[Footnote: Paper read before the Institution of Naval Architects, March
27, 1885.]
By Mr. A.E. SEATON.
My attention was first called to the modern triple compound engine by the
published reports of the trial trip of the yacht Isa, and in it I plainly
discerned the germs of a successful new type of engine; but it was not
until I had seen the engines of the screw steamer Aberdeen erected in the
workshops of Messrs. Robert Napier & Sons that I became convinced that it
was the engine of the immediate future. It is, however, due to the
farsightedness and enterprise of Mr. C.H. Wilson, M.P., that I was
enabled to try the merits of the new system and compare it with the old.
Mr. Wilson had already viewed the triple compound engine with more than
ordinary interest, and it required little persuasion on my part to allow
the company to which I have the honor to belong to construct a triple
expansion engine in lieu of the ordinary compound for one of four sister
ships which it then had in hand for Messrs. Thomas Wilson, Sons & Co.,
the latter only stipulating that it was to be of the same power as the
engine already contracted for. As I was quite convinced that economy was
due to the system rather than to the higher pressure, it was decided not
to increase the boiler pressure more than was necessary to suit the
triple system. The other three ships already alluded to were being fitted
with engines having cylinders 25 inches and 50 inches diameter by 45
inches stroke, and supplied with steam of 90 lb. pressure from a double
ended boiler 13 feet 9 inches diameter by 15 feet long, having a total
heating surface of 2,310 feet, so that these engines have every
qualification for being economical so far as general proportions go, the
stroke being an abnormally long one and the boiler of ample size.
Experience has since shown that these engines are economical in coal,
and the wear and tear exceptionally small.
The new engines for the fourth boat were made with considerably shorter
stroke, and the cylinders proportioned so as to give equal power; they
are 21 inches, 32 inches, and 56 inches diameter by 36 inches stroke, the
high pressure cylinder being supported on columns immediately over the
medium cylinder, and in other respects these engines were made as near as
possible like the other ones above named. Steam at 110 lb. pressure is
supplied from a double ended boiler 12 feet 9 inches diameter and fifteen
feet long, having a total heating surface of 2,270 square feet, and
identical in design with the boiler supplied for the other engines. The
propellers were made exactly alike in all respects, and the ships being
likewise precisely alike, a comparison of the performances of the one
fitted with the triple engines could be made with as little grounds for
differences of opinion as is possible. One of the ships fitted with the
ordinary compound engines was named the Kovno, that with the triple
compound engines the Draco. Their dimensions are as follows:
Feet. Inches.
Length between perpendiculars. 270 0
Breadth. 34 0
Depth of hold. 18 3
And of 1,700 tons gross register. They are ordinary cargo boats, built of
steel, having a raised quarter deck and long bridge amidships, but
nothing about them otherwise requires comment.
After making a voyage or two to the Baltic, and finding that everything
was working satisfactorily, the Kovno was loaded with 2,400 tons dead
weight, and sailed in January, 1883, for Buenos Ayres; the Draco was
loaded with 2,425 tons dead weight, and sailed March, 1883, for Bombay,
the distance in both cases being about 6,400 miles. It was thought
advisable, for purposes of comparison, that the ships should steam at as
near as possible the same speed; and to attain this object, we considered
the safest plan was to instruct the engineers as to the average amount of
coal they were to burn per day, and experience with these ships on their
Baltic voyages had fixed this at 12 tons in the case of the Kovno and 10
tons in the case of the Draco. During the voyage each ship seems to have
had fair average weather, and equal care was taken in getting the best
results possible. The average speed of the Draco was, however, 8.625
knots, or 207 miles per day, the engines making on the average 57.5
revolutions per minute, while the Kovno did only 8.1 knots, or 194 miles
per day, the engines making 55.5 revolutions. The coal used was ordinary
South Yorkshire, just as it comes from the pits for bunker purposes. The
indicated horse power in each case would average about 600. The total
coal consumed was 326 tons in the Draco and 405 tons in the Kovno, or a
saving of 19.5 per cent. over the ordinary compounds, with an increase of
speed of 6.5 per cent.
In December, 1883, one of the others, the Grodno, sailed from Bombay, and
attained an average speed of 8.5 knots, or 204 miles per day, the engines
making 57 revolutions, with a coal consumption of 12.8 tons per day, or
469 tons on the voyage. The Draco's consumption is therefore 30.5 per
cent. less than that of the Grodno on the round voyage, and 20.3 percent
per day.
The success of the triple compound engine was in these instances more
than had been anticipated, and induced Mr. Wilson to go a step further.
The S.S. Yeddo had been refitted with boilers made for a working pressure
of 90 lb. per square inch, but owing to the size of the shafting the
working pressure was limited to 70 lb.; the average consumption of coal
under these circumstances on two voyages was 17 tons per day. These
boilers had a margin of safety beyond what was required by the rules when
made, and as the Board of Trade rules had been modified in the mean
while, it was found that they could with safety be worked at 100 lb. per
square inch. A third cylinder was now fitted on the top of the original
low pressure, and the safety valves loaded to the 100 lb., and the ship
was dispatched to Cronstadt. After making two voyages under similar
circumstances to the two previous ones, the average consumption was 13.5
tons per day only. In this case it was the same ship, same boilers, same
engines, same propeller, and same men, the only difference being the
addition of a third cylinder and the increase of pressure.
So far all the trials had been made with two crank engines; so it was now
decided to construct another set of engines for 150 lb. pressure, having
a crank to each cylinder. These engines had cylinders 20½ inches, 33
inches, and 58 inches diameter by 36 inches stroke, and were fitted into
the screw steamer Rosario, whose dimensions are 275 feet 3 inches between
perpendiculars, 34 feet 3 inches beam, and 19 feet 2 inches depth of
hold, 1,862 tons gross, and the deadweight capacity 2,550 tons. In March
last year she was loaded with 2,530 tons deadweight, and did the voyage
to Bombay at an average speed of 8.6 knots on a consumption of 10.5 tons
per day of South Yorkshire coal, and burnt on the voyage 347 tons. This
result is superior to that of the Draco when the size of the ship is
taken into account, but is not so much so as might have been anticipated
from the increase of pressure and the rate of expansion, which was 14.4
in the Rosario and 12 in the Draco. Another set of engines was made from
the patterns of those of the Draco, but with the high pressure cylinder
20 inches diameter, steam at 150 lb. pressure being supplied from two
single ended boilers, having a total heating surface of 2,200 square
feet. They are fitted in the S.S. Finland, a cargo boat 270 feet long, 35
feet beam, by 18 feet depth of hold, and 1,954 tons gross register. In
January she was loaded with 2,500 tons deadweight, and sailed for
Rangoon. The average speed attained was 8.42 knots per hour, or 202 miles
per day, on a consumption of 10.3 tons of Welsh coal per day, the rate of
expansion being 12. It should be mentioned that all these ships named are
fitted and steered with steam stearing gear, so that in comparing these
results and those published of the engines made by an eminent engineer in
the north of England, an allowance should be made, as in that ship there
was no steam stearing gear.
I have chosen to make all these comparisons by reference to the ships'
logs, and to give results such as a shipowner looks for rather than those
which engineers prefer to use in forming a judgment on the merits of
different engines. I do this for two reasons: first, because the
commercial success of the triple compound engine depends on the saving it
can effect in a long voyage; and secondly, because I had no reliable
indicator diagrams from which the consumption per indicated horse power
could be calculated with any degree of accuracy. On trial trips with the
steamers already named, the consumption of ordinary South Yorkshire coal
was 1.6 lb. per indicated horse power, and the consumption of water per
indicated horse power calculated from the high pressure indicator
diagrams was 1.41 in the Draco, 13.2 in the Rosario, and 13.16 with the
Finland, or taking the medium pressure diagrams, it was 12.2, 1.30, and
11.95 respectively. Twelve months ago we constructed for Messrs. Thomas
Wilson, Sons & Co., two sets of triple expansion engines of 600 indicated
horse power, one having two cranks and the other three cranks, the
engines, boilers, and propellers being otherwise exactly alike and fitted
into sister ships. The water consumed in the three crank engine is 12.93
lb., against 13.0 in the two crank, but the former drives its ship nearly
½ knot per hour faster than the latter does its, and when both ships are
driven at the same speed the consumption of coal in the three crank ship
is considerably less than in the other.
We have now entirely given up the construction of two-crank triple
expansion engines, because of the impossibility of equally dividing the
work between the cranks; for, although the engine when running appeared
to be perfectly balanced, the wear of the brasses of the crank having the
two cylinders was always considerably more than that of the other.
Placing the high pressure cylinder over the low pressure cylinder seemed
to give the most satisfactory results, but even these were far inferior
to those once obtained with the three cranks. We have lately constructed
some very small three-crank engines from which exceedingly good results
were obtained; the cylinders are only 11½ inches, 17 inches, and 30
inches by 18 inches stroke, which developed 218 indicated horse power
with a consumption of 12.8 lb. of water per indicated horse power, and
this, together with some other observations, leads me to believe that the
best economical results will be obtained by running triple expansion
engines at a much higher number of revolutions than is usual, and with a
rate of expansion not less than 12 for a steam pressure not less than 140
lb. (155 absolute). The largest engines we have made of this type so far
are those of S.S. Martello, which have cylinders 31 inches, 50 inches,
and 82 inches diameter by 57 inches strokes and indicate at sea 2,400
horse power when running at 60 revolutions with steam of 150 lb.
pressure; the consumption of Yorkshire coal is 37 tons per day average
throughout a New York voyage. Had Welsh coal been used in every case, the
results would have been very much better, for, in addition to the
superior evaporative power of Welsh coal, it is slow burning and much
more easily controlled, especially on the comparatively short grates of
these modern boilers, the quick-burning Yorkshire coal causing the safety
valves to frequently blow off when working near the load pressure unless
great care is taken by the firemen.
I trust these few particulars may be of interest to the Institution, and
especially to those members of it who are particularly interested in the
commercial success of our mercantile navy. I have purposely avoided
engineering details and technicalities of any kind, giving only such
information as will tend to give British shipowners faith in that form of
engine which will undoubtedly help them to successfully tide over bad
times, and keep the bulk of the carrying trade of the world in their
hands.
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