Page:EB1911 - Volume 24.djvu/1017

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STRENGTH]
949
SHIPBUILDING

Propulsive Thrusp "T 'MMM is obtained which fypeofship- N§ .'2§ ;;, °' C°~==fH;=1e“tf'”'f““"..Y“'“'*' Ded':?“°“» a.'3:i§ ., . R-mm “ab, ..E'§ “';;';€ ~ »tion. Sym oically, if

1 15 12 I or Inner screws 'wi » ¢Q1'€S¢l1t t e Battleship (turbine driven) 4 47 2 0 16 PM Outer screws load' shefumg force Ilgvattleship (older types) . 2 -47 -I4 -17 -95 ang begging m0<;{1€IE. irst-ciass cruiser 2 -53 'IO -Io -99 . an x e C0'°T U13 C Second, , . . ' 2 -48 ~o6 -ro '95 ' °f length. Third, , . 2 -48 -05 » -08 -97 . 4F ' 4M Torpedo-boat destroyer 2 -62 -or -02 -97 . 'w =2§ and F = 'jf'-Mail steamer (turbine) 6 -30 -17 I-08 Inner screws , 4 4 -22 -20 »98 Outer screws EE? °°“d't?°“s '°f E?" Cargo vessel . . 2 .. -20 -14 I~O3 . Hlfnnnv Vlz- (G) t at Sloop 1 .45 .21 .17 1.00 the total weight and Submarine (on surface) . 2 . -I6 'IO I'04 buoyancy am equal. H (diving) 2 .20 .12 1.05 and (IJ) that the centre of gravity and the

The above figures refer to full speed and are affected by alteration of speed. Y S 1 Higher values have been obtained for the propulsive coefficients of the most recent turbine-driven ships. Strength.

The forces tending to strain a ship's structure include (1) the static forces arising from the distribution of the weight and buoyancy when aiioat, and the weight and supporting forces when in dock or ashore; (2) the dynamic forces .arising from the inertia of the ship and its lading under the accelerations experienced in the various motions to which the ship is liable, such as rolling and pitching in a sea. Way; and (3) local forces' and water pressures incidental to (a) propulsion and steering, and (b) the operation of the various mechanical contrivances which it carries.

The straining actions of the forces, due to the distribution of the weight and buoyancy of the ship at rest and to the inertia. of the ship in motion, constitute the only part of the problem of the strength of the structure which can be considered theoretically with any generality; the character of-the internal reactions arising in the structure is so complex, that simplifying assumptions have always to be made in order to enable them to be calculated.

The results of theoretical calculations as to the general structural strength of ships are therefore of value for comparative purposes and to some extent for the approximate estimation of stresses actually liable to occur in the structure. The comparison of the theoretical calculations with the results of experience forms an invaluable guide to the proper distribution of material. In making such a comparison the necessity of providing sunicient strength, on the one hand, and of keeping down the weight, on the other hand, has to be borne in mind; the latter point being especially important in a ship, since its economical performance is roughly dependent on the difference between the weight of the structure and the total available displacement. The greatest straining actions, to which vessels of ordinary forms and proportions are subject, are due to inequalities in the longitudinal Lough distribution of the weight and the buoyancy. Let WWW mam" (fig. 54) represent the weight, and BBB; .the buoyancy bending per foot run of a ship plotted along the length AC; over the lengths Aa, bc, de. fC the weight is in excess of the buoyancy; while from a to b, z: to d, e tof, it is in defect. Acurve LLL, whose ordinates are equal to the differences between those of WWW and BBB, is termed a 'curve

5 ' of loads, and represents

H the net load of the ship

° regarded as a beam

subject to longitudinal

bending. Shearingforces

are produced whose

resultant at any trans-I

° verse section is equal to

W ~ 1| the total net load on

f H ~ ' either side of the section; tlgey arehrepresenged by

f f ' ' the “s earin orce"

aff.° C t curve FFF whose

r ordinate at any transverse

section is proportional

to the area

of the “ loads " curve

plotting the areas of the

shearing force curve as ordinates, a “bending moment " curve Fig. 54.

LLL. . .up to that section. Similarly, on centre of buoyancy are

in the same vertical

transverse section, ensure

that the end ordinates of the shearing force and bending moment curves are zero.

These curves are usually constructed for three standard. conditions of a ship, viz. (1.) in still water; (ii.) on a trochoidal wave of length equal to that of the ship

and height fifth of the

length, with the crest §

amidships; and (ia) on /"' “

a similar wave with the -;

trough amidships. The f

curve of wei ht is obtained

by distributing

each item of weight over

the len th of the ship

occupier? by it and summing

for the whole 'ship.

Such a condition of the

ship as regards stores,

FIG. 55.-Cruiser of 14,000 Tons on Wave Crest.

coal, cargo, &c., is selected,

which will produce

the greatest bending »

mgment in each gage, The ordinates of the curve of buoyancy are calculated from-the areas of the immersed sections, the ship being balanced longitudinally on the wave in the second and third conditions. The shearing force and bending moment curves are then drawn by successive in- =

tegration of the curve ofloads. Typical curves

are shown in figs. 55 to

59 for a first-class cruiser

on wave crest, a torpedo boat

destroyer on wave

crest (bunkers empty); "'.

and in trough (bunkers /

full), and a car o vessel

on wave crest (old and

bunkers empty) and in

trough (hold and bunkers A

full). From these curves

it is seen that the maximum

bending moment

occurs' near amidships; its effect in figs. 55, 56 and 58 is to cause the ends to fall relatively to the middle, such a moment being termed “ hogging ”; the reverse or a “ sagging ” moment is illustrated in figs. 57 and 59. Curves of a similar character are 'obtained in the still-water condition, but the bending moments and shearing forces are then generally reduced in amount. The maximum bending moment is frequently expressed as a ratio of the product of the ship's length and the displacement; average values for various types of ships are tabulated below:- FIG. 56.-Torpedo Boat Destroyer 'on Wave Crest.

Whether Hogging

(on Wave Crest)

or Sagging

- (in Wave Hollow).

WXL

Maximum B.M.

Class of Ship.

Mail steamer .... From 25 to'3o' ' H Cargo vessel .. .. From 30 to 35 A ' " H Battleship (modern) . ., About 30 i H Battleship (older types) About 40 H First-class cruiser . . About 32 ' ' H Second-class cruiser . About 25 S Scout ..... 'About 22 H

Torpedo-boat destroyer . I§ E;):lt52to 25 Ig Torpedo boat ... About 23 H-V

About 23 S I