FIG. 1.--RECKENZAUN'S ELECTRICAL HORSE POWER DIAGRAM.
Draw a square, A B C D--divide B C into 746 parts, and
C D into 1,000 parts, or, generally, let a division on C D
be 0.746 of a division on B C, so that we can use the
horizontal lines cutting A B as a horse power scale.
A B, in the above diagram, gives 1,000 horse power, if
the line B C represents 746 volts, and C D 1,000 amperes.
Let x = any number of volts, y the amperes,
and h the horse power, then
h/x = y/100 :. h = xy/746
A fine wire or thread stretched from o as a center to the
required division on C D will facilitate references.
Whatever force may be imparted to the water by a propeller, such force can be resolved into two elements, one of which is parallel, and the other in a plane at right angles to the keel. The parallel force alone has the propelling effect; the screw, therefore, should always be so constructed that its surfaces shall be chiefly employed in driving the water in a direction parallel to the keel from stem to stern.
Fig. 2--KAPP'S DIAGRAM.
It is evident that a finely pitched screw, running at a high velocity, will supply these conditions best. With that beautiful screw lying on this table, and made by Messrs. Yarrow, 95 per cent. of efficiency has been obtained when running at a speed of over 800 revolutions per minute--that is to say, only 5 per cent was lost in slip.
Reviewing the various points of advantage, it appears that electricity will, in time to come, be largely used for propelling launches, and, perhaps, something more than launches.
In conclusion, quoting Dr. Lardner's remarks on the subject of steam navigation of nearly fifty years ago, he said:
"Some, who, being conversant with the actual conditions of steam engineering as applied to navigation, and aware of various commercial conditions which must affect the problem, were enabled to estimate calmly and dispassionately the difficulties and drawbacks, as well as the disadvantages, of the undertaking, entertained doubts which clouded the brightness of their hopes, and warned the commercial world against the indulgence of too sanguine anticipation of the immediate and unqualified realization of the project. They counseled caution and reserve against an improvident investment of extensive capital in schemes which still be only regarded as experimental, and which might prove its grave. But the voice of remonstrance was drowned amid the enthusiasm excited by the promise of an immediate practical realization of a scheme so grand.
"It cannot," he continues, "be seriously imagined that any one who had been conversant with the past history of steam navigation could entertain the least doubt of the abstract practicability of a steam vessel making the voyage between Bristol and New York. A steam vessel, having as cargo a couple of hundred tons of coals, would, cæteris paribus, be as capable of crossing the Atlantic as a vessel transporting the same weight of any other cargo."
Dr. Lardner is generally credited with having asserted that a steam voyage across the Atlantic was "a physical impossibility," but in the work from which I took the liberty of copying his words he denies the charge, and says that what he did affirm was, that long sea voyages could not at that time be maintained with that regularity and certainty which are indispensable to commercial success, by any revenue which could be expected from traffic alone.