Many references to H26/260 being "tender". Is there a technical specification for this attribute? I suspect that it is very relative I.e. Very tender when compared to a full keel boat but maybe not so tender when compared to a centreboard dinghy. Being a "swing keel" boat, My Mac/Venture 25 is some where between a keelboat and centreboard dinghy. I look forward to experiencing how they compare and will report my experience.
There is indeed a technical specification of this characteristic; its called the GZ curve, and it measures the righting moment of a boat at any angle of heel.
The very early part of this curve is called 'initial stability' (say 0-5 degrees), and is mostly a factor of hull (or form) stability. Beamier, flatter hulls will have more, generally speaking.
The main sailing part is called 'secondary stability', and is mostly a function of the location and density of the ballast but also of hull form. Deeper, heavier ballast resists heel more as the angle increases and the lever arm gets longer. Water ballasted boats, not as much.
The strongest part of the curve (MAX GZ) is typically around 60 degrees. Here the boat is at its stiffest. But note that this is WAY past its optimal sailing angle, as well as past it comfort angle!
At a certain point, the boat actually will be happier upside down. This is called the AVS, or Angle of Vanishing Stability. Bad boats close to 90, better boats 115+.
This curve can be calculated by the designer and on modern boats normally is as part of the performance design and validation. On modern ocean race boats (IMOCA 60, Class 40), they will do an actual test, and 99% of the time, the on-water test matches the expected performance by a degree!