Quote:
Originally Posted by tanngrisnir
The capsize screening formula only takes into consideration displacement and beam, so your keel type has no affect on the values. It was specified after the Fastnet disaster to screen out boats not considered suitable for all conditions. Anything less than 2 was acceptable for racing at the time. As for the difference between a 1.7 and a 1.65, It's not really meant for determining which is safer/better, just means the lower number has more form stability for a given displacement.
It is only one data point in considering seaworthiness and a fairly blunt instrument at that. AVS & STIX are probably not available for boats of that age.
At the end of the day if you are anything like me, you are compiling a list of parameters that you will give different weights to to help you justify your choice.
Perhaps look for existing owners and ask them about how the boat sails in different weather conditions and try to get a ride. |
A lower screening number is indicative that the boat has more ultimate stability (ballast) and LESS initial stability (form).
AVS is a mediocre number to use for predicting capsize resistance. Its good for weeding out smaller boats with very low AVS that might be capsized by
wind, and/or moderate sized waves, but for boats with AVS over 125-130 other things are much more important.
STIX is an EU mishmash of a bunch of designers' pet theories about what makes a boat resistant to capsize the components of which are weighted by a formula developed by committee vote maybe?... One of the big components of STIX is righting moment or area under the righting moment curve. This is a mistake as I will explain shortly.
Following Fastnet '79 a bunch of testing was done on model hulls in a special tank where they could reliably create scale breaking waves. They characterized the size of the wave needed to capsize the models against various things like length, beam, displacement, etc. Apparently later in testing somebody thought to include a scale
mast just to be thorough. All of a sudden they needed larger waves to capsize the models. Once they thought it thru they realized several things.
A. Having a mast decreased stability: higher AVS, higher peak righting moment, greater area under the righting moment curve.
B. Having the mast increased capsize resistance.
C. Stability is a static phenomenon. More stability primarily means ability to carry more sail in more wind before needing to reef.
D. Capsize is a dynamic phenomenon.
E. Inertia resists acceleration, in this case roll moment of inertia resists accelerated rolling.
F. Stability can
work against you based on the shape of the stability curve. Form stability is not relative to the center of the earth, it is relative to the surface of the
water. When a
hull is on a wave, form stability tries to have the boat float perpendicular to the face of the
water which is inclined, and the inclination increases at the wave approaches. A narrower boat that uses more ballast stability and less form stability will be slower to start rolling regardless of roll moment of inertia. That is why narrower boats are favored in the capsize screen.
A higher displacement implies the boat has a heavier mast and therefore more roll inertia which is why displacement is in the formula. Even if the boat doesn't have the heavier mast, the increased displacement improves inertia, just not as much as the mast. The mast is 2-4% of displacement but somewhere near 50% of the roll moment of inertia.
If I was going to design a performance cruising boat I would use a very light, moderate beam
hull, moderately deep keel with a bulb or tip weighted fin, and a fairly heavy un-tapered mast
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