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Paddle steamer
-BOILERS
By W. Melville
FIG.
I
M
a few locomotive boilers into service
in light fast vessels, and tubular
boilers of various types became
common by the 1840s. The most
popular variety resembled the modern
Scotch return-tube boiler, except that
they were square or rectangular in
cross-section, with furnaces of similar
shape, and were often dry-bottomed.
The haystack boiler, said to have
been invented by David Napier,
came into existence about this time
and enjoyed a great vogue on the
Clyde where it was usually associated
with steeple, oscillating and single
diagonal engines. The haystack
boiler was built as late as the present
century for pressures exceeding 100
p.s.i. in at least one case, though 50
p.s.i. was usual.
Its construction is shown in Fig. 2,
which reveals that it was essentially a
water-tube boiler, an arrangement
made possible by the pure feed water
obtained from the surface condenser.
Its making must have been a feat even
in these early days when the boiler-
makers were evidently undeterred by
the most intricate plate work. They
were built of considerable size, a fast
steamer carrying two boilers, perhaps
ANY
readers will be
interested in building a
boiler for Edgar T. West-
bury’s workmanlike diagonal en-
gines. Before we consider the
model some remarks on full-size
boilers may be of value.
Paddle propulsion, of course, forms
the whole early history of marine
engineering, but as that is too wide
a field I shall limit myself to the boilers
used in coastal passenger craft,
which Mr Westbury obviously had in
mind when he designed his engines.
In the very early days the flue
boiler was universal, with side-lever
jet condensing engines. As the pres-
sures were around 5 p.s.i., the engi-
neers of these days were not restricted
to the circular shell, and designed their
boilers in the most convenient shape
to suit the space available. A flue
boiler is shown in Fig. 1, but their
shapes and internal arrangements
were legion. They were still built as
late as the 1880s for harbour craft.
The feed water was salt and muddy
and a surface condenser was con-
sidered an unnecessary luxury.
Increase in steam pressures brought
s
FVRNACE
PAN
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8-B
A-A
29 MAY 1958
MODEL ENGINEER
boilers appeared in the Clyde fleet
in the 1880’s, with one short-lived
example of a watertube boiler,
installed in the Caledonian Steam
Packet Company’s
Caledonia.
The
Scotch and navy boilers have persisted
to the present day, and will never be
superseded as the Clyde steamer has
nearly run her course.
One feature which came in with the
horizontal boiler was forced draught,
which the limited grate area and high
gas resistance of this type made
necessary if it were to hold its own
with the haystack; it had to do this,
as speed paid a premium on the Clyde
until the decline began in 1914. The
closed stokehold system was universal.
Some of the Scotch boilers were of
enormous size. The
Juno,
owned by
the Glasgow and South Western
Railways, had a double-ended boiler
nearly 20ft in diameter, with eight
furnaces, and others were nearly as
large.
As nearly all diagonal engines got
their steam from Scotch or navy
boilers, it seems that one or other of
these types should be chosen for
Mr Westbury’s engines. The Scotch
boiler is the shorter and more corn-
pact, and is easily modified to make an
efficient small generator. I have,
therefore, selected it for the model.
The engines have been designed as
a small working set of diagonals,
preserving the character of the original
without being an exact model. I have
tried to follow a similar course with
the boiler.
12ft 6in. dia. and 14ft 6in. high,
one forward and one aft of the engine
room.
The haystack boiler was well
adapted to its work. The large grate
area, effective tube heating surface
and free draught made it very power-
ful for the space it occupied. The two
boilers in McBrayne’s famous
Columba
made steam at 50p.s.i. for
a pair of oscillating engines indicating
about 2,000 h.p., which drove her
finely-modelled hull at nearly 20
knots. The latest haystack I have
seen,
and possibly the last ever
built, was fitted in the small North
British Railway steamer
Lucy Ashton,
and survived until the 1940s. It worked
at over 100 p.s.i., and so far as I
know was unique in driving compound
diagonal engines.
Compound engines and higher
pressures brought the need for more
robust boilers, and Scotch and navy
t
THE MODEL
The full-size boilers were, of course,
orthodox in type, generally double-
ended. The model has one furnace,
and the wet-back combustion chamber
has been replaced by a system of
watertubes which is just as efficient
when fired with liquid fuel and is
based on the full-size Howden-
Johnson improved Scotch boiler.
It is designed to carry 60 p.s.i.,
and to provide ample superheated
steam to drive the engines at 400 r.p.m.
which should drive a 6 ft model at a
scale speed of over 20 knots. It may
appear small by normal model stand-
ards, but with a good workmanship
in the engines and a suitable burner
it will make steam to spare.
In the absence of a special burner,
this type of boiler is best fired by a
blowlamp. giving a short, fat, flame.
A suitable burner, for paraffin, is
produced by the makers of the
“ Monitor ” blowlamp, and resembles
a Primus roarer with a short barrel-
shaped flame tube. A blower will be
found necessary, as a compact and
efficient model boiler lacks natural
draught, but the merest wisp of steam
through a No 70 jet will be ample
when standing, and the engine ex-
MODEL ENGINEER
687
29 MAY 1958
Paddle
steamer
boilers
continued
haust will serve when under way.
Many model marine boilers would
be improved by some attention to the
draught, as the appalling roar made
by a blowlamp does not necessarily
indicate that the flame is receiving
enough oxygen to burn efficiently.
I have not shown any mountings
except the main stop and safety
valves, to which I have tried to give a
marine flavour. LBSC has described
so many thoroughly practical examples
that the constructor should have no
difficulty in providing his boiler with
the full complement.
Construction is quite straight-
forward all the joints being silver
soldered. It
IS
essential that the
combustion chamber baffle plate be
thoroughly soldered to the water-
tubes, as it does not only deflect the
flame but acts as extended heating
.
The securing of this plate
should be the last soldering operation,
for the joints where the tubes enter
the backplate are obscured when it is
fitted.
No method of fixing the boiler in
the hull is shown, but it should sit
on a pair of cradles and be secured
by ties to the frames of the vessel.
The ties are inconvenient in a model,
and a better arrangement would be to
strap the boiler to the cradles with
brass bands.
I hope many model engineers are
turning to the paddle steamer. The
beautiful ships of the classical days
have gone, but perhaps they will have
little sisters to keep their memories
green.
The superheater area may have to
be altered on trial; I cannot pretend
that the surface is right, but it seems
reasonable. The design allows any
auxiliaries, such as the blower and
possibly a steam pump, to be supplied
with superheated steam, to the benefit
of their thermal efficiency. But if a
bilge ejector is fitted it will need
saturated steam.
688
MODEL ENGINEER
29 MAY 1958
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