Avoiding capsize is more strongly correlated to the boat‘s Captain, than its engineering ratios. It’s genuinely not hard to avoid the kind of conditions needed for a capsize. Darn near impossible to think of it happening anywhere near shore, as one would never volunteer to sail in such extreme conditions. The rule of thumb to capsize a monohull is a breaking wave whose height is half the length of the boat and catches it on the beam. Again, if the Captain is caught in such severe conditions, by surprise, they should understand the various ways to avoid being exposed on the beam (sea anchors, drogues, running on a storm sail, etc). Just avoiding open ocean, out of reach of a safe harbor, during severe storm seasons, is basic seamanship too.

 

Ignore that factor. You’re not going to capsize a Beneteau or any other large sailboat.

 

^ thats what I meant to say.

 

This is a draft article that I had written for another purpose. It covers the topic pretty well but also includes the Motion Comfort Index since typically, when questions come up about the Capsize Screen Formula, the Motion Comfort Index usually gets dragged into the conversation.

Within the yacht science and yacht design community, these formulas have largely been deemed to provide no useful information. By and large these surrogate formulas do not include most of the critical factors that impact motion comfort or the likelihood of capsize (such as damping, horizontal nd vertical buoyancy and weight distribution, waterline plane, the extent to which the beam of the boat is carried fore and aft, and roll and pitch moments of inertia) and in the case of the impact of beam, both have beam as a negative while the current science suggests that beam is helpful in resisting capsize and that waterline beam is more critical for motion comfort. The net result is that these formulas tell almost nothing about the reality of a boat's likelihood of capsize or its motion comfort. In fact they provide so little indication of a boat's behavior that to rely on them in any way borders on the dangerous.

To explain, both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. The formulas were intended to provide quick and easy to calculate rules of thumbs based on the hull forms, weight distributions and rig proportions of the day. The half century old Motion Comfort Index and the 40 plus year old Capsize Index preceded the many decades of research and the resultant scientific understanding that has occurred since they were penned. As a result, at best, these formulas have limited utility in comparing boats other than those which are very similar in weight and buoyancy distribution to each other.

The reason that it is useless to apply these formulas to compare boats with differing hull forms is that both formulas over emphasize displacement, which turns out to play a minor role in either motion or capsize likelihood. And both treat beam overly simply, and in the case of the capsize screening formula has beam as a negative rather than a positive it is now understood to be. Neither formula contains almost any of the real factors that control motion comfort, the likelihood of capsize, or seaworthiness. Neither formula contains such factors as the vertical center of gravity or buoyancy, neither contains weight or buoyancy distribution (of the hull both below and above the waterline), the extent to which the beam of the boat is carried fore and aft, and neither contains any data on dampening, all of which really are the major factors that control motion comfort or the likelihood of capsize.

The disconnect between these formulas and real life in large part results from the way that they were derived. The data used to create these formulas were based on the hull forms that existed at the time, which in large part were traditional heavy displacement for their length cruisers, CCA era race rule beaters, and IOR race rule beaters. The inventors of the formulas tried to boil down the easily measured characteristics of these three hull forms, noting that the traditional heavy displacement for their length cruisers had the smallest chance of capsize and the most comfortable motions, and IOR boats the worst behavior with CCA era boats somewhere in between. They interpolated the results of a few easily measure factors and that became the formulas. In the days before computer simulations and real life documentation, that approach made sense as a stop gap. It no longer does.

I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 500 pound weight at the top of the mast. (Yes, I know that no one would install a 500 lb weight at the top of the mast.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth.

And while this example would clearly appear to be so extreme as to be worthy of dismissal, in reality, it may not be that for off. For example, if you compared two boats, the first has a very heavy interior, shoal draft, its beam carried towards the ends of the boat near the deck line, a heavy deck and cabin, perhaps with traditional teak decks and bulwarks, a heavy rig, heavy deck hardware, a hard bottomed dingy stored on its cabin top, and the resultant comparatively small ballast ratio made up of lower density ballast. And if we compare that to a boat that is lighter overall, but it has a deep draft keel, with a higher ballast ratio, the bulk of the ballast carried in a bulb, its maximum beam carried to a single point in the deck so that there was less deck area near the maximum beam, a narrower waterline beam, a lighter weight hull, deck and interior as well as a lighter, but taller rig, it would be easy to see that the second boat would potentially have less of a likelihood of being capsized, and it is likely that the second boat would roll and pitch through a smaller angle, and would probably have better dampening and so roll and pitch at a similar rate to the heavier boat, in other words offer a better motion comfort....And yet, under the Capsize Screen Formula and the Motion Comfort Index it would appear that the first boat would be less prone to capsize and have a better motion when obviously this would not be the case.

There are some better indicators of a vessel’s likelihood of capsize. The EU developed their own stability index called STIX, a series of formulas which considered a wide range of factors and provides a reasonable sense of how a boat might perform in extreme conditions. Unfortunately meaningful results require a lot more information than most folks have access to for any specific design. The Offshore Committee of US Sailing developed the following simplified formula for estimating the Angle of Vanishing Stability (Sometimes referred to as the ‘AVS’, ‘limit of positive stability’, ‘LPS’, or ‘Latent Stability Angle’ ):

Screening Stability Value ( SSV ) = ( Beam 2 ) / ( BR * HD * DV 1/3 )
Where;
BR: Ballast Ratio ( Keel Weight / Total Weight )
HD: Hull Draft
DV: The Displacement Volume in cubic meters. DV is entered as pounds of displacement on the webpage and converted to cubic meters by the formula:
Displacement Volume in Cubic Meters = ( Weight in Pounds / 64 )*0.0283168
The Beam and Hull Draft in this formula are in meters. These values are entered in feet on the webpage and are converted to meters before SSV calculation.
Angle of Vanishing Stability approximately equals 110 + ( 400 / (SSV-10) )

It should be noted that the AVS is only one indicator in evaluating the likelihood of capsize, meaning it only predicts the point at which the vessel wants to turn turtle. It does not predict the amount of force that would be required to heel the vessel to that limit, nor does it predict how the shape of the boat might encourage wave action to roll the boat closer to the angle at which it no longer wants to return. It also does not include the impact of the freeboard of the boat, the change in pitch and buoyancy distribution due to heel or pitch, the positive and negative buoyancy due to the cabin structure or the cockpit. Those characteristics require a much more nuanced calculation based on the specific design boat in question, but that more detailed analysis is what it takes to produce truly accurate and meaningful results.

Jeff

 

^ thats what I meant to say

 

Soooooooo,
Is the sa/ disp formula any good?
Thks

 

There are a number of formulas that are intended to be non-dimensional. In other words they are meant to allow comparisons between boats of different lengths. SA/DISP and D/L are two of those formulas. But interpretation of what the resultant number means in real life is not always self-evident.
For example, when you see a boat with a large SA/D you would expect it to heel a lot. But more often than not there's a bit of a chicken and egg thing happening such that boats with lots of stability often also carry lots of sail area, and may actually heel less in changing conditions.
Or boats with a small D/L would seemingly be light weight and perhaps a have a less comfortable motion. But it may be that these boats have the similar length on deck and displacement but the seemingly light boat has a longer waterline and therefore probably a better motion comfort.

Jeff

 

In my experience, no amount of wind can capsize a monohull when sailing. At some point the windward rail will cut the pressure on the sails. If all the hatches and ports are closed and secured, you get to tell the tale of your big knockdown. However, if one is sailing with the companionway hatch open, and perhaps some ports undogged, well, I sure hope you know how to swim.
I know many an ICW traveler who have made the mistake of leaving their foredeck hatch open for ventilation, only to encounter a wake that soaks their fore cabin. And then they'll blame everyone for their mistake except themselves.
Anyway, unless you are in some pretty large and perfectly shaped waves, which are generally pretty hard to find, you shouldn't give much thought to capsizing a sailboat.
If you really need something to worry about, falling space debris would be good. That stuff can get you anywhere (except for maybe a cave).

 

If you really need something to worry about, falling space debris would be good.

I wouldn‘t worry about that either. Odds of being hit are 1:1 trillion. Odds of being hit by lightening are 1:1.4 million.

If you want to fret, the planet is overdue for a super volcano eruption that would put us into an ice age for decades. Although these occur every half billion years or so and, while statistically overdue, scientists don’t believe this will recur in our lifetime. But it will eventually. My principal is I’d rather be sitting atop it, when it blows, than deal with the after effects. 🙂