Although there is a emphasis on the Waratah in this post, the point is clearly made regarding top heavy steamers.
The West Australian, 13 April, 1912.
STABILITY OF SHIPS AND
LAWS OF STORMS.
- To the Editor.
"Sir, now I that the missing Koombana
may certainly be listed as lost, like the
Waratah and Yongala, the travelling public
might very well be interested in studying
for themselves the simple proposition of
stability in ships. This proposition lies buried
in scientific jargon as far as the man in the
Street is concerned, and can easily be
demonstrated in ordinary language.
It is a long time since Archimedes proved.
that a floating body is exactly the same
weight as the water it displaces. A steamer,
with whatever cargo or ballast she may
have in her, is exactly the same weight as
the water she displaces. The water she
displaces is what would fill the cavity her
weight and shape impose below the water
line or surface of the water. Exactly in the
centre of this cavity, in which the floating
ship fits lies the vessel's centre of buoyancy,
and through this centre there acts an upward
pressure from the sea in its endeavour to
become level against the weight of the ship.
When a steamer is floating upright, this
centre of buoyancy lies midships in a
vertical line or plane, dividing the vessel
in two. If one thinks of a partition being
built from stem to stern amidships from
the keel upward, then in this partition lies
the centre of buoyancy when the steamer
stands upright, and it lies nearly half-way
between the keel. and the water line.
The water line is a imaginary line or plane
joining, from side to side through the vessel
the surfaces of the surrounding sea. Not any
of the painted lines on the hull often alluded
to as the "water line." When a steamer
heels over; that is lists or rolls from side to
side part of her hull comes out above the
level of the sea on one side and another part
sinks further in on the other side. When
this happens the centre of buoyancy changes
position in the hull while always retaining
its position about the centre of whatever
portion of the hull is immersed. Thus, as
she rolls to starboard, it leaves the assumed
partition amidships, moving to starboard,
returning to partition, and then towards
port as she rolls from starboard, through
upright, and then to port.
As before explained, there is always an
upward pressure from the ocean in a
straight line perpendicular to its level
surface through this moving centre of
buoyancy and that line always passes
through a given point in the midships
partition above it. This point is termed
the "meta centre" by experts and it will
readily be imagined that the centre of
buoyancy swings from side to side like
a pendulum suspended from it, when
the vessel is rolling at sea.
There is now the centre of gravity, which
everyone nowadays understands is simply
the centre of weight to be considered as it
must readily be realised that a vessel's
centre of gravity depends upon the amount
of cargo or ballast she may be carrying and
how such is stowed or disposed in her holds.
But once the cargo or ballast is placed, stowed,
or, disposed in the vessel, the centre of
gravity remains constant, and does not
shift (unless the cargo shifts), like the centre
of buoyancy. If the vessel be stowed properly
it will be found somewhere in the assumed
midships partition and at a point below meta
centre point. The pressure from the weight of
the ship is always in the direction of an assumed
plumb line hanging from the centre of gravity point,
and as the vessel rolls at sea this plumb line or
direction of pressure swings from side to side
in harmony with the line of buoyancy, exactly
coinciding when the vessel is upright, and parallel,
with an increasing distance between them as the
vessel rolls to one side.
The degree of stability- that is safety from capsizing
- depends on the distance of the meta centre above
the centre of gravity. This distance is termed the meta
centric height. (GM). The force downward from the
centre of gravity is exactly equal to the force
upward exerted by the ocean endeavouring to
get level; and these two forces tend to right the ship
when the undulations of the sea swing her away
from the upright. The greater the metacentric
height which is the same as saying the greater
the safety from capsizing, the more uncomfortable
the vessel to travel on. (not so - greater GM equates
with brisker, less comfortable rolling) The more
leverage the forces of buoyancy and gravity have
the more quickly they can right the vessel swaying
on the undulating surface. Too great safety from
capsizing brings about other dangers. Sailing vessels
with heavy dead-weight cargoes have been
known to lose their masts and strain their
hulls to such an extent, in so rolling, their
masts out, that they have sprung a leak and
foundered.
Iron and such like heavy cargoes have often to
be stowed in narrow trunkways or on platforms
especially built in the ship to keep her centre
of gravity higher when loaded. When we hear
people say that such and such a vessel is a
grand seaboat, etc., etc., such a vessel may
have been very unsafe on that particular voyage,
her very unsafety contributing to the comfortable
travelling.
It may be taken for granted there is very little
difference in modern cargo vessels when
carrying complete cargoes that nearly fill
them. With like loading they may safely
be expected to behave much the same in
similar storms. The common design for such
vessels provides a breadth equal to about
twice the moulded depth below the main
deck,and as there are no passengers
carried there is very little superstructure
above the main deck. When we consider
passenger steamers, however, the tendency
to build additional decks and keep the
passengers' accommodation all above the main
deck is most noticeable. Everyone prefers a
nice airy cabin to the stuffy ones which were
once the vogue, and all below the main deck.
Here it is an open question whether we are
not sacrificing safety for comfort and carrying
capacity.
The fact that we have had the Waratah,
Yongala, and Koombana mysteries in these
latitudes during the last three years is sufficient
excuse for the public requiring some practical
and expert investigation made on their behalf.
In the case of the Waratah it seems abundantly
clear, from the evidence given at the Law Courts
that this vessel was not considered to have
sufficient ballasting powers when sailing without
cargo, to counteract the weight of superstructure
supplying the passenger accommodation. She
was to some extent in the same predicament as
our famous sailing clippers of last century, which
needed nearly half a cargo of ballast to go seeking
for cargoes from one port to another.
When the Waratah was lost she had nearly a
full cargo on board and whatever her degree
of stability was when empty, had surely nothing
to do with her degree of stability when loaded.
Yet, as far as the writer can learn, there was
little or no evidence forthcoming as to the weight
and disposal of the cargo she lad when lost.
In the case of the Koombana there is
considerable food for reflection. She was
probably carrying less than 500 tons of
cargo and:appears to have been engulfed
in the centre of a "willy-willy." It the opinion
of the writer that the Koombana in light trim
was not fit to encounter a hurricane centre.
The writer has been caught near a cyclone
centre off Mauritius in a sailing vessel, and
remembering how that vessel, although in
ideal load and trim was.smothered under
almost bare poles with bulwarks under water
and hatch comings awash, he cannot conceive
it possible for a steamer like the Koombana
in light trim, exposing such an area of
superstructure to such a force of wind, to live
through it.
The question is, 'Is it a legitimate risk to send
such a vessel in such a trim into hurricane
latitudes in hurricane seasons?"
It must be remembered that the law of
storms is getting on towards being an exact
science, with barometers to provide indications
of approach; and with means to indicate the
vessel's position from the centre and to show
from collected data the most probable path of
the centre, a good steamer with an experienced
and expert master should easily avoid being
caught. It is a matter of vigilance and judgment
just as is the case with a pedestrian avoiding
motor cars. These remarks are applicable.
to cyclones, typhoons, and other well
known and studied storms, but have we
done our duty with regard.to the Nor'-West
"willy-willy" ?? Is there a published hand
book with information; instructions; and
suggestions as is the case in other
hurricane parts of the,world for the
safety of mariners. If not, is it not a work
worth taking in hand at once?
'Yours; etc.,
LONGSHOREMAN.'
Fremantle, April.
 |
| SS Waratah |
 |
| SS Koombana (courtesy wikipedia) |
No comments:
Post a Comment