Then the writer considered our present marine engine as to its
efficiency and capability of further improvement. The weight of
machinery, water, and fuel carried for propelling ships has not had due
attention in the general practice of engineers. By the best shipping
authorities the writer is assured that every ton of dead weight capacity
is worth on an average £10 per annum as earning freight. Assuming,
therefore, the weight of the machinery and water of any ordinary vessel
to be 300 tons, and that, by careful design and judicious use of
materials, the engineer can reduce it by 100 tons, without increasing
the cost of working, he makes the vessel worth £1,000 per annum more to
her owners. That there is much room for improvement in this direction is
shown by the following statement, giving, for various classes of ships,
the average weight of machinery, including engines, boilers, water, and
all fittings ready for sea, in pounds, per indicated horse power:
Lb. per I. H. P.
Merchant steamers.......................... 480
Royal Navy................................. 300
Engines specially designed for light draught
vessels...................................280
Royal Navy, Polyphemus class (given by Mr.
Wright).................................. 180
Modern locomotive.......................... 140
Torpedo vessels............................. 60
Ordinary marine boilers, including water... 196
Locomotive boilers, including water......... 60
The ordinary marine boiler, encumbered as it is by the regulations of
the Board of Trade and of Lloyds' Committee, does not admit of much
reduction in the weight of material or of water carried when working.
The introduction of steel has reduced the weight by about one-tenth; but
it will be the alteration of form to the locomotive, tubulous, or some
other type, combined with some method of forced draught, to which we
must look for such reductions in weight of material and water as will be
of any great commercial value. The engine may be reduced in weight by
reducing its size, and this can only be done by increasing the number of
revolutions per minute.
It has hitherto been the practice to treat the propeller as dependent
upon the size of engines, draught of water, and speed required. This
process should be reversed. The propeller's diameter depends on the
column of water behind necessary to overcome the resistance in front of
it due to the properties of the vessel. This fixed, the speed will then
fix the number of revolutions, which will be found much greater than is
usual in practice, and from this the size of the engines and boilers
will be determined. Great saving in weight can be effected by careful
design and judicious selection and adaptation of materials, also by the
substitution of trussed framing and a proper mode of securing the engine
to the structure of the vessel, as worked out in H.M.S. Nelson, by Mr.
A. C. Kirk, of Glasgow, and in the beautifully designed engines by Mr.
Thornycroft, in place of the massive cast-iron bedplates and columns of
the ordinary engines of commerce. The same may be said of the moving
parts. In fine, the hull and engines should be as much as possible one
structure; rigidity in one place and elasticity in others are the
cause of most of the accidents so costly to the ship-owner; under such
conditions mass and solidity cease to be virtues, and the sooner their
place is taken by careful design, and the use of the smallest weight
of material--of the very best kind for the purpose--consistent with
thorough efficiency, the better for all concerned.