St4fM E
(Aned SLL's
Urtc
ON THE EFFECT OF SIZE - AS RELATED
0 CAPSIZE RESISTANCE
INTRODUCTION
This note is intended to clarify the
effects of various yacht parameters, insofar as
we Understand the effects presently, in
determining the relative resistance to a single-wave-impact capsize.
Race organizers have frequently invoked a limit for offshore races which establishes a
minimum size of yacht which is allowed to enter and participate in t-)e race, and one of the
motivations for such a minimum has been safety.
Traditionally this has involved leñgth, and
in response at least one case exists where an entrant added a false nose piece to qualif' for
the race: Cohoe in the 1950 Bermuda Race, and
then discarded it for- the ensuing Transatlantic
Race.
By-and-large the limit has represented the best judgement of the organizers and has been selected without specific numerical basis; not to be arbitrary, but because little exists to
guide-race committees in defining such a limit.
Recall that any such limit is, however,
moré-or-less arbitràry since it is unlikely that
two identical yachts, save one-inch in stern overhang, will respond differently in bad
weather. However, given that some hard limit is desired (or desireable) it remains to develop the best possible measure.
Although length has enjoyed great
popularity, it is rather indefensible. Just as handicap performance "ratings" take into account
many characteristics of a yacht in deerr.ining
its equivalent size, so should a size limit aimed at safety consider more than one dimension. Note that many rating systems have
characterized the rating of a yacht in feet which is equivalent to stating that the yacht will sail as fast as one of some standard
proportions and that length.
In a like manner, it should be possible to
state the equivalent size of a yacht for safety
considerations, and length alone cannot be any
more appropriate for this than for rating.
Until now, we have been denied sufficient information to construct a safety "rating', but recent advances in our knowledge of bad weather
capsize mechanics make possible a try, at least-,
to begin to define size in some multi-variable way.
by
Karl L Kirkman
i Phillips-Birt, Douglas, "British Ocean Racing', Adlard Coles, Ltd., 1960.
39
Faculty WbMT
Dept. of Marine Technology
Mekelweg 2,-. 2628 CD Deift
The Netherlands
This note will develop the basis for making the determination, present the data used in
determining the effects of the selected
variables, and suggest a formulation for
characterizing size from the standpoint of
capsize resistance.
Why -Size?
-The recurring success of voyagers in quite small yachts seems irreconcilable with the
notion that larger is safer; is it possible that
the small yacht will bob like a cork, resisting
the ability of the elements to "get- ahold" of it whereas they destroy its larger counterpart? Could it be that the sthall yacht rolls with thé punches while its resistful large counterpart i-s torn by Targe forces?
The laws of physics say no, àt léast for
the simplified case of
a prismtic yacht form
struck by an incident wave: modeling of the ratio of gravity to dynamic- forces (which is known as Froude Scaling after William Fraude) dooms the outcome to a sithple linear scalar
relationship: the yacht twice as large will survive twice as large a wave.
TECHNICAL APPROACH
In short, what is required parallel.s the
rating situ-ation which -traditionally utilizes a
rating rule to measure t-he equivalent length of
the iÏportant design variations, and a time
allowance table to account for the-
performrice-effects of lehth variations.
Rolling these together- in a situation
analogous to the MHS "VPP" vs. traditiOnal rating schemes seems beyond the reach -of our
present knowledge; but on the other hand this would spare the organizer of a race the task of
specifying storm weather conditions.
-A most difficult philosophical problem also
presents itself. Careful study of this note
will réveal that the practical means of
modifying a yàcht to "optimize" its capsize
rating are bounded to the extent that a twenty
footer cannot be made to start a particular race where bad weather is expected with the same
One consequence of this may well be that
the small yacht have more strict provisions for
recovering from and surviving a capsize than its
large counterpart for equal safety; an approach which is much more difficult to quantify than
some of the physical capsize phenomenon.
Another might be to try to achieve an
equally small probability of remaining stable
inverted regardless of size; that is, the smaller yacht might be required to possess a
larger range of positive stability than its
large counterpart. Such a standard is
hypothesized herein.
DEVELOPMENT OF A SIZE DEPENDENT CAPSIZE RATING
Hydrodynarnic Considerations
From consideration of the hydrodynamics,
from analysis of incidents involving capsizes,
and from model tests, we have developed and
refined an intuition regarding the parametei-s
which affect capsize resistance (and which do
not significantly affect it).
a) YEL001TA MAPS
Ef
FIGURE 1 JET VELOCITY IN BREAKING WAVE
b) VELOCITY PROFILES
At this time, the significant factors
appear to include the following, each of which
will be discussed in a subsection:
Mass moment of inertia/gyradius
Displ acement Be am
Vertical center of gravity
In considering the single wave impact
as a single degree of freedom (roll)
spring/mass response, some insight into the
important dimensions can be gained by
mixing test observations with first principles.
First, recall the data on breaking wave velocity maps and velocity profiles
from Reference 2 which is reproduced as
part (a) and (b) of Figure 1 respectively. Piecing that data together schematically as
in Figure 1, part Cc) the wave jet can be simplified by considering the flow to be a
2 Stephens, 0.J., II, Kirkrnan, Karl L., and Pete-son, R.S., 'Sailing Yacht Capsizing' CSYS 1981.
14
r !JUaV.us. I
. ..
I-'
FIGURE 2 YACHT HULL POSITION WHEN STRUCK BY WAVE
a) TIP.E SE0505CE SEETCH DF MODEL CAPSIZE EXPERIMENT
high
veioity,
fairly
localized
uniform
stream
havin
avelocity,
Vj,
in
the
horizontal plane.
Note that the graphical
data
Onvelocity
profiles
does notcorrespond ectly to the velocity maps but
is used for illustrative purposes.
A review of capsize model
tests
suchas characterized by the data
in
Figure
2(part
(a)
and(b) taken from references 3
and 4 respectively) also show that the hull
at
aconceptualized "moment of impact" has
already
rolled
at
afairly
significant
angle to the vertical, even in the absence
of
windThis
position
is
shownschematically in part (c) of Fïgue 2.
Combining
this
physical
notion
of
ahigh velocity jet area from Figure
1 andthe sigñificänt roll
angle at the instant
of
impact fom Figure
2,the geometry at
the
instant
of impact,
andthè fesulting
forces
and moments can behypothesizd
based upon the sketch shown in Figure 3.
FIGURE 3 - DEFINITION SKETCH FOR. ROLL PHYSICS
When
these
are
combined,it
is
possible.
to
hypothesize
a frameworkof
forces and moments which fit the equation
of motion:.
F Ma,
for
the
system
in
roll.
With
sithplificatï'ôn,
the
equation
canreformulated at the instant of impact:
1/2 pVj2SR
lxx Q
where 1/2 pV'j2 =q of jet
S = drag area
r = moment ,m of jt forcé
lxx -= roll moment of inerti.a
Cr
roll acceleration
Clearly this rèpresentation Includes a
number of gross simplifications including:.
The transfer of ñiomentum from the
jet to the hull
is. neglected
Added mass in roll is neglected
'41
(3)
Roll
momentsfrom
other
fTowpatterns are neglected.
It
should
beappreciated
that
this
representation
is
suggested
primarily
tò
give
aninsight
into
the
relative
importance
of
the
various
parameters
andnot to estimate actual values.
Equation
[2)
can befurther
manipulated
to
iepresent
alevel
of
critical acceleration which fOr a specific
design, exceedance will result in "capsize
i.e.
roll
motion
beyond somearbita-y
thréshold:
&Crjt
e1/2 2Vj2Sr
lxx
If the jet velocity
is then taken
aspropotioñed to the square root of the wave
height,
and some constants
are d1àrded,
the equation becomes:
O'crit
lxx
where h = wave height
s = struck. area
r = ïadiu
of action of striking
fOfè about VCG
Let
us nowsimplify further
byholding
freeboad as
a fixed proportion of length.
(This
i
done because afreeboard
changeimplies a change in range of stability añd
hence in threshold of capsize, o cnt.)..
As an
side, earlier test results (Ref
3, Figures 21, 2? ãñd 26) show a pronounced
lack
of
sensitivity
tOTikélihood
of
capsize
fOr
fairly
large
variations
in
freeboard.
A
final
simplification
involves
decomposing
the
capsize
armr
into
components related to hull georitetry. 'For a
fixed
VCC, andfreeboard,
this
is
asfollows:
3
Kirkman, K.L., Nagle, T.J., and Salsich, J.O.,'
Sailing Yacht Capsizing',
CSYS 1983.
more particularly: r
b'
+ Cwhere
b = Beam, and
c
Height,
center
of
pressure
above VCG.Substituting into (4),
' cnt is
directly proportional
to
Band (VCG-CP) and Thversely p,oportional to
lxx.
J.(j2
'lxx
(6)
Let us consider now this
is
born out
by model test results.
(1)
C5)r
s orne bet
) IMODEL TESTS
Model tests involving a variation in
wave height have been condîjcted by Salsich
and Zseleczky at the U.S. Naval Academy
Hydrodynamics Laboratory (Ref. 4) to
explore the sensitivïty of a simple
prismatic nodel to capsizing with variation
in a number of parameters: displacement,
gyradius, beam, appendage area, and
freeboard.
Shortcomings of wave family
In pHnciple, the data should allow
for the deriviation of variations in susceptabi-lity, and a simple measure would be as an equivalent change in wave height. Because of the means by which the diffeent
waves were generated as detailed below,
such a direct measure was not possible, and at first glance, these data did not support
the rather severe sensitivities
hypothesized by fundamental considerations described above. This was the cause for
concern which has only tentatively been
assuaged by the following consideration.
The method of generating breaking waves in the laboratory was dïscùssed ih detail in Reference 3; in
sumary
thedispersion relationship for water Waves was utilized to generate a train which converged
and boke
in the test sectionarea of the tank,. Although rigoroús
procedure would have involved developing a
new wave train for each wave height and demonstrating that this family was similar to the others, expendency dictatêdthat the height variations be achieved by tampering with the amplitude of all waves generated
but with the phasing relationship
unchanged.
As a result, there is no assurance
that
the
breakers
are
correctly
scaledversions
of one another, in fact that would be unexpected. Thus, while the data showthe effect of wave size, the nominal height
measured and plotted cannot be used as the scalar of the wave size variation in terms
of energy available to capsize because this is such a sensitive phenomenon.
In the context of the original
experiments, nOne of this presénted any
problem since the data were intended to
répresent more or less severe waves, but it does limit the utility of the data for this
new purpose of estimating numerical values
of
sensitivity.L
However, as the waves of various sizes were repeatable, the notion
Of uing the
data to check ratios of ensitivity One to another in hypothesis and then in the experiments was introduced. For example,
the ratio of effect of dislacement to
gyradius as postulated iti (4), was checked
using the test data and cnfirrñed approximately, and such checks will have to suffice until new tests can be conducted.
As with many puzzles, unlocking one
relationship allows for clearer understanding of others not previously
possible and this is particularly true in
the case of unlocking the relationship of
beam to capsizing.
The data in Figure 4 (from Reference
4) were presented to show the effect of
beam, but due to the scaling of the models
'N:;
FIGURE 4 - ThE EFFECT OF BEAM VARIATIONS ON CAPSI RESISTANCE
included an unintended displacement-length ratio variation. The result of this seems to have been to disguise a rather marked sensitivity to beam whih can be shown by
manipulating the data as follows:
The beam variation testéd was generated by taking the baseline model and
changing the half-breadth coordinates while
holding the depths to yield variants with
25-percent less and 25-percent more beam. As a result the displacement per unit
length changes proportionately.
Using the relationship that the effect of displacement is linear, and that of gyradius squared, the square root of the ratio of displacements was used to correct the plotted gyradius values, thus creating data for models of equal displacement and
gyradius. The results are shown in Figure
5 and the supremacy of the narrow-beam
model is far more clear than as previously presented.
4 Salsi.ch, J.O. and Zselec-zky, J.J., 'Experimental Studies of Capsizing in Breaking Waves, AIAA/SNAME AI-XIII, October 1983.
o
o
11 12
WAVE HEIGHT IN INCHES
FIGURE 5 - THE EFFECT OF BEAM VARIATION WITH CAPSIZE RESISTANCE WHEN CORRECTED FOR DISPLACEMENT
Recall that one of the sharp design
t-rends shown in Reference 2 and reproduced
herein as Figure 6 was an increase in the
beam of recent racing yachts, and that one of the most definitive relations between
fastnet capsizes and design parameters was shown in the same reference to be with
beam, Figure 7.
At the same time, freeboard has been shown by limited tests to be a relatively unimportant variable and this was
attributed to the bilance bétween the
disadvantage of high freeboard in impact
area balanced by the advantage, of freeboard
on the lee side in avoiding tripping over
the lee rail.
LEGEND ONE TON CHAMPI.ONS
REPRESENTATIVE DESIGNS
FIGURE 6 - TRENDS IN IOR YACHT BE.AM
43
FIGURE 7 - RELATIONSHIP BETWEEN FASTNET CAPSIZES
A1i ¿ÈÁM
Size Rating Rule
Accepting that the' physical model
matchés (6), and the desire to determine an equivalency on a length basis of equal
capsize resistance, one way of expressing the length (using the concept of a base boat of normal proportions) is the fol lowing:
L' =
(.JB)2
+()2.I
(7) where: L = capsize length, feet
L measured length (MH '1). feet base beam given by + 2, feet B = measured beam (MIlS 4"MB), feet
CB base center of pressure above YCS given as 2, feet
C = estimated center of- pressure above VG' giv'en by 2-CGTOT, feet
I = estimated roll omment of inertia, f't' lbs (See Ref S) = base roll moment of inertia given by .135 L4-5, ft2 lbs
This calculation has been performed for the MHS fleet with the results as shown
in Figure 8.
5 'Safety from Capsize Project - 2nd Interim Report of the Directors", USYRU, June 1984. 0.05
o
0. 0'4 -J 0.03 C-0.02 0.01 o '60 '65 '70 '75 '80 7 6 5 4 :AIUS IN NC H ESBALLAST
-POSITION:
MEASURED LENGTH
FiGURE 8 - COMPARISON OF'CAPSIZE LENGTH TO
MEASURED LENGTH F(R IWtCAL FLEET
me role of range of stability
The phyicai model postulated in
equation (3) and developed thereon to the size rating rule of equation (6) aid the L'
size equivalent was based upon a critical level of roll accé]erati.on, frcrit, which represented a threshold of capsize. It
would seem, thén that in additioh to the size equivalent, L', noted, range Of positive stability may also bé important in
the caØize.
BILGE
SOURCE: UNPTBLISHED RESULTS BY SALSICY & ZSELECZKY
AGRA. AFORO LAB
100 110 120 130 RANGE 0F POSITIVE STABILITY
TOP OF
KEEL
BOTTOM
OF KEEL
140
FIGURE 9 - VARIATION OF CAPSIZE TEÑDANCY WITH RANGE OF STABrLITY
Further, a small range of negative Stability is most important in avoiding
stable inveted equilibrium. Cohsider the
data irs Figure 9 and 10 in support of this role of range of stability:
\ BOTTOM OF'0
KEEL
iDO 110 120 130 140 RANGE DF STABILITY IN DEGREES
FIGURE 10 VARIATION OF TENDANCY TO' REMAIN
- STABLE INVERTED WITh RANGE OF
STABILITY
Observation of the capsize components seems to indicate that two phases of the
phenomenon: à "snap-roll' phase where inertia domihates and then à phase where
the yacht is left to its Own devices as the
dyna,Úi -forces dissipate and classical static stability considerations achieve dominance If this is the case, a yacht
with a larger range of static stability (other things being equal, eg: roll moment
of inèrtia, displacement, beam, etc.) wiH
be in a better position to use this
characteristic to return to upright as the static stability takês over. This leaves
the issue, what is the role of stability in
capsize resistance and to answer this both
phases must be considered.
It exceeds the scope of. this note to
develop the statistics of this secOnd phase
in detail and Figure 11 of Reference 10, and accompanying text may be useful for background. Suffice it to say that any
experiments, and indeed real life, may be biased by the reàlity that thé band of
disturbance energy present
is quite nàrow
and probably lies adjacent to thè upper
edge of the range of stability of the
yachts of interest so that specific test
results which measure percentages of yachts trapped inverted stable will probably not
match the 'wheel of fotune'! analoy.
10 Kirkman, K.L. "On the Avoidance of Inverted Stable Equilibrium",
A1AA/SNAfr1E, Anciént Interface XIII, Oct 1983.
: / / / /
/
o o ooa
/ o o o // 0O
Oc C. oI
/
/
I
/
/
/
o I/
o C_I
LE G'E N D -C 10G 80 60 40 20 BALLAST POS ITIOU SOURCE\
N BELGE\
N TOP OF UNPUBLISHED KEEL SALSICH B ZSELECZF.Y, U.S.N.A. HYDR0 LAB\
\
\
MISCELLANEOUS ULDB'S SOR I 1Q 20 30 40 50 60 o000
/
0/
/
00
/
: r
30 20 10/
90 80 lo 60 50Specifically, Figure 11 shows some test results for yachts with masts and
various ranges of stability and the quality
of fit to the analogy. Because the wave used in the tests was sizes to 'just capsize the models a wide range of
disturbance energies was not present and
the result should not be so expected to fit
the dotted line.
C EN T I'; VED T ED SAD.0 0.5
EiLEAIU!
1.0 L 1 I I.Q. .(_j... 90 100 110 120 130 150 160 170 180 RANGE 0F STABILITYFIGURE 11 - COMPARISON OF VARIATION OF TENDANCY TO REMAIN STABLE INVERTED WITH "WHEEL OF FORTUNE" ANALOGY
Another reason for introducing the role of range of stability as related to
re-righting into the problem of resisting capsizes in the first place is simply this:
with no constraints from either practical
construction, sail carrying power, or
concern from re-righting, it will appear
that an "optimized" configuration considering only the snap-roll phase
involves a high VCG with a maximum roll
moment of inertia. In particular, for a
boat with a small range of stability (i.e.,
a relatively high total VCG, say at the OWL) it may lower the capsize vulnerability
by raise the ballast further! Consider
Figure 12 which indicates a break-even
range of stability of about 120-125
degrees. If the contours of capsize probability are accepted for the moment, either raising or lowering the ballast
seems helpful; but if one refers back to Figure 11 the ballast raising alternative
carries a high probability of inverted
equilibrium whereas the lowering
alternative reduces the probability of same to an extremely low valve. As a practical
benchmark, the yacht tested with a VCG=0 starting point represents a 46-percent
ballast ratio with the VCG of the lead at the top of the keel - terribly close to
contemporary competitive tOR boats.
WHEEL 0F FORTUNE
45
-
BEAM DISPLACEMENT T'ACE OF PRACTICAL BALLAST SHIFT GYRADIUS IN INCHESFIGURE 12 - VARIATION IN PERCENTAGE OF CAPSIZES WITH GYRADIUS AND VCG
It must be appreciated that the shape of contours may reflect an inadequacy of
the method of accounting for some effects
other than VCG and lxx, and that the implications discussed below as related to
range of stability are only that.
Two lessons can be inferred from
Figure 12, the certainty of which must be tempered by the speculative nature of the
probability of capsize contours:
1) Depending upon the initial valve,
range of stability has a weak influence on
probability of a capsize after inertia effects have been accounted for separately. Further, loweriig the range of stability by. raising the VCG for a yacht with an
excessively low range (from a practical
standpoint) initially may have a large
pay-back in capsize probability reduction, but the yacht will be unable to carry sail,
cannot withstand knockdowns of an
'aerodynamic sort, and will almost certainly stick inverted once pushed down. However,
(2) Practical VCG shifts, as shown by the trace labelled "ballast shift" not only
increase the range of stability but also
tend to lower the probability of capsize.
o Raising ballast from typical
positions has a very weak tendency to
improve probability, so weak as to be
practically negligible.
o Lowering ballast from typical positions has a vèry weak tendency to improve probability, so weak as to be
practically negligible.
o A neutral zone where ballast shifts have a small effect exists with the total VCG near the DWL.
Of course, all of the above are for a single yacht configuration, displacement, beam, appendages, freeboard, etc.
= 150
r
140 130 120 110 X 1 00P
Based upon this,
it seems justified to
delete from the capsize 'size" formulation
any measure
of
rangef
stability
to
account for the shift in & cnt value with
this change.
If we accept the contours in Figure 12
for probability of capsize as a function of
range of stability (for fixed length, beam
and
displacement)
aprobability
of
capsizing and remaining stable inverted
is
calculable as follows:
If
wetake
the
probability
of
acapsize as a funtion of range of stability
from the data in Figure 9 añd shaped as
in
Figure 12, we get a curve as
in Figue 13,
part (a).
Then,
the.
probability
of
inverted
equilibrium,
if capsized, taken from Figure
10 can be taken as in Figure 13, part (b).
The
product
of
these
or
joint
probability of capsize and inverted stable
equilibrium is as Figuré 13, part (c).
This result,
to wit, that a range of
positive stability of 140-degrees implies a
vanishingly small
probability of
inverted
stable equilibrium corresponds well to the
findings
of
Reference
11: "Fromobservatioh
of
the
tests
it
is
&pparent
that those (yachts-ed) that have angles of
vanishing
stability
less
than
150-160 degrees can beleft flòating
upside down
after encounter'ing a breaking wave."
PROBABILITY 0F INVERTED EQUILIBRIUM
0INT PROBABILITY 0F CAPSIZE AÑO ISVERTED
STABLE EQUILIBRILJII
FIGURE 13 - PROBABILITY 0F IÑVERTED EQUALIBRIUM WITH RANGE 0F STABILITY
THE NATURE 0F BREAKING SEAS
To
this
point,
the
subject
has beentotally
restricted
to
the
rating
of
relative size; how big a breaking wave can
a specific design withstand.
The time has come to answer a far more
complex
question:
For agiven
set
of
charactéristics
for
adésign
underconsidération
how much
bigger or
smaller
standard yacht has the same prObability of
a
capsize?
This
is
the
time
allowance
table analogy to handicapping.
A-
clear
understanding
requires
anappreciation
of
the
basis
of
rough seageneration because breaking waves represent
än instability in nature bth in the
iicro
(a pile of water of
a steepness beyond its
dynamic
equivalent
of
"angle
of
repose")
and
in
the
maceo sense. (A.transient
oceanographic phenomenon, unlikely to occur
for extended timés.)
HOW ARE BREAKING WAVES. FORMED?
In dealing with -bOdy generated waves,.
the wäke of a power boat in a harbor (or a
duck swiasning
along
apier)
is
putting
energy into the water via wave-making drag
in a very steady manner.
Ocean waves
of
interest
in
capsize
phenomenon are not so nicely ordered; they
are generated by unsteady phenomenon, they
interact
with
oneanother,
andthe'
dispurse
(that is
they travel
across the
ocean
at various
speeds)so that
to
the
observer they appear quite complicated..
Asa result of this behavior we tend to deal
with
thehiih statitical or probabilistic
way. Theclassical
oceanengineering
treatment of
irregular waves assumes that
the.
systemcan
bedesignated
by a"photograph"
which
fixes
it
in
time yet
this does not apply in the case of breaking
wave phenomenon.
Let us consider some basics to see how
all of this applies to our capsize problem.
So much of what we utilize in working
with
wavesis
expressed
in
terms
of
frequency
that
wé must
have afeel
for
these unfamiliar measures.
Figure 14 shows
socs general descriptions of wave types and
indicates
bot-hthe frequency,
ci,and the
physical
length of thesE.
Notethat the
descriptions overlap and the zones are not
well
divided;
that is
not important since
the purpose of this graphical presentation
is only to give a féél for the waves under
discussion.
Theieason
for
introducing
this term for frequency.,
ai,will
be clear
shortly.
PROBABILITY 0F CAPSIZE
110
id
1 50 170 180WM'E
HEIGHT
FIGURE 14 - RELATIONSHIP BETWEEN LENGTH MD
FREQUENCY OF 'WAVES
If we were to go back to our view of a
rather confused ocean surface,
freeze the
waves
in place and make a saw cut section
through
the
surface
aview
of
this
cut
would appear as Figure 15.
This irregular profile can be shown to
be made up of a
large number of component
waves äll superimposed upon one another in
the
oceahresultiñg
in
the
confused
appearance.
FIGURE 15 - IRREGULAR WAVE HEIGHT TIME HISTORY
In fact,
a Figure 16 reproduced from
Refèrence
6 showsjust
such aseries
of
regular waves which are these components in
part
(a)
and a meahsof expressing
this
information
called
'a'spectrum"
in
part
(b).
Thespectrum
is
a way
of eaily
portraying a confused sea where the energy
t each fequency, w,
is plotted vertically
as
in a histogram and the envelope of these
represents the spectrum.
This curve has a
number of useful properties beyond that of
compressing
the many
items
regarding
the
regular
wave componentsinto
asingle
display;
the
area
beneath
the
spectÑl
curve is a direct measure of the enègy in
the spectrum and the shape tells us how the
energy is distributed.
7
Van Dorn, William,
York 1974.
TIME
6
Principles 'of Nãval Architecture, Snarne, 197O.
i) TYPICAL ENtP.GY SPECTRUM, SHOWING
APPR0Y.iMATI0 B.Y A FINITE SUM 0F
coM:pouEN IS
b) SCALE 0F FREQUENCY SPEC'T'RUM
FIGURE 16 - ,KE-UP OF IRREGULM SEA
The grOwth of a spectrum is 'also key
to
understanding
b'eaking
wavebehavior.
Waves fori as a by-prbdùc.t of the shéàr set
up when wind blows across the sea surface.
This
begins
with
a complexpattern
of
ripples
which
eventually
growto
sirtall.waves, etc.
The presence of the ridges of
water causes
a pressure distribútion which
leads to energy being transféred into the
ridge frOm the wind.
However, many practical
sea states do
not grow 'slowly enough, nor does thè wind
blow steadily enough
to reach an orderly
spectrum.
'A meansof
estimating
the
effécts of non-u'niforn conditions
is
iven
by thecumulative sea, state (CSS)
diagram proposed by VanDorn7.
'Such adiagram
allow's. one
to
estiifte
the
relàtive
severity of
asea state with an unsteady
wind time history.
An example
of
the
useof
the
CSSfollows' 'as related 'to Fastnet 79.
A tifflé' histOry of the wind experienced
by thé Fasthet fleet and
as charàcteHzéd
by a number of sumftiary analysis is given in
Figure 17.
What is then required is to go
fròrn
such acontinuous
estimate
of
windspeed to a wave characteration.
Oceanography and Seamanship", Dodd, Mead & Co., New
47
FIGURE 17 - OBSERVATIONS OF FASTNET WIND
STRENGTH TIMEHISTORY
The method suggested by Van Dorn
entails breaking the wind speed record into discrete time steps with an equivalent wind
speed throughout, and entering the CSS with
the energy from the last segment as a
starting point to build upon. For example,
if the first step is taken as twenty knots for four hours the CSS diagram, Figure 18,
is entered along the "Wind Speed, V
(KNOTS)' contour labeled "20" for a
duration of four hours. This energy level is then maintained while shifting laterally
to the next wind speed (in this case .28
knots-, (2) and along that contour for its
duration. When completing the entire process, (3) the significant wave height
can then be determined using the right-hand auxiliary scale; ih this case the Fastnet
conditions probably were equivalent to a
fully-developed sea (FDS) with a
significant wave height of about 37-feet.
148
RESPONSES
RESPONSES
ESTIMATED SIGNIFICAIJT WAVE HEIGHT IN FEET
a) SIGNIFICNAT WAVE HEIGHT
OCEAN SCIENCE CSS ESTIMATE ESTIMATE / /
/
O 10 20 30 40o/°
/
.7.ç
O 10 20 30 40 50 60It is interesting to correlate this
50
/
o/
/
O\
\
055 diagram estimate with observations, and
the Inquiry8 gives a great deal of data from participants questionnaires to allow
40 O
/
X/
\
this.The participants were queried
regarding the significant and the maximum
30 O
/
/
A wave height. Not surprisingly, the estimates by the respondents ranged widely
as shown in Figure 19, but the circular 20 data points define a frequency polygon in
each case which is near the CSS estimate; in fact the tendency to overestimate wave
10 height
is belied by this comparison. Perhaps the most surprising factor is that
a measurable portion of the respondents
cited a significant height so far from the likely value that one must wonder whether
the question was widely minunderstood. NO0 13 JUNE MIDNIGHT 1 14 JUNE NODO TIME IN HCU
FEET ESTIMATED MAXIMUM WAVE HEIGHT IN FEET
WIND DURATION IN HOURS b) MAXIMUM WAVE HEIGHT
FIGURE 18 - CSS DIAGRAM OF FASTNET WEAThER FIGURE 19
- OBSERVATIONS OF FASTNET WAVE HEIGHTS
8 Forbes, Sir Hugh, Laing, Sir Maurice, and Myatt, Lt.-Col. James, '1979 Fastnet Race Inquiry", RYA RORC, 1979.
SEARCH RESCUE ESTIMATE
\
0
o o/
/
This same CSS method will be employed
later in consolidating our capsize data from a number of catastrophies.
Another feature of the behavior irregular waves which causes great
difficulty in the practical realm is the
rapid appearance of large waves through the dispersive property of components. Since a particularly large wave forms from the
instantaneous combination of a number of
components each traveling at a different speed prior to their combination at a
location in the ocean, they appear seemingly without warning, and this feature
has been a striking one mentioned by a
number of observers. No better example of
this property can be given than to show the test tank surface where the capsizing tests
PHOTOS COURTESY U.S. NAVAL ACADEMY RYDROMECHANICS LABORATORY
FIGURE 20 - PHOTOGRAPHS OF DISPERSIVE PROPERTY 0F WAVES
If 9
for this work were conducted; the scheme involves generating a series of waves which combine at a predetermined location in the tank to give a brEaking wave front. The
photogPaphs in Figure 20 show clearly how
quikly
this large breaker appears; thetime values given are for a full-scale
breaker related to a 40-foot yacht and a
foot breaker.
As mentioned earlier in this paper,
the transfer of energy from wind into waves
is complex and many (most?) actal sea states are not "fully developed' in the terms of the oceanographer; i.e., the
product of a steady wind of unvarying
direction for sufficient duration to
transfer the energy into the waves in
equilibrium. The CSS diagram gives a hint
of how long this might take if the leveling
TIME AT FULL SCALE
6 SEC BEFORE BREAKER
START OF BREAKER
of the contours is used as a measure of '-maturity.
During the non-stationary parts of
irregular wave (i.e. when equi1ibriu between wind and waves does not exist) behavior, a number of phenomenon with
importance to capsizing take place.
The first of these is the rapid appearance of large and steep waves upon increase of the wind speed in an already rough sea. The Dec 81 capsize of a 40-foot
sloop in the Gulf-stream reported in
Reference 9 happened within a short time
after an increase in the wind strength.
Data taken from an offshore tower 2 in
the path of hurricane Camile show the
largest waves appearing before the spectrum had matured as shown in Figure 21 below; and that there waves were of short period
and an accompanying great steepness.
20 22 20 1! 16 12 10 0000 1000 1200 1600 TIME (AUGUST 17. 1969)
FIGURE 21 - TIME VARIATION OF WAVE HEIGHT PARAIIETERS DURING HURRICANE CAMILLE
Van Dorn has shown a relationship
between wind strength and wave breaking behavior which is in concert with this kind of interpretation. Based upon his reported study of a large number of aerial
photographs, he postulates an equation for
the proportion of the largest waves that will be breaking according to the fol lowing:
LB = 2V-20, o<V< 60 (8)
His sample photographs are reproduced n Figure 22, and part (b) is truly awesome
to a sailor. 70 60 50 60 30 20 IO
a) SEA SURFACE UNDER 10-KNOT WINDS. LESS TUA i PERCZNT OF HIGHER
WAVES ARE BREAKING
b) SEA SURFACE UNDER 60 KNOT WINDS 100 PERCENT OF HIGHER WAVES ARE
BREAKI NG
9 Kirkman, K.L., Nagle, T.J.,and Salsich, J.0. "Sailing Yacht Capsizing', CSYS, 1983.
FIGURE 22 - PHOTOGRAPHS OF SEA SURFACE IN VARIOUS WIND STRENGTHS
The Variation of Danger with Sea State.
All of the foregoing data has tended
to argue that the prediction of dangerous
breaking waves is a complex problem; it now falls to us- to make some application of this data in a form which is to sufficient
simplicity to be attractive to the yachtsman.
This task is paradoxical - we know
that the phenomenon cannot be extricated from their time dependence (that is: a
scientific prediction of dangerous breaking
waves depends upon the time history of the
wind and sea conditions, bottom shape, and
currents, yet the user cries for an insight
based on little more than a marine weather forecast.
the most basic diffiulty lies in the fact that large and breaking waves depend upon two independent conditions: large
waves in a mature sea state are not the
threat that is represented by more moderate waves with a sudden increase in wind
strength.
With this qualification to what follows out of the way, let Us prodeed to
make the best of a difficult situation.
Again Van Dorn provides a crucial key in a proposal for estimäting catastrophic
probabilities. The rilethod Should atuà1ly
be studied in detail in the context òf
Reference 7 and will only be summarized
here.
Van Dorn uséd an estilnaté of the most probable value. for a maximum expected wave height passing a stationary observer:
Hm = 2 J E logs N, (g)
and combined that with the formula which
relates the average wave period to the time
between waves to arrive at the following expression:
0.54 Hm2
y (V/lO)
t = O.0O08(-) (10)
and further presented the results as
contours of encounter expectancy as shown
in Figure 18.
Note that thé only tie to breakiñq wave probability is the subjective oiithàt
is storm conditions the larest waves will
oe breaking. (See equation 8)).
60 , 50 40 30
t
A 3i 20
t. ENCOUNTER EXPECTANCY IN HOURS 20 40 60 80 100WAVE HEIGHT IN FEET
FIGURE 23 r- CATASTROPHIC PROBABILITY DIAGRM FOR LARGE BREAKING WAVES Y METHOD OF
VAN bORN
51
Figuré 23 thén gives contours of
equi-catastrophy, that is lines which are estimates of the time befor ea catastrophy
is expected. In the case of Van Dorn,
Figuré 23, the measure of catastrophy was wave height but he points out the rather direct tie of this to a vessel characteristi.c length as a predictor of capsize.
Another way of integrating the Van
Dorn catastrophic probability diagram is to
rilaké a slice at fixed FDS aììd examine the
variation of catastrophic frequency with
vessel size.
PART III - DANGER AND SIZE
The Variation of Danger with Size
Let us now attempt to apply this general data to the case whee data exists from a wide range of yacht sizes caught in
more-or-less the same conditions.
First, à view of the homogeneity of
the. Fastnet storm requires expansion; much
has been said about how the larger yachts escaped the worst conditions in Fastnet by virtué of their location on the course relative to the smaller yachts, but lit-tie
in the analysis of the race supports this
unequivocally. The data within the body of the Fastnet Inqu-iry related to the °worst" weather shows a widespread occurrence of
Beaufort 10 and 11 winds, and the
reanalysis projected the strongest winds to
have occurred through the arCa so that, while the phasing was indeed not
simultaneous, pretty much of the course was
swept by winds of sufficient strength to
cause widespread breakers. On the other
hand those around the rock and reaching for
home might arguably have been on a favored cOurse.
In view of these conflicting data, and
the difficulty of so-doing, no attempt has
been made to correct the raw percentages
of
knockdown by size bands for lòcation on thecourse.
If the Fastnet data on 81 and B2
knockdowns are plotted against a background
of contours of encounter expectancy from
Figure 23, some degree of correlation
exists if the following specific
rnanipulaiton are accepted:
The actual value of encounter time from the Fastnet data is
divided by à factor oF
The encounter time from Fastnet. is taken class by class and is
defined as:
t=
1 .3 0.1 LE SE T
32 tÜDO
G 51 iOC2C
/
G
0.0) 10 20 30 40 50BOAT LENGTH IN FEET
FIGURE 24 FASTNET Bi AND B2 KNOCKDOWN ENtOUNTERS AS A FUÑCTIOÑ OF BOAT
LENGTH
when t = encounter time in
hours
% capsizes = % of yachts experiencing at least one capsize by class
Duratiôn = length ôf storm, taken as 10 hours
The corélation ïn this case specifically shows that except for Class O the Bi knockdowns fit the slope reasònably
well of a curve for wave height i.25.L in terms of variation of encounter tirne with
size and the data for 82 knockdowns fit the slope reasonably well for a curve of
wave height = 1.5L.
Since certain aspects of this analysis
are totally arbitrary (ex: duration of
storm 10 hous) the values of wavé height
ara considered to represent only a relative difference; i.e. a 20% increase tn. wave size seeriis to möve many resùlts frdm 90
degree wave input knockdowns to full capsi zes.
Twb cbmmnts. should be mäde about the
exclusion of Class O data:
Class O was small enough (about one-forth, the. size) comparèd to the others to give significantly
less weight to the results; in fact the B2 knockdown in O
represented a single incident.
Class O was argued by some (as discussed above) to have been in
different weather/on a diffèrent point of sail.
In the context of all of these
mnipulations and qualifications, the
Fastnet data seems to Support the
equi-catastrophic proposal of Van Dorn in terms of effect of physical size.
Reanalysis of Incidents
-l.5L
In light of this
limted/ualified
success,, is it possible to move to
a more
general characte°ation of the
vàriation of with size? 60 NAME Doubloon Puffin Aba Tilly Twin Wok 000ilmç. Aen pu s Morning Cloud Ear le Sting Soutn er, Riidef Mirebel Halcyon
In order to answer this broader question, the author re-analyzed all
available published accents of accidents at sea related to overwhelming, knockdowns and
capsizes within his possession. These
included:
'TYPE
19-foot daysaik.r' 40-foot yawl - foot yawl 40-foot ketch LOCATION Florida Coastal Gulf stream Medi terranean Biscay Channel Ou 1f s tream DATE 1977 1964 1966 1960 1956
30-foot boop Gulf stream 1964
40-foot schowner Du 1f s tre am 1981
In each case, the FDS value was
estimated using. the method introduced with Figure 18 previously, and the type of event was tabulated. These data, when plotted by
vessel watèrline length make-up Figure 25
which shows, in addition the following
i t ems.
(i) A band of values
representing Fastnet 79.
Çontours of equi-catastrophy
as suggested by Vàn Dorñ; a first through
the knockdowns and overwhelrinings and a second greater by the factor deduced in
Figure 24 which seems to fit the capsizes.
A "zone of potential
capsizes" wherein, for the cohditions of
existing ea state plus high winds capsizes have occurrèd in thepast.
This single graph, then, seers to
provide a practical guideline för
characterizing the variation of danger with yacht size.
It must be emphasized that this graph,
while appealingly simple must be carefully
interpreted with the f011owing specific qualifications in mind:
60-foot ShOar Du 1f stre an
29-foot sloop Flri60 Coastal 1h53
(i)
The'Zone of Potential Capsizes"
is
not
to
be confused with
aprediction
that
they
will
occur.
Asexplained
in
great detail with the text,
rather
short-lived
mixesof
strong
winds
ontop
of
existing niòderatewaves seem to be a most
dangerous combination,
but others possibly
are important.
(2)
Thelength
scale
for
specific
boats should probably be a weighted length
as
given
is
the
section:
"Size Rating
Rulé", (7).
The
variation
of
dangerwith
range
of
stability
A danger
associated
with
range
of
stability
coniesfrom
the
needfòr
anextraordinarily
large
waveto
re-right
ayacht with
great stability
inverted which
conies
with
asmall
rangeof
positive
stability.
'èAPS
IZES-\
\
B2 IMPACT CAPSIZE I J 50 BOAT LENGTH IÑ EETFIGURE 25 - RELATIONSHIp OF REPORTED
CÀÏASTROPHIES TO FDS WIND SPEED AND BOAT SIZE
53
This danger coÌTIes from:
o
loss
of
confidence
in
the,
yacht
resulting
in
subsequent
i nappropriate
seamanship,
o
trapping
of
crew membershooked-òn on deck,
o problems
associated
with
watertight integrity, and
o
damage/destruction
of
mechanical
systemsintended
for
upright
operation,
ex:
loss
of
engine,
crankcase
oil.
Using
data
from
'Reference
10, and,combining this with the results in Figure
23, a
feel
for the time trapped inverted
The -esults
of uing these data
are shown i Figure 26 and the ftllowing can beconcluded from that figure:
o In conditions where capsize of a yacht become likely, great danger of relatively
long period of inverted
stable equilibrium is
associated with a low range of positive stability.
o The behaviôr i.s highly
non-linear, so that as a
practical mattér all yachts
with a range of stàbility exceeding 140-degrees will
reright almost instantly.
o Yachts with a range typical
of the lower edge for IOR
yachts may be trapped for period aprbaching 5 minutes
with 2 minutes likely.
TIME TPADPED 1'VCP.TEE ii
'i
JTES 20 18 16 i 12 10 8 RANGE 0F STABILITY 40-FOOT YACHT AD KNOT SOS WINDS PE E D
O FASINET
EXPEP.-IENCE
FIGURE 26 - VARIAtION OF TIME STABLE INVERTED WITH RANGE OF STABILITY
To those who find the derivation of
this estimate too torturous to accept at face value, consider the data from the Fastnet fliquiy which was discussed in
detail ïn Reference 11. The times dan only
be guesses under such conditiön, but, to
quote the inquiry:
!'These five reports (of inverted
stable equilibrium-Ed) give grounds for concern abqut the ultimate
self-righting ability of certath: boats and
a full stability analysis of two boats, one of a type which reported
remaining inverted för five minutes
and another which reported very rapid
self-righting was commissioned.'
The results of that stability study aré shown on Figure 26 labelled 'Fastnet Experience.
514
An Èqui-Safety Standard
Let us now investigate the
implications of some sort of size-varient
standard of capsize safety; the form chosen
for illustrative purposes is that of a
constait time for probable stàble equilibrium regardless of size.
Simply stated, the lârge vessel i.s
allowed to remain stable inverted for an amount of time porportioned to its
advantage in expected encounter of a
capize -
if it i teh timés more resistantto capsize in a givèn sea condition as
measured by the expected ençounter time
it-may therefOre stay upside down ten times as long when the capsize cOmes.
this notion may seem illogidal, but
perhaps it is no less so than allowing boats with a two order of magnitude greater
proclivity to capsize over other to race
Side-by-side as in the case if a 30-fOoter
sets out with a fifty footer.
Be that as it may, the results of udh a standard give sothe rètïonàl basis to
support the notion that somehow a l&rge yacht be given some credit for its greater
capsize resistance.
Using the data for cpntructihg the
curve of inverted equilibrium time, a
relationship of ranöe of positive stabilty with size can be defined as in Figure 27. Note that the 30-footer is allowed an
equilibrium time of a few tenths of a
minute while the 40-footer gets a minute,
the 50-footer about nine minutes, and the
60-footer an hour-and-a-half. Note also that it is the slope of the line which is
of importance, and the locàtion is arbitrary. RANGE DF POSITIVE STAB I LI TV
N
4. 30 FOOTERN
40 FOOTERN
N
4- 50 FOOTERN
N
6Ò FOOTER 'I-¿ z:: I - I "+ 20 FOOTER NMINUTES STABLE INVERTED
FIGURE 27 - RELATÏONSHIP OF SIZE WITH RANGE OF STABILITY FOR EQUAL TIME INVÉRÏEb
11 Claughton, A., and Handley, P., 'An Investigation into the Stability of Sailing Yachts in Large Breaking Waves", University of SöuthamPton,
Jan 84.
If
we nowdesire
to
consider
aspecific value of range of stability with
size,
areplot of the. data cah be rnade
as
in Figure 28.
In this case, the values for
equal
probability-time,
inverted
is
repesented by the curved contour bounding
the zone labeled
capsize zone'.
LENGTH IN FEET
FIGURE 28 - PROPOSED CRITERIA FOR RANGE OF STABILITY OF OFFSHORE YACHTS
17
-,
it
Frdm other wörk on knockdown capsizes
appears from energy consideratiàns
that if
yachts
are
to resist
knockdowns,especially when carrying light
sails,
the
righting
momentcharacteristics
needattention in a manner which make a minimum
value for range of positive stability fall
out in the area of 115 to 120 degrees.
För
those who
cannot
appreciate
the
need for this
in
big boats, a withdrwal
eport by a 60-föoter in the St. Pete-Fort
Lauderdale Race of 1984 is instructive:
"At
approximately
1340 ESTwhile
i-urining under spinnaker ad full main, boat
entered violent roll cycle.
On deep roll
to
port
boat
buried
endof
boom andmainsail
preventer
held
boii
in
water.
Boom apparently collapsed as boat took deep
roll
t
starboard
(windward)
andlay
on beam ends(80-85°
angle
of
heel).
Boat remainedin
this
position
with
a goqdportion of spinnaker and
10 or so of pole
in
water
until
force
onpole
causedspinnaker car control chain to break.
Pole
then thot up vértically with reference to
water pláne relieving pressure on spinnaker
sufficiently
to
allow
boat
to
right
itself.'
"The "death roll" was so violent and
deep thatcrew members on the starboard rail
were
completely
submerged and'grinders"
were hanging vertically from the pedestals
(and
maintaining
adeath
grip
onthe
handles, one Shoúld add)!!
The
weather
associated
with
this
incident was reported as wind 20 gusting to
30.
If äne postulates a constant range of
stability minimum to prevent knockdowils of
120-degrees this
is shown
in Figure 28 as
the
straight-
upper
bounddf
the
zohelabeled "kñockdòwn zone".
If
one thenaccepts
aslope midway
between the two as reasonable and selects
an ánchor point for- à 30-foot yacht at
130-degrees, the relatiOnship:
reg'd = 160-L'
when &=
minimuÌrequi'ed
rahge
of
positive stability
L = equivalent length as pér (7).
This
would seemIike
arational
minimuol for yachts
whfth will participate
in true offshore events.
Let us check the implication of such a
standard upon
anexisting Grand-PHx
tORyacht.
Figure
29 shows asketch
of
currently competitive
tOROne-Tonner,
andthe result of requiring a range of stability
of this
iaçht of 130-degrees.
The designer
has -estimated that the result would be to
reduce internal ballast to an amount usually
associated
tk,trimming ballast and that
the keel piôfile would be altered as shown
by the dashed line.
12
Kirkrnan, K.L., "Ultirate Stability in Small Cruiser/Racers", July 1983.
REQD = 160-L. MINIMUM, 1200 KN0C00W1 CAPS I ZE ZONE ZONE 10 20 30 40 50 60 ipo 160 140 C = 120 100 80 -C 60 4° 20 CCONCLUSIONS
Thi.s
report
presents
a meansfor
estimating
the
équivalent
size
of
aspecific
yacht
asrelated
to
capsize
vulñerab.ility,
gives
data
onthe
environmental
conditions
wherecapsizes
might
beexected,
énd showsthe
particularly troublesome result of
a smàll
range of stability.
It wOuld seem a simple matter to make
these calculations fOr
a paticulér yacht
and decide on this basis to stay home from
any races where a creditable storm having
an FDS
in
the zone of capsizes for that
size
is
likely;
bût
of
course
with
anuhderstanding
of
the,complexity
of
this
latter
processwill
probably
comereluctance to so simplify.
Of what use,
the,
is the infomation
contained herein?
The
note
began as aneffort
to
determiné
whether
historical
pecedents
related to a minimum size of entrant is an
offshore race was based upon solid ground.
.t isi
With this iñ hànd,
an attempt was made
to quantify this 'size
measure on
a more
rational
basis than length and a formulae
has been
presented
to
doso;
aformula
which
is
kind
to
roll
moment of inertia,
displacement, and to low center of gravity
and harsh toward wide beam.
Further,
an accompanyiiig rangeof
stability is suggested.
Finally,
the
reanalysis
of
storm
incidents has shown that whenever the wind
blows
for
sustained
periods
at
over
40knots the zone pf potential capsizes is
at
hand
for
manyyachts.
This
condition,
.56
FIGURE 29 - IMPACT OF STABILITY CRITERIA ON ONE-TONNER
while not sufficient to
mancate a capsize,
suggests
that
one might occur
andis
soconsnon to off,hore races that yacht design
must
reflect
whatis
knownrègarding
reducing risk through
a changein
design
parametrs
andthe
construction
andoutfitting of yachts must
prepare them for
this ultimate tèst.
The particular problem of iverted s.tà
bility is so debilitating as to cry for
imme-diate rule chanês tO reduce beam and lower
VCG.ACKNOWLEDGEMENTS
This
paperis
anoverview of
récent
research
in
capsizing
and summarizescontributions
from
Joé Sal.sich and JohnZseleczky of the U.S.
Naval. Academy Hydro
Lab and Andrew MacGruder, Kn Weller,
andJohn Wright of USYRU.
BIOGRAPHY
The
author
is
égraduate
of
WebbInstitute of Naval
Architecture and is
anactive offshore .ai1or.
He is a member of
the SNAME Small
Cràft COmmittee,
Chéirmanof T&R Panel SC-i (Sailing Yachts & Ships),
a
Director
of
the
SNAME/USYRUJoint
Research Project on Safety frOm Capsizing,
a member
of
the USYRU MHS Committee, a memberof
IOR Research Còmrnittee andSafety-at-Sea
Committee, andthe
CCATechnical
Corrimittee and Bermuda RaceCommittee.
BEFORE MODIFICATION ATER MODIFICATION
RANGE .0E STABILITY 118° 1 300
OUTSIDE BALLAST 4300 6600 INSIDE BALLAST 28 OO 500 LOA
10-O"
LWL 31-3"
BEAM13-3'
DRAFT7'-5"
DISPL14,400"
BACKGROUND:
Simplified
screening
formulae
have
proven
use-ful
in
race.
management
as
a means of testing the majority of the population
on
a
simple
basis,
requiring only those caught by the sieve to undergo more
rigorous e amination
Such an approach seems appropriate to dealing with
capsize resistance.
This
note
presents the basis -for the development o-f
a
capsize
resistance
screening
formula suitable -for application
to
Category
races
n accordaflce, with the ORC Special RegulatIons.
F'PROACH:
The published capsize research data has shown two characteristics
o-F
danger
related
to
capsizing
the risi
u-fbeing
unduly
easily
capsized.
and
the
risi
of sticling in the inverted position
for
an
extended
time
period.
The
measures of these have been
shown
to
be
"capsize
size".
(referred
t
herein as L).
which is a
function
of
length,
beam,
roll moment of inertia,
and VCG location, and range of
stability, a function of beam arid VCG location.
fri