Vanishing angle of stability

Peter Crouch writes…

If you read the newspaper articles or the Marine Accident Investigation Board (MAIB) interim report of the tragic sinking of the Bayesian off Porticello in Sicily last August you probably noted the significance of the raised keel, the windage of the tall mast and the vanishing angle of stability.

If your thoughts turned to Carina, with her lifting keel and tall mast and her tendency to heel over to the point where the windward rudder is skimming the tips of the waves, read on.

The Vanishing Angle of Stability (sometimes AVS rather than VAS) is the righting lever created by the displacement of the centre of gravity, which always remains in the same place close to the deepest part of the hull, and the centre of buoyancy.

The recorded values of AVS for the Bayesian were between 84.3º and 92.3º for different loaded conditions of fuel and water with the keel down. In other words she could heel over until she was almost flat in the water and, provided no flooding occurred, would recover to an even keel.

Analysis for her condition at the time of the sinking, with low consumables and keel up, indicated an AVS of 70.6º which could be reached in a wind of about 65 knots.

The Southerly 110 derives her stability from the grounding plate (2020kg) and the swing keel (over 1048kg). The published vanishing angle for the design is nearly 150º and, because the keel swings on a low mounted pivot rather than being lifted up into the hull, reduces by only about 15º with the keel fully raised.

Two thirds of her stability is derived from the grounding plate so she will sail safely with the keel raised but perform and steer better with the keel fully down. The other comforting feature is that at large heel angles the leeward rudder is still close to vertically downwards and the steering is responsive.