584. Q.—­If in any screw vessel the power of the engine be diminished by shutting off the steam or otherwise, you will then have a larger screw relatively with the power of the engine than before?

A.—­Yes.

585. Q.—­Was any experiment made to ascertain the effect of this modification?

A.—­There was; but the result was not found to be better than before.  The experiment was made by shutting off the steam from the engines of the Rattler until the number of strokes was reduced to 17 in the minute.  The actual power was then 126.7 horses; thrust upon the shaft 2 tons, 2 cwt., 3 qrs., 14 lbs; horse power of shaft 88.4 horses; ratio of shaft to engine power 1 to 1.4; slip of the screw 18.7 per cent.  In this experiment the power utilized was 71 per cent.

586. Q.—­Was any experiment made to determine the relative performances in head winds?

A.—­The trial in which this relation was best determined lasted for seven hours, and was made against a strong head wind and heavy head sea.  The speed of the Rattler by patent log was 4.2 knots; and at the conclusion of the trial the Alecto had the advantage by about half a mile.  Owing to an accidental injury to the indicator, the power exerted by the engines of the Rattler in this trial could not be ascertained; but judging from the power exerted in other experiments with the same number of revolutions, it appears probable that the power actually exerted by the Rattler was about 300 horses.  The number of strokes per minute made by the engines of the Rattler was 22, whereas in the Alecto the number of strokes per minute was only 12; so that while the engines of the Alecto were reduced, by the resistance occasioned by a strong head wind, to nearly half their usual speed, the engines of the Rattler were only lessened about one twelfth of their usual speed.  The mean thrust upon the screw shaft during this experiment, was 4 tons, 7 cwt., 0 qr., 16 lbs.  The horse power of the shaft was 125.9 horses, and the slip of the screw was 56 per cent.  Taking the power actually exerted by the Rattler at 300 horses, the power utilized in this experiment is only 42 per cent.

587. Q.—­What are the dimensions of the screw in the Rattler?

A.—­Diameter 10 feet, length 1 foot 3 inches, pitch 11 feet.  The foregoing experiments show that with a larger screw a better average performance would be obtained.  The best result arrived at, was when the vessel was somewhat assisted by the wind, which is equivalent to a reduction of the resistance of the hull, or to a smaller hull, which is only another expression for a larger proportionate screw.

588. Q.—­When you speak of a larger screw, what increase of dimension do you mean to express?