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THEORETICAL]
933
SHIPBUILDING


where I, , I, are the principal moments of inertia of the water-p Hence

x= -%-l, ,-sin ¢;

y= %»I, -cos ¢;

2 = § $(l, cos' ¢-l-I, sin' ¢) .

lane.;

Eliminating 0 and ¢, the locus of the centre of buoyancy for small inclinations of the ship becomes the elliptic paraboloidT unaltered. The resultant couple can be readily found, but in this case it bears no simple relation to the indicatrix, as before; it may be shown, however, that the plane of the couple is conjugate to the axis of inclination with respect to the confocal ellipse x2 y2

Ei-% a -constant.

V

ln the case when GM =O, the ship being in neutral equilibrium for that direction of inclination, the resultant couple is parallel to the axis Ox', i.e. perpendicular to the plane of the indicatrix. Numerical va ues of the meta centric height GM, the angle of gllgeegpgtion to the indicatrix referred to axes parallel to Bx, By is Obliquity a or QOM (equal to tan-117251 and the angle rp are given JC' in the following table for a ship whose transverse GM is 4 ft., longir[W+, / -°°“Sfa“*» iudinal GM 400 ft., and BG 10 ft.:- ¢ oo Io 5° loo 20° 300 40° 500 600 70° 80° 900 GM 4' 4-1 7' 16 50-4' 103' 168' 237 300 354 388 400 -= 0° 60° 78-5 76-8 68-5° 59° 49~3° s9~s° 29'7° 19-8° 9-9° 0° it 90° 29-o° 6-5° 3-2° 1-5° 1-o° o-7° o-5° o~3° o-2° o-i° 0° and the indicatrix is therefore similar and similarly situated to the momental elli se of the water lane and the surface of buo anc is P 'P » . . Y

everywhere synclastic and concave to all points within it. The quantities I, /V and I, /V are evidently eq)ual to BM, and BM, (referring to inclinations about Oy and x respectively); and the indicatrix and momental Ellipse become § %f- +§ yTjI- =constant.

Il I

f

The angle rp that BB, (the projection of BB' on the plane of the indicatrix) makes with xO is given by tan ¢= -%=-f. cot ¢;

Il

hence the direction is conjugate to that of the axis of rotation with respect to the indicatrix.

This is illustrated in fig. 25, where

  • , ¢ - * the ellipse shown is the indicatrix

f

OPx

COU-

jugate radius, and ORMy' the

pendicular on the tangent. Draw QN

parallel to OM to meet OP. The triangle OMQ is similar to BB1B2; and

they can be made equal by giving a

per-

f ' .

Lf' the axis of inclination, OQ the

is '°

suitable value to the constant in the indicatrix equation. In that case

QN is the projection on the plane of

the figure of the normal to the surface at Bl, and the shortest distance between the normals at B and B1 is equal to ON =MQ= FIG. 25.

B, B, =l;/2, since ON or the axis of inclination is perpendicular to them both. Also, the length B'M of the normal at B' intercepted between B' and the foot of the common perpendicular is equal to gg since 0 is the angle between the normals at B and B'; it follows BB I

that B'M'=-f=§ ,

an expression analogous to that obtained before for.the case of small inclinations in the direction of the principal axes of the water plane. It is worthy of note that the radius of curvature p of the normal section of the surface of buoyancy through Oy' is, in general, OM' .

less than BM; the latter being equal to -;£-, and p being equal to (-gg; p is also obtainable by Euler's equation=cos'¢ sin'¢

Vffsnjf

becoming equal to BM for inclinations about the principal axes. Similarly the radius of curvature of the normal section through Q is, in general, greater than BM.

If the centre of gravity G of the ship is coincident with B, the arm

of the righting couple is OM or ie?—0; and there is also a couple of lever ON or $0 in a perpendicular vertical plane. The resultant couple lies in a plane containing OQ, having a lever equal to OQ or %/i, '=+P= org/ 1, = cos =¢+.1,2 Sin =¢ In the general case when G is situated at a distance a above B, the I

righting lever becomes <%—a) 0, and the perpendicular couple is The greatest angle of obliquity (a) occurs in this case when ¢ is about 5§ ° and the plane of the couple is nearly coincident with the middle line plane for all angles of QS greater than about 3l0°. It follows that if a weight is moved obliquely across the s ip the axis of rotation is approximately longitudinal, except when the line of movement is nearly fore and aft; and in the latter case a small deviation from a fore and aft direction produces a large change in the position of the axis of rotation. The direction of the axis of rotation is above expressed with reference to the position of the inclining couple in relation to the indicatrix of the surface of buoyancy; as, however, the couple is assumed small, the direction of the axis and the amount of inclination may equally be obtained by resolving the couple in planes perpendicular to the principal axes and superposing the separate inclinations produced by its components. It has been shown above that the positions of uilibrium are found by drawing all possible normals to the surface 2? the buoyancy, and the condition for stability for an inclination in any direction is that the centre of ravity shall lie below the corresponding meta centre. The height of tie meta centre varies with the moment of inertia of the water-p ane about the axis of inclination, and the maximum and minimum heights are associated with the maximum and minimum moments of inertia, which again correspond to inclinations about the least and greatest axes of inertia respectively. If the centre of gravity lies below the lowest position of the meta centre (the transverse meta centre in the case of a ship when upright) the equilibrium is stable for all inclinations, and the condition is referred to as one of absolute stability; if it lies above the hi hest meta centre, the condition is one of absolute instability; if it between the highest and lowest meta centres, the condition is one of relative stability, the ship being stable for inclinations about a certain set of axes, and unstable otherwise.

The foregoing remarks apply to a vessel whose axis of inclination is fixed so that the component couple perpendicular to the plane of inclination is resisted. If, on the other hand, the vessel is free to move in all directions the resultant couple does not in general tend to restore the original position of equilibrium, although the component in the plane of inclination complies with the conditions above stated for absolute stabilit . If ml and mg be the greatest and least values of GM, the ratio of, the component couples perpendicular to and in the plane of inclination, or tan a. (fig. 25), is greatest when

m

tan ¢= gi; and then tan a.=;%#. If mg/mi be small, this 1 2

ratio is large, being equal to 4-95 in the numerical example above. In such cases the extent of the movement that can result from a small initial disturbance cannot be readily determined by statical method, but the investigation of the work done in moving the vessel from one sition to another appears to meet this difficulty. This process is employed by M. Guyon in his T héorie du na:/ire, the stability of a shi in any condition being treated throughout from the d namical standpoint. He proved that:- I. hor changes of displacement, without change in inclination, the ~ potential energy of a system consisting of a floating body and the water surrounding is a minimum when the weight of the body is equal to its displacement.

2. For changes of direction, without chan e of displacement, the potential energy of the system is equal to the weight of the body, multiplied by the vertical resolute of BG; when this distance is a minimum or a maximum the stability is respectively stable or unstable. A statical proof of this has been given in the two dimensional case.

The potential energy is thus equal to the dynamical stability