Tensas gazette. [volume] (St. Joseph, La.) 1886-current, 1917-03-30

This Is another example Of the old rule of thumb
being supplanted by the certainties Of science.
Without going too far into the history of things.
1t is sufficient to know that an Englishman, DR..
William FrOude, discovered back in the seventies
that there was a comparable relation between the
power required to draw a small model through the
water and the motive energy needed to propel a
big craft Of similar form. The principle which he
discover I'd later became known Froude's law or
the law of comparison. His studies were first made
with miniatures and then verified by the towing of
a large vessel formed on identical lines.
Within the last twenty years there have been
created and developed many vessels of novel types
and the results obtained would have been virtually
out Of the question but for the model tank Mil
lions Of dollars would have been wasted in blind
cropping instead of obtaining positive assurance OF
what the finished vessel would do before a single
cent was spent in her building.
One illustration will suffice. Among the vessels
in the naval review is the gunboat Nashville de-
signed in 1894. She had displacement of about
1875 tons and with $500 horse power, makes 16
knots Last year the model tank developed the
hull form for the gunboat Sacramento, vessel
Of 300 tons more displacement but OF speed OF
10 knots upon a development of only 1900 horse
power This saving of 500 horse power, calculated
at $50 horse power, meant an economy In first
cost of machinery of $4,000; but it also meant
that the larger gunbOat could be driven ut speed
equal to her smaller sister ship.
Upon 300 tons the bigger Sacramento could carry
a more powerful armament go farther upon the
same supply of fuel and be more weatherly In
stormy seas; In brief, be much better unit OF the
fleet. And all of this was made certain by the ex-
penditure of a few hundred dollars for wooden
models and their testing, the lines of the hull being
altered during these tests until the best length and
shape were obtained.
All of the models tested at Washington are Of a
uniform length of 20 feet and the purpose of max
ing them large is to reduce the possible error In
translating the performance figures Of the model
into those of the full-sized vessel. These models
are made of wood and are fashioned In a machine
especially designed for this work.
Next the model is painted and carefully var
nished, in order to obtain q very smooth surface;
and with this done various waterlines are marked
upon the white paint, so that instantaneous photo
graphs taken during the towing tests will show just
how and where the waves and the hoows pro-
duced by the miniature in motion are developed
When the model Is ready 1t is carried to the
basin and there loaded in the balancing tank with
bags of shot until 1t has the right weight or dis
placement and proper trim or poise upon the wa
ter With this done 1t is ready to be attached TO
the towing carriage and tested.
The towing carriage is sort of a mobile bridge
which straddles the main tank and 1t Is driven ny
electricity, the different speeds being under very
nice control. Upon the towing carriage are all OF
the operative switches and measuring instruments
and also the recording mechanism which marks
the speed of the model and its pull or resistance
when drawn through the water. The aim is TO
obtain ship form which will show the least pun
Or resistance at the desired maximum velocity. The
actual speed of the model is a mathematical ratio
IF the relation between this miniature and the full
sized ship and therefore the model does not travel
Fast II you consider what the big craft will really
10.
Tho towing basin Is $40 feet long and 43 feet
wide and the maximum speed of the carriage is
relatively far In excess of the probable speeds Of
anything but freak crafts.
After a model of satisfactory form has been
developed and tested then comes the further task
IF making the figures of the trials applicable to
he intended ship.
Now there is just one part OF this work whicn
3oes not follow Froude's law Of comparison This
!, the factor Of the friction set up between the