stop the engines they coarsened the pitch so as to bring the screw right
fore and aft, so that they never altered the way of the ship in changing
from steam to sail alone. The reason why twin screws had been adopted in
the navy was that if one was damaged there was the other still available.
But it gave them a still further advantage, as it enabled them to have a
fore and aft bulkhead, which with a single screw was difficult. The
mercantile marine had not as yet looked favorably on twin screws. Their
finest and fastest ships were single screws, probably because, in very bad
weather, the single screw was better.

Mr. Spyer said that in designing propellers for ships of war, they were
obliged to attempt to obtain the highest possible speed, and that was not
necessarily coincident with a propeller of maximum efficiency. On the other
hand, for mercantile purposes, coal consumption was obviously of paramount
importance, and the speed of any particular vessel must be obtained with
the smallest possible amount of indicated horse power, and a propeller of
maximum efficiency. Regarding the position of the propellers in a small
pinnace, the propellers were shifted six or seven inches further out, and
with about ten per cent. less indicated horse power she obtained three
tenths of a knot more speed.

Mr. Barnaby asked Mr. Linnington whether, in designing twin screws for a
vessel of 8,000 i.h.p., he would make each screw, which would have to take
4,000 i.h.p., of the same diameter as a screw for a single ship of 4,000
i.h.p., of the same speed. Unfortunately in high speed vessels, from one
point of view, the faster they went for a given power the smaller the
diameter of the screw had to be, and the larger the pitch, so that in very
high speed twin screw vessels the ratio of pitch to diameter would be found
to come out very great indeed. In a twin screw torpedo boat, to be tried
shortly, they had a ratio as high as 1.64. In the case of the Inflexible it