661
OF ENGINEERING FORMULÆ.
Resistance of Vessels—continued.
Altering the trim of the vessel caused no great difference
in the resistance. Superiority In resistance seems to be indi-
cated in deep rather than in broad vessels.
It is calculated that the power Is absorbed as follows,
assuming the gross indicated horse-power = 1-000:—
Ship’s net resistance....................... ‘426
Augmentation of ship’s resistance by nega-J
tive pressure caused by screw .. .. f
Water friction of screw .. __...............‘ 043
Friction of engine .......................... ’MS
Friction of working load................... • 143
Friction of air-pump and feed............... ’075
Total .. ..1-000
or the proportion of gross indicated horse-power to effective
horse-power as 2-317 to 1; and if to the gross indicated
horse-power be added 10 per cent, for slip, the proportion will
be 2 • 58, which neatly agrees with experiment.
In the experiments with H.M.S. ‘Iris,’ the following
results were obtained:—
1. With twin four-blad<d screws, a speed of 16'577 knots
per hour was obtained with the expenditure of 7503 I.H.P.
2. On removing two of the blades of each screw, 15-IZ26
knots per hour were obtained by the expenditure of only
4361 I.H.P.
3. With new screws slightly modified in form from experi-
ment No. 1, reduced in diameter about |th and pitch in-
creased by abuiit ^th, 18-57 knots were obtained with
4572 I.H.P.
4. On removing two blades from each screw in No. 3,
18-59 knots were obtained with 7530 J.H P.
The skin resistance is nearly as the area and V8 (the square
of the speed), but experimeiits approximate more nearly to
Vl-85or yi-9thanto V*.
At moderate speeds the resistance coincides with the calcu-
lated resistance due to surface friction alone; but as the speed
increases, the entire resistance becomes more and more in
excess of that due to surface friction. At low speed the fric-
tional resistance of the engine is greater than the resistance of
the ship.
2 O