Here''s an interesting website that discusses AVS (angle of vanishing stability) LPS (limit of positive stability) RM or righting moment and GZ (the distance between CG and CB). Note the graph showing GZ as a function of heeling angle.
Here''s another interesting page with an estimation formula, and a table of recommended values for small sailing vessels. Note that for vessels of about 30 feet, an LPS angle of 143 degrees is called for for unrestricted offshore use. Extrapolating to 28 foot would probably bump that to 145 degrees.
You can always measure your righting moment directly (GZ times displacement equals righting moment; crudely you can attach a spinnaker
pole of known length to the mast base, attach the other end to a halyard, tie a tension gauge to the spinnaker
pole end and the dock, and measure the tension at various angles of heel up to about 80-90 degrees. Tension in lbs. times pole length equals righting moment; divide by displacement to get GZ), but estimating the curve past 90 degrees is problematic because of cabin top buoyancy.
BTW, when you use the US Sailing online calulator below
and enter data for the Newell Cadet 27 (Disp. 6900, ballast 2700, beam 7.75)
You get an obviously incorrect answer (negative). When I plugged values into the formula I got a value of about 113 degrees, but this may well be off by 15-20 degrees or more.
Here''s a Powerpoint presentation from Navy ROTC at MIT showing lots of diagrams and info about vessel stability.