had a tendency to keep the boat from capsizing due to the conservation of angular momentum
Not totally intuitive but apparently true. People who have lost their mast have reported that their boat seemed even more prone to role subsequent times after the mast was cut away.
This has interesting consequences of course.
One I have never heard discussed is the possibly of hoisting something into the rigging to add weight.
It sounds counter productive as the goal is usually to reduce weight aloft. However maybe it is an appropriate technique in special circumstances.
For example would extra weight aloft stabilize a boat in certain kinds of rough anchorages.