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I-When the elements are in an angry
mood or rage sooner or later, depend-ing on a boats size, the wind strength, wave configuration and other factors.
the ships crew may well face most
serious difficulties in keeping the boat under control and afloat.
Some-times extraordinary precautions must be taken, such as streaming the warps
astern against potentially dangerous or even disastrous broaching in a
gale.
Let us limit our attention to the
controllability or directional stability problem which may arise in the case when the crew has decided to use the technique of running before a
storm as a survival tactic. There seems to be some misconception about this technique even amongst the best experts in heavy weather
sailing, and this explains why one can
quote most controversial and
contra-dictory advice from the books written
on this subject.
One thing is certain, if a boat
mani-tests broaching tendencies, it means that the steering power available in given weather conditions is not suffi-ciently large to compensate the dis-turbing yawing moment. This might
happen when, for some reason, either the efficiency of the rudder is reduced
or a disturbing yawing moment in-creases beyond control or both. The problem is to find out the reasons and conditions in which the direc-tional instability in a yacht motion is most probable to occur to such an extent that the boat becomes
un-manageable.
We shall consider two extreme
cases:
a yacht running before a strofa wind, with sails hoisted, in
shel-tered waters where waves are relatively small and insignificant as a disturbing factor.
a yacht running before a gale under bare poles in following and steep waves, which over-vihelmingly determine a yacht's
behaviour. Case i
Fig. i depicts, in a simplified man-ner, an equilibrium of moments when
a yacht sails upright. The yawing moment due to the action of the
aerodynamic force TA, shifted by dis-tance a relative to the centre of
lateral resistance, is compensated by the rudder action. The boat will
main-tain her straight course as long as teere is an equilibrium of moments TA > a F x b. By proper distribu-tion of the sail area between a main
and a
headsaji, the aerodynamic moment can be reduced almost to nilor otherwise can greatly increase if the yacht carries a mainsail only.
by C. A. Marchaj
An analysis of hie causes of broaching in yachts
When, for purely aerodynamic reasons, an overcanvassed yacht
be-gins to roll heavily, a large variation in the yawing moment immediately
occurs.
Measurements taken ¡n the wind tunnel during tests on the rolling rig show that an increase in the yawing moment due to rolling can easily be twice as large as the initial yawing
moment vihen there is no rolling. The
yawing moment is at its maximum
when the yacht is heelina towards the side on which the boom is rigged, see
Fig. lb. In order to correct unwanted deviation from the course caused by
aerodynamic moment, the helmsman must increase adequately the rudder
force F by increasing the angle of incidence of the rudder. However, when the yacht heels, the effective rudder force F.. Fig. io, decreases proportionately to the cosine angle
of heel, so also does the steering
effi-ciency. Futher limitations on the effective rudder force and directional controllability of the boat may be reached when the rudder stalls or
ventilation takes place. The smaller the area and the higher the aspect
ratio of the rudder, the more probable
are stalling and ventilation to occur.
C
Yawing moment due to
action of aerodynamic force TA
Compensating moment due to rudder action
cheepsbouwkfl
Technische Hogeschool
AIIDDLI3
Perhaps some explanation of these
terms would be in order. Stall: When a hydrofoil or airfoil is set at a cer-tain critical angle of incidence, the flow separates over the suction side of the hydrofoil. The hydrodynamic force decreases rapidly and
sLibse-quently its hydrodynamic efficiency also decreases. Ventilation: When a
spade rudder is set at an angle cf incidence large enough, the suction
near the leading edge can be so large that it may cause the atmospheric air to communicate throLigh a spiral
vor-tex with the suction side of the
rud-der. Subsequently the force generated
by the rudder can drastically be re-duced, and this is called ventilation.
One might expect, for example, that the rudder configuration incorporated initially in Noryema V/I, Fig. 2, a boat
designed for maximum racing per-formance, may not fulfil its duty in heavy weather conditions. For the sake of safety as a justifiable racing penalty, a skeg introduced later,
undoubtedly serves to reduce the rudder aspect ratio, and for that
reason also the probability of early ventilation and stall. There can also be other reasons for steering
de-ficiency n unfavourable conditions, Figure 1 b b Fef Angle of heel Yachting World
but those already mentioned should be sufficient to illustrate the point. The effect of skeg or dorsal fin extensions in front of the rudder is
shown in Fig. 3. We can see that such
a configuration has quite powerful
anti-stall and stabilizing properties at
a large angle of incidence, although
it does not improve the effectiveness
of the rudder at small angles of inci-dence. A conclusion one can reach
from the case discussed above is that
a yacht will broach-to even in com-pletely flat seas if the de-stabilizing
yawing moment due to rolling is larger than available steering power
neces-sary to compensate it. The coupling
between rolling and broaching s very strong indeed.
Case 2
A yacht running before a gale
with-out sails cannot basically be
direc-tionally de-stabilized for aerodynamic reasons, however, it is well known that
the yacht may broach-to because of
wave action. Why? In order to answer
this question, we should remind
our-selves briefly of basic principles of wave motion. These are shown ¡n Fig. 4. Perhaps an analogy will help
us to understand the essentials better than any words. When the wind blows
over a field of corn, each ear waves to and fro, its neighbour closely fol-lows suit and so they seem to follow the wind across the field, though in fact each remains fixed by its roots.
Similarly, the water particles in a sea wave do not follow it across the water
surface, but each one describes a
vertical orbit, more or less circular, as the pressure disturbance passes. In Fig. 4 the wave profile is marked by a broken curve, while the points marked on the circles represent the positions of free surface water par-ticles at a specific time. If after a certain lapse of time the water par-ticles moving in their orbits change
their relative positions, as in this
example, by 1/12 of the circumfer-ence of the circle, then the crest of the wave will shift by the equivalent distance. The new position of the wave is marked by the continuous curve. lt must be emphasized that a
travelling wave is a passage of motion
only, not of water. The actual move-ment of the water particles that com-pose the wave is relatively very small.
While the water particles are exe-cuting one orbit about their position
at rest, the crest of the wave will shift
from position ito position 2, by a full
wave length L,,.. The period during
Figure 3 June, 1972 C) 'J o C) -o Figure 2
which the wave traverses one wave length, in other words the period of passing of two successive wave
crests through one fixed point, is
called a wave period T. if the height of the wave H,, which is equal to the
diameter of an orbit 2r, and the wave period T are known, the orbital
velocity at the surface U0 is found from
U, = 2r/T =
H,V/TTherefore the higher the wave, the laster is the orbital rotation of its water particles. The tremendous im-portance of this is that it leads to the formation of orbital flow, of
vary-ing velocity and direction, dependvary-ing
on which part of the wave surface s
involved. The intensity of the surface
currents moving with the wave is represented in Fig. 4 by arrows of varying thickness, the thickest
por-tion of the arro'/ corresponding to the point at which the speed of the
surface flow is highest.
From Fig. 4 it can be seen that the
influence of the surface current on sailing craft will depend on:
i course sailed
2 ratio of hull length to the wave length
3 position of the boat in relation
to the crest
4 magnitude of the current flow or wave steeoness ratio.
By analysing Fig. 4, bottom sketch, one can deduce that a de-stabilizing yawing moment will be at its worst when the wave length is
approxi-mately twice the hull length, so that
when the forepart reaches the trough the afterpart of the hull is in the crest.
In this position, quite apart from the strong de-stabilizing effect caused by
Angle of incidence
Without skeg
the surface flow, rudder efficiency is reduced, because the orbital flow at the rudder depth can exert consider-able influence on the magnitude of those local flow velocities on which rudder control depends. Since the side force generated by the rudder is proportional to the square of the
velocity
of the flow at the rudder
depth, then, for example, if that velocity is reduced by 40 per cent. theforce generated by the rudder will decrease by 2/3. So only 1/3 of the normal' steering power will be
avail-able. By normal' we mean one which
would be developed by the rudder if the flow velocity in the rudder depth is equal to the calm water speed of the boat, not affected by the orbital
velocity.
It s believed that broaching or
dis-concerting behaviour can not take place if the helmsman keeps a yacht running dead before the wind and
the waves are coming from the stern. However, this idea cannot be trusted since the sea waves are never regular,
so an initial de-stabilizing yawing moment may always occur, particu-larly when the boat is on the crest of
a wave and both transverse and
directional stability are reduced. Once an effective angle of yaw existsrelative to the wave formation, the major influence on the liability to broach is the dynamic pressure exerted on the immersed hull, and this may be very large. In that con-dition the efficiency of the steering system and the helmsman's
re-sponses are deciding factors.
The more rapid the deviation from the course due to the wave action, the more difficult is the helmsman's
task to respond quickly enough against broaching. An unavoidable
time delay in rudder action may be of
three kinds: firstly, a psychological delay in observation of the yacht's tendency to broach and a decision
to react adequately: secondly, a
physical delay in making a correction
when turning the wheel: and thirdly, hydrodynamic delay due to the fact
that the generation of the hydro-dynamic rudder force, proper for a
given angle of rudder incidence, also
requires time. All these factors are commonly labelled as bad or good helm sm a n ship.
In his book, Heavy Weather Sailing,
Figure 4
Crest
Fa ce
Adlard Coles, when referring to the two schools of thought, says the subject of tactics in weathering gales and storms is one about which
yachts-man like to argue . .. Moitessier
de-scribes yachtsmen believing in run-ning at speed as belonging to the
Dumas school. I am impressed,' says
Coles, 'by it myself, because I think
the danger when running in gales is
not due to speed alone, but to loss of
control, which may be attributable to lack of speed as much as to exces-sive speed. However, I shrink from
recommending the method of running
at speed, because ¡f it proves wrong it could lead to loss of life. For those who aro in doubt, especially when
caught out in ordinary gales, I recom-mend the well-tried expedient of streaming warps, following what
Moìtessier calls the Robinson .-hoo! of thought,' i.e. reduced speed."
Robinsons tactics of survival
would fit the idea of basic seaman-ship recommended in the Yachting World Handbook: Trailing warps in a heavy sea have an effect similar to that of a sea anchor, but ¡s more often used as a method of reducing the boat's speed and minimizing the risk of broaching-to.'
Let us illustrate both these tactics more dramatically by quoting the heroes who survived gales of
supreme violencewarps had been
streaming astern and Moitessier
found the vessel somewhat sluggish on the helm. He felt great anxiety that he might be pitch-poled by one of the enormous grey-beards which carried the boat forward at great speed, the rush of water completely engulfing the hull so that only the masts were visible. Of a sudden, he wrote, he appreciated the wisdom of Dumass technique of running free and taking the following seas at a slight angle. Immediately he cut his warps adrift and the vessel,
becom-Wind llflhIIHIIIflPI' Wave length L
ir
/Th
Trough;:puhu.inhuIu:lulIIIurn'..'1w_
...udsiflfflffuIIIIIIIIfflh'
ing responsive to her helm, could be
handled with safety Moitessier makes
¡t clear that he owed the survival of his wife, himself and the yacht to
this decision.' This ties up very
closely with the experiences of W. Brown, who ran in Force Seven
be-fore a hurricane at speed taking seas
on the quarter.
Now, for a comparison, an opposing
conceptfrom
Robinson'sexperi-ence when sailing VaruaThe seas were so huge and concave at this
point that
the whole upper third
seemed to collapse and roar verticallydown on us. Our oil had little or no effect now . . .' Robinson unlashed
the wheel and ran her off downwind dead before the storm, gathering speed under bare poles to 6 or 7 knots. As he considered this dangerous, he let go five 75 feet lengths of 2m
warps plus loo fathoms of smaller lines. This reduced her speed to 3 or 4 knots and she steered under
perfect control . . . Nevertheless, at
times she ran down a sea and buried
her bowsprit in the trough before
rising again . . If Varua had not been
trailing drags,' says Robinson, 'she
might have been run down.'
If one agrees with Adlard Coles that the men who actually survived the exceptional storms or hurricanes
probably did the right thing and were the best judges of what could be done
in particular conditions and seas, a question arises, why is there a
dis-crepancy in recommending one tech-nique rather than another? Certainly, they must refer to different conditions,
but what is the clue? Before we will attempt to answer this question we should perhaps remind ourselves of
some fundamental principles applic-able to our case:
a safety depends upon giving to the seas and not standing up against them or, in other words, the boat speed relative to the
Effect of orbital flow
at the surface ,.iuiuiUUuuiüh Direction of
orbital velocity
Back 2 C restwave train velocity should be
possibly at minimum,
b it is essential to keep the yacht under directional control n
order to avoid broaching-to and throwing her flat on her beam ends. This can be done either by maintaining relatively high speed and relying upon the
rud-der, or by application of
addi-tional stabilizing moment by means of warps or a drogue to
keep the yacht going straight and the rudder, as a steering device, becomes more or less of secon-dary importance.
Sometimes the combined tech-nique of maintaining a boat at speed and at the same time towing a drogue is used on lifeboats in very steep and
confused seas. According to the re-port of one of the most experienced
lifeboat sailorsthe faster you go,
the steadier the boat sits in the water.As soon as you slow up to get the drogue in, the stern is all over the place again. The main thing which is keeping the lifeboat going straight is the terrific strain on that drogue.'
Now, in order to clarify our
prob-lem further, we need to resort to some experiments and statistical data
con-cerning wave geometry. From the series of experiments carried out in conditions simulating the regular fol-lowing sea on models of high speed hull form, one can deduce a general trend in broaching tendencies of any
sailing craft. The two factors of primary importance are incorporated in Fig. 5. namely the boat speed
wave velocity ratio and the wave
steepness. The worst condition facili-tating the tendency to excessive yaw
or broaching occurs when the boat speed/wave velocity ratio is about unity. In this case, as we mentioned earlier, the rudder will be liable to lose a considerable amount of its
steepness and, at the same time, the de-stabilizing yawing moment, due to
orbital flow being at its maximum, is
much accentuated by a relatively long time during which the boat is exposed
to its action. For boats which are 30-60 feet long, the most dangerous waves would be of 50-100 feet in
length respectively, including to some extent, surfing effect which increases the so-called displacement or smooth water speed of the boat. Such
relatively short waves may appear: a at the beginning of a storm,
b when the wind blows against the tide,
c when a yacht running for shel-ter enshel-ters shallow washel-ters and the waves become shorter and shorter, and finally break up. In these conditions, see Fig. 5, even
when the steepness ratio of the wave
train is relatively small, a broaching
tendency can be quite substantial and may increase beyond control in steep
waves. When surfing down the for-ward slope of the wave, due to the
gravity force, the boat's speed can in-crease to such an extent that the sea becomes effectively a head sea, then
the forward part of the hull
com-mences to penetrate the back slope of the wave in front. In such
circum-stances the yacht may bury herself in the rising slope of the next wave with further ultimate consequences of wild broaching or being pitch-poled.
Referring back to Fig. 5, this explains why broaching tendencies persist
when the boat speed/wave velocity ratio is higher than unity. One can clearly see that in short and steep waves the only sensible tactic left
to the crew willing to sail activeTy is the application of streaming warps technique, which in tact stabilizes the boat directionally and prevents
dangerous surfing. If warps or drogue
towed behind the stern produce a large drag and the resulting stabiliz-¡ng moment is also large, the boat can be sluggish on the helm, this means that the boat is steered
auto-matically. Playing adequately with the
sail area and the number or
length of streaming warps, the crew can find the most desirable balance between these two factors for given weather conditions and a particular boat. This tactic of survival, which corresponds to the Robinson school of thought, can be useless ¡f, due to
a long lasting gale, the waves become
longer and longer. Why? In order to answer this question let us analyse the curve in Fig. 6, which depicts some geometrical properties of waves. It is based on statistical ob-servations. On the vertical axis the wave steepness ratio is marked, on the horizontal axis there are num-bers indicating the age of the wave system' in the form of wave velocity! wind velocity ratio. This term, which is perhaps not familiar to the sailing
fraternity, requires some explanation. If strong winds begin to blow, the rate of growth of the wave height is much
greater than the subsequent growth of the wave length. The wave velocity/wind velocity ratio is small.
since corresponding wave velocity of
short waves is relatively low. Conse-June, 1972
Figure 5
>'40
quently the 'young waves' generated
by the wind at the beginning of a gale are relatively steep and their steepness grows until wave velocity/ wind velocity ratio reaches a value
f about 04, then gradually the waves
become longer and longer and their steepness decreases. When the waves become older their velocity in-creases foilowing the growth in wave length.
Looking back at Fig. 5, we can
deduce that when the waves become
longer and faster, and the boat velocity/wave velocity ratio de-creases, the tendency or liability to broach also is reduced. Putting it
another way, for a given wave length broaching can only occur when a cer-tain wave steepness is surpassed. Let us assume, for example, that the average length of a well-developed wave system is of an order of 500 feet, then the wave velocity is about 30 knots. If in that condition a boat
sailing under bare poles makes 5 knots, it means that the boat speed! wave velocity ratio is of an order of 1/6, i.e. well below a critical
condi-tion in which de-stabilizing yawing
moment due to surface current might
be dominating, see Fig. 4 bottom
sketch. In SUch circumstances the
1/10 1/15 1/20 1/50 Figure 6 No broaching B ro a cli i n g 1/4 1/2 3/4 1
Boatspeed/wave velocity
Vproblem of primary importance is to reduce the relative velocity between the boat and the overtaking seas. lt can only be done by sailing faster. The slower the boat is sailed, the
higher the possibility of being pooped-in or having a boat danger-ously hit by a breaking crest, the
destroying power depends on velocity squared. Therefore a logical piece of
advice would be to sail as fast as
possible without exceeding, of
course, the natural or 'displacement speed' proper for a given boat. The
longer the waves, the more justifiable
is the tactic of sailing at speed. Let us illustrate this by quoting once again W. Brown's experience when sailing Force Seven in a gale. 'The problem facing us was keeping the boat going fast enough in order
to keep out of the viay of huge
break-ing seas by slidbreak-ing down their sides
and keeping them on the quarter.' Perhaps it sound strange if we
con-clude that both techniques, slowing
down with streaming warps astern or
sailing at speed, can be used as a survival tactic by the same crew in the same storm, the first one at the beginning of a gale, the second one later, when the wave system has
sufficiently been developed.
0-2 0 4
06
08
i O 12 1 41.6
Wave age C/v - (wave velocity/wind ocity)
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