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11 September 2016 Updated Pictures

Stability - introduction.

I've always known that boats turn over when they are top heavy, but never really understood the science behind it. Having a boat capsize is not my idea of a good day out. So this little corner of the site is Dr. Holt's attempt to understand model boat stability - without getting too scary or geeky.

There are two 'types' of stability, these being longitudinal and transverse. Longitudinal stability dictates the attitude and behavior of a boat from bow to stern i.e. lengthwise. Essentially we're talking about trim and pitching behavior. On the basis that normal boats don't go 'arse over tit' I'm going to ignore longitudinal stability, other than saying that the moments involved can be calculated. Transverse stability is more interesting because its more of a threat.

Transverse stability affects the way a boat rolls and whether or not she rolls over. A boat is said to be 'stiff' if she is very stable - and will roll quickly through small angles. A boat is said to be tender if she is less stable - and will roll slowly through large angles. The diagram below shows the forces acting on a stable boat when upright.

The next diagram shows the forces at play when a stable boat rolls to 20 degrees. Distance GM is called the Metacentric height and is a measure of stability (the bigger it is the more stable the boat is). Distance GZ is called the Righting Arm or Lever and is the 'torque' which (in this case) returns the boat to the upright condition.