Monday, 29 May 2017

STABILITY EXPLAINED.


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. 

With Waratah's markedly improved GM (1.9 ft.) on the final crossing from Australia to Durban, this factor might have contributed to the snapping of a steel ladder as well as the 'jerky' recovery.

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. 

Coal in Waratah's 'tween decks equated with this principal, further compounded by the wind factor against her prominent upper decks and funnel, which counteracted the reduced righting force. 

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.

Very valid points made. Although there was great difficulty in establishing exactly how much cargo Waratah carried and the weight thereof, there is more than enough circumstantial evidence to support the fact that she was heavily loaded with at least 9000 tons of cargo, and the all-important 1300 tons of lead concentrates, midships.

waratahrevisited.blogspot.co.za/2016/03/9000-tons-cargo-from-agents-mouth.html
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.



No comments: