Conference unifix::sailing

193.0. "resistance to capsizing" by PULSAR::BERENS () Mon Oct 21 1985 11:14

The October 15th issue of Practical Sailor includes a report on the
latest results of the SNAME and USYRU capsize study. The results are
summarized in a rather simple (and somewhat approximate) formula: 

                   Bmax
	SF = ----------------
             (DISP/64)**(1/3)

where SF is the screening factor
      Bmax the maximum beam of the boat
      DISP is the displacement of the boat (including equipment)

Boats with a large screening factor are likely to stay upside down if
capsized and are also more likely to be capsized. Note that recent
designs with large beam and light displacement will have a large
screening factor. This is bad news for a lot of boat owners. Keep
reading. 

The study suggests that any boat with a screening factor greater than
2.00 not be allowed to participate in Category 1 offshore races (those
races well offshore where very severe weather may be encountered and
outside help is not readily available). Any serious cruising boat faces
the same circumstances at times. 

The article lists the screening factors for a number of boats. J-Boats
are well over 2.00. A number of other popular boats are also over 2.00.
The Valiant 40, several of which have circumnavigated, two singlehanded,
is well under 2.00. The boat with the smallest screening factor listed 
is the Naval Academy Luders 44 yawl. 


  Bmax      Minimum DISP for SF = 2.00 

    6               1728  lbs
    7               2744
    8               4096
    9               5832
   10               8000
   11              10648
   12              13824
   13              17576
   14              21952

Calculating the screening factor for some of the currently advertised 
boats shows that many of them are rather over the limit of 2.00. To be 
sure, the screening factor is not the only thing to consider when choosing 
a boat, but perhaps it should be kept in mind by those of us foolish and
intrepid enough to sail offshore, either on our own boats or as crew. 

Alan

It has been pointed out to me that the actual shape of a hull and the
depth of the keel affect the stability of a boat, the probability that
it will be capsized, and its behavior once capsized. Therefore, it was
suggested, the screening formula discussed previously is invalid.

Maybe, maybe not. It is well known that (often very light) trimarans
capsize with considerable frequency, and, once upside down, stay upside
down. Trimarans clearly have a very large screening factor. I have the
impression that a lot of monohulls are capsized in bad weather but since
most of them are relatively narrow and heavy, they come back upright,
often without much damage. Some of the boats capsized in the infamous
1979 Fastnet race had wide beams and deep keels and some of them stayed
upside down for some time. The screening factor equation gives, for the
first time so far as I know, a way of numerically comparing the relative
capsize probability of two different boats. 

For a given displacement there will only be so much weight in the keel 
(typically less than 40% in cruising boats and less than 60% in racing 
boats). As beam increases, the righting moment due to the weight in the 
keel becomes less and less effective and the boat becomes more and more 
stable upside down. A thought experiment: which would you rather try 
standing on in the middle of the ocean, a 4x8 foot sheet of 3/4 inch 
thick plywood or a 2x4 inch x N foot long plank weighing the same?

... ever sailed on a Catalina 30?  Now that's got to be beamiest 30' that 
you'll find on the market, short of a multi-hull.

-RPR-

Catalina 27 (with an 8'10" beam and a displacement of 6850 lbs) gives a
SF ~=1.9.  Nice to think about when the wind is gusting to 50 knots.


Phil