The Capsize Screening Formula is a quick and dirty formula for indicating whether a naval architect should do more analysis of a boat's capsize resistance.
The more involved analysis looks at the roll moment of inertia of a boat to determine its susceptibility to capsize due to wave action. The CSF uses displacement
and weight together as a proxy for roll moment of inertia.
Stevensuf's statement that 'bigger is always better' in regards to length and beam is incorrect. Consider a Westsail 32 vs a Santa Cruz
37, the SC is longer and wider, but which is more likely to roll over in a storm? Or a WS32 and an SC40? Or a Pearson
Triton 28 and a Kiwi 35?
WESTSAIL 32 sailboat specifications and details on sailboatdata.com
SANTA CRUZ 37 sailboat specifications and details on sailboatdata.com
SANTA CRUZ 40 sailboat specifications and details on sailboatdata.com
TRITON (PEARSON) sailboat specifications and details on sailboatdata.com
KIWI 35 sailboat specifications and details on sailboatdata.com
Mitiempo tries to use an absurdity to try to make his point. The formula is intended to be used on a normally ballasted monohull
in sailing condition. An extra 1000lb up the mast
of most boats less than 45'-50' would roll them over at the dock
, that's hardly sailing condition. This would be akin to saying a Toyota MR2 sportscar is bad car because it doesn't have the ground clearance to go off-roading. In reality it is a fine car for going fast on paved roads. Likewise if you use the formula in a manner outside what was intended you get bogus answers.
By way of comparison, consider the alternative: remove the mast. The removal
of that weight would increase the peak righting moment of the boat and increase the angle of vanishing stability. Physical testing has shown that boats without their masts are more likely to capsize in breaking waves contrary to the intuitive expectation.
So then the question becomes what is the appropriate amount of weight aloft for capsize resistance? 1000lb is obviously absurd, but no weight aloft is not that good either. The answer is complex and involves tradeoffs between capsize resistance and ability to carry sail.
I have read that in centuries gone by hoisting small to moderate weights into the rigging
was a way to prevent capsize and calm the motions of boats. I do not recall
where I read this, I do not know if this is apocryphal. The only mention I can find currently is in Richard Henderson's book on single
handing where he indicates on pg 166 that Voss and Waller used anchors hoisted aloft to ease their boats' motions.
Once again, the CSF should be used as a pointer as to whether more in depth
investigation is in order, not as a be all and end all evaluation of capsize resistance.
Generally the best way to use the formula for the lightship weight with engine
aboard but no water, fuel
or other gear
. This should give you the most conservative number. Generally as you load the boat you will keep the weight low so roll moment of inertia increases with little or no decrease in general stability. If you are not loading the boat this way, you should give some thought to doing so.
I would not expect the CSF to work with the Macgregor boats, they rely on water ballast and the assumptions about ballast location and roll moment of interia that went into the CSF do not apply.
There are actually 2 distinct 27' Catalina
hulls, the Cat27 and the Cat270. The 270 is wider and lighter which would give a higher CSF number.