Excerpt from Papers and Discussions on Screw Propulsion: Reprinted From Transactions of the Institution of Naval Architects
The main purpose of this paper is to place on record the facts respecting the steam trials of deep-draught twin-screw ships of the Royal Navy, and to compare their performances with those of similar ships propelled by single screws. From this comparison it may be possible to deduce some measure of the relative efficiency, as propellers, of single and twin screws; and the discussion will probably have a practical interest for ship builders and ship-owners generally.
The opinion appears to be generally entertained that in deep-draught ships it is preferable to employ a single screw, because any assigned speed can be attained with a smaller engine-power than would be required with twin screws. This opinion is based rather upon a priori reasoning than on actual experience; very few trials having been made with twin screws in deep-draught merchant ships. Moreover, recent experience with ships of war points in the contrary direction; and, after a careful analysis of all the facts which I have been able to collect, my clear conviction is that in vessels of deep draught and large engine-power twin screws have hitherto proved to be more efficient propellers, than single screws.
Before proceeding to give the principal data upon which this statement is based, I would remark that although trials on the measured mile may not compare, in scientific accuracy, with experiments like those conducted by Mr. Fronde, yet the broad deductions from a large number of such trials may be trusted. Recorded performances on the measured mile include in one result the net resistance of the ship, the augmentation of the resistance due to the action of the propellers, the efficiency of the engines, and that of the propellers; errors of observation, the influence of tides or currents, and many other circumstances, must also be guarded against It is unquestionable that a separation of these items in the performance, and an analysis of each is the process likely to yield the greatest benefits, and to secure a just appreciation of the real causes of good or bad performance. It is not easy, however, to arrange experiments which shall correctly indicate the value and tendency of each of these constituent parts, as well as eliminate disturbing influences; and for our present purpose it must suffice to have recourse to the results of carefully conducted measured-mile trials, like those to which I now invite attention.
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