Friday, 26 February 2016

STABILITY FACTORS, EXPLAINED.

The West Australian, Saturday 13 April, 1912

STABILITY OF SHIPS AND
LAWS OF STORMS. 

- To the Editor.

Sir, Now 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, which in the floating ship is called
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 amidships 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 forward, 
then in this partition lies in 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 are 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 of 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", 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. 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 such as 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 
'midship' 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 vessels 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 metacentric height. 
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 meta centric height which is 
the same as saying the "greater the safety 
from'' capsizing, the more uncomfortable 
the vessel to travel on. 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 break, they have sprung a leak. and
foundered. Iron and such like heavy
cargoes have often to be stowed in
narrow trunkways or on platforms
specially 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. 

This important point highlights that when the Waratah was tender (first three voyages) this should have created a more comfortable voyaging experience for her passengers  - as reported by Mts. Agnes Hays.

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.

I have often wondered if the Waratah's beam of +/- 60 ft. was too narrow. According to the writer the beam should have been roughly twice the moulded depth. The Waratah's moulded depth was 38.5 ft. which should have had a corresponding beam of 77 ft., not 60 ft.. However, Waratah's actual depth of hold was 35.1 ft. (3.4 ft. accounted for by the space between the double hulls) which gives us a beam of 70 ft.. In theory this might have been the case but in practice many steamers with similar dimensions to Waratah had similar beams in the region of 60 ft..

On the flip side of the coin better GM stability is achieved with a deep, narrow hull which would confirm a beam of 60 ft. rather than 70 ft. taking into consideration the destabilising triple deck superstructure.

When we consider passenger steamers, however, 
the tendency to build additional decks and keep the passenger 
accommodation all above the main decks. Here it is 
an open question whether we're 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.

This valid point was observed in the construction of the Waratah's successor, SS Ballarat (http://waratahrevisited.blogspot.co.za/2016/01/ss-ballarat-time-to-learn-from-mistakes.html) 

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
power when sailing without cargo, to counteract 
the weight of superstructure supplying the passenger 
accommodation. She was to some extent in the same 
predicament as out 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 loadedYet, as far as the writer can learn, there
was little or no evidence forthcoming as to the weight 
and disposal of the cargo she was lost.

Absolutely! The Waratah was heavily laden and no concrete proof establishing the degree to which she was functionally overloaded.
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 is the
opinion of the writer that the Koombana
in terms of stability 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.

There was very little ambiguity in the case of the Koombana which was significantly top heavy (to clear sandbars at ports along the Northwest coast).

The question is, "Is it a legitimate risk to
send such a vessel in such a trim to hurricane
latitudes in hurricane seasons?" 
It must be remembered that the law of
storms is getting on towards being an exact
science with barometer 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 in 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 seafaring mariner?
If not, is it not a work worth taking in hand at once?

In my opinion, Captain Ilbery had no intention of subjecting his steamer to the gale forces moving up from the southwest. He had a fire to contend with and reinforcing the opinion of the writer came about in the hope of outrunning the storm and making it safely back to Durban.

Yours; etc.,
S LONGSHOREMAN.
Fremantle, April 4





SS Koombana

4 comments:

Mole said...

Excellent post and some very good points made here. Thanks Andrew.

andrew van rensburg said...

Thank you Mole.

Stuart Flood said...

One thing I read about the Lusitania recently suggested that she and her sister ship were built with fairly minimal width for stability for the purpose of creating a fast hull. However she survived several major battering's in the North Atlantic including a freak wave greater than bridge height so I am not sure how accurate that statement is.

andrew van rensburg said...

I agree, Stuart, and in addition to speed I would imagine reduced fuel consumption was a significant factor, which suited the Lunds just fine. With suitable dead weight ballasting I'm sure these vessels could have withstood battering to a point. Andrew