SCREW PROPELLER BLADES. 1 59

It is, therefore, no matter for wonderment that with its superiority
in efficiency the Griffiths screw was retained by the Admiralty for
all classes of ship. In spite of the claims of other propellers, to
the present day, so far as shape is concerned, the Griffiths idea is
followed. In the case of the " Iris" the performance of the original
screws with two of the blades of each removed was very good,
compared not only with that of the screw bearing the whole four
blades, but even when judged by that of the new propellers. In fact,
had those blades been given a foot or two more pitch there would
have been no justification for new ones, except, perhaps, on the
ground of vibration. Again, it is noticeable that of the new screws
made specially to replace the condemned ones, the two-bladed
Griffiths, in spite of its huge diameter, gave the highest speed with
the best speed coefficients.
To-day we have yachts and other cruisers for waters where coaling
stations are few and far between, and other ships with which quickness
of passage is not so important as cheapness. Such ships continue to
be fully masted and have full sail power, and yet require screws which
combine minimum of obstruction with the maximum of convenience
for adjustment so as to yield the best combined steam and sail results.
They are therefore generally fitted with Bevis' feathering screw (see
fig. 47) which allows of this, inasmuch as the two blades can be
turned so as to partly fill the stern apertures and yet be masked by
the stern-post.
