One of the most important results deduced from experiments on model screws is that they appear to have practically equal efficiencies throughout a wide range both in pitch ratio and in surface ratio; so that great latitude is left to the designer in regard to the form of the propeller.  Another important feature is that, although these experiments are not a direct guide to the selection of the most efficient propeller for a particular ship, they supply the means of analyzing the performances of screws fitted to vessels, and of thus indirectly determining what are likely to be the best dimensions of screw for a vessel of a class whose results are known.  Thus a great advance has been made on the old method of trial upon the ship itself, which was the origin of almost every conceivable erroneous view respecting the screw propeller.  The fact was lost sight of that any modification in form, dimensions, or proportions referred only to that particular combination of ship and propeller, or to one similar thereto; so something like chaos was the result.  This, however, need not be the case much longer.

In regard to the materials used for propellers, steel has been largely adopted for both solid and loose-bladed screws; but unless protected in some way, the tips of the blades are apt to corrode rapidly and become unserviceable.  One of the stronger kinds of bronze is often judiciously employed for the blades, in conjunction with a steel boss.  Where the first extra expense can be afforded, bronze seems the preferable material; the castings are of a reliable character, and the metal does not rapidly corrode; the bronze blades can therefore with safety be made lighter than steel blades, which favors their springing and accommodating themselves more readily to the various speeds of the different parts of the wake.  This might be expected to result in some slight increase of efficiency; of which, however, the writer has never had the opportunity of satisfactorily determining the exact extent.  Instances can be brought forward where bronze blades have been substituted for steel or iron with markedly improved results; but in cases of this kind which the writer has had the opportunity of analyzing, the whole improvement might be accounted for by the modified proportions of the screw when in working condition.  In other words, both experiment and practical working alike go to show that, although cast iron and steel blades as usually proportioned are sufficiently stiff to retain their form while at work, bronze blades, being made much lighter, are not; and the result is that the measured or set pitch is less than that which the blades assume while at work.  Some facts relative to this subject have already been given in a recent paper by the author.