difference in the revolutions.  It will be observed that F is a vessel of finer form and a little less displacement than E, and, therefore, has less resistance.  Although E has the greater resistance and the screw the smaller pitch/diameter, the higher revolutions permit the use of a smaller screw.  But from this example the influence of the high revolutions in diminishing the size of screw does not appear so great as some empirical rules would indicate.  The screws for A and B are also worthy of attention.  Although the ship A has a much greater resistance than B, the screw of the former is much the smaller, both in the blade area and the disk.  A’s screws, however, in addition to 22 per cent. more revolutions than B, have a much larger slip, and the blades have rather a fuller form at the tips.  Compared with the practice in the mercantile marine, the revolutions of these screws are very high, and from the foregoing remarks it may appear that much larger screws would be required for a merchant ship than for a war ship of the same displacement and speed.  There would, however, be several items favorable to the use of small screws.  For a given displacement the resistance would be less in the mercantile ship, and with the lower revolutions the proportion of blade area to the disk could be increased without impairing the efficiency.  Thus in passing from the war vessel to a merchant ship of the same displacement, there are the lower revolutions favorable to a larger screw, but, on the other hand, the smaller resistance, larger proportion of blade area, and the coarser pitch, are favorable to a diminution of the screw.  The ship B has a very large screw at 88 revolutions, but the tips are very narrow.  If the blade were as dotted for a diameter of 16 ft., the same work could be done with the same revolutions, but with a little coarser pitch and a little more slip.

There is something to be said for large screws with a small proportion of blade area to disk.  For instance, two bladed screws have frequently given better results than four bladed screws of smaller diameter, neglecting, of course, the question of vibrations.  Twin screws, however, should, as a rule, be made as small as possible in diameter without loss of efficiency.  The advantages of small twin screws are the shorter shaft tubes and stern brackets, deeper immersion, and less exposure as compared with large screws.  The exposure of the screws is usually considered an objection, but, perhaps, too much has been made of it, for those well qualified to speak on the subject consider that careful handling of the ship would, in most cases, prevent damage to the screws, and that where the exposure is unusually great, effectual protection by portable protectors presents no insuperable difficulty.

High speed twin screws.