# Ship Stability at sea - all models are wrong

Here is the standard, typical stability diagram;

In this model an external force causes the ship to incline putting the ship out of equilibrium, the GZ will then act as a restoring force and return the ship to upright.

This model is helpful to a point but showing the sea as a straight, horizontal line is not always the case.

This is from the book “Know your ship

This explains why a stiff ship, one with high GM in a swell sometimes snap rolls to large angles. The ship is trying to match the wave slope. The epitome of a stiff vessel is a raft as it always matches wave slope.

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1896 to 2019 and the first line is still true today:

“Experience is a wonderful teacher, though often a very slow one.”

Thanks for linking the book @Kennebec_Captain. Both prefaces made me chuckle about my old stab and trim tyrant. Something to skim in the library besides my LaDage.

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La Dage is good on this too. I think a diagram like would be more helpful than just text.

Here is the text:

Effect of GM on rolling. The first point to be made quite clear is that GM is by no means the only factor involved in the manner in which a ship rolls, although it is an important one. We know that with increased metacentric height a vessel will roll more quickly; that is, her period of roll in seconds will be short. The effect of GM on amplitude is less well known. It should be clearly recognized by ship’s officers that a stiff ship in heavy weather not only has a short period of roll but also a large amplitude. Conversely, a tender ship is apt to have a long period of roll with small amplitudes.

To understand the reason why a large GM produces large amplitudes, we can compare a stiff ship with a raft. The raft as a type of vessel represents the acme of stiffness. And how does a raft behave in waves? Does it not assume exactly slope of the seas, thus inclining to large amplitudes? The stiff ship attempts to do the same. She is quick and alive, responding immediately as a wave rolls up her side and under her bottom. The stiff ships bobs about like a cork. The tender ship, on the contrary is sluggish. She lags behind the motion of the waves and thus tends to roll to lesser amplitudes.

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