# Page:EB1911 - Volume 24.djvu/1000

938
[ROLLING or SHIPS
SHIPBUILDING

If at any instant

~ 9A=0C""0B&I'ldd%=%%g-'Q(%'a

it follows that

1129; d20¢ 112011

TiT=Ti'F"¢W;

whence the above three relations hold at the successive instants and consequently for all time. Hence the rolling of C differs from that of B in having the free oscillations of A in still water superposed upon it. If, therefore, it is possible to obtain any one motion in the swell, any other motion due to a different phase relation between ship and wave slope can be at once determined. A convenient motion in the swell to form a basis for obtaining other motions is the forced oscillation proper to the swell, Le. the particular oscillation that is recurrent in the period of the swell. The amplitude of roll at an instant is therefore the sum of the amplitudes due to the forced, oscillation and to an arbitrary free oscillation in still water. If the latter component be regarded as perfectly arbitrary there is no limit to the angle of roll obtained by postulating suita le initial conditions; to determine the practical limitation of rolling, however, it may reasonably be assumed that at or near the commencement of the motion there will be a brief period of no roll, and t hat the maximum angle of roll obtained will occur at no great interval of time after this period. At the instant when there is no roll, the forced and free oscillations are equal in magnitude and opposite in phase, and the period of maximum (termed the “ criterion ') amplitude 9, will occur as soon as the two components are in phase; T

the time interval between the two conditions isnT, where n = iifjI It is assumed also that during the above interval-(I) the effect of the swell was sensibly the same as that of a simple harmonic wave, A being the amplitude of the forced oscillation (and of the initial free oscillation); (2) the extinction equation of the free oscillation -E2-?=a9+b92 can be replaced by the simple form -§ =E6, where E =a+b9¢ approximately; this has been implied by the absence of terms containing 2 in the differential equation above. The amplitude of the free oscillation during the maximum roll is, from equation (8) Ae'"f; whence

Qc =A(I ~|-6"“).

Also, from equation (9), the forced oscillation is given by -1-W ea->2

From these equations 91 can be determined if T, T1, a, b and 90 are given; conversely if Ge is known, 61 can be tentatively obtained. The followin table gives the criterion angle (®e) and the angle of steady roll (A) for the ' Revenge, " both without obtained on the above-mentioned assumptions: and with bilge keels,

Maximum Wave-Slope, 3 Degrees.

I'§ =1-3. %:=I'2. ér;=I'I. T1£=r-o.

n=3-33. n=5. n=1o n=°o.

l

=-“sis 2% s-sa as?

33 v T12 vm '22 vm R22 om

3 ~';~ eu gi 10% as -" >. 3 U1 'gn >-I/I

(D ID V)

"ReVenge” (deep deg. deg. deg. Ydeg. deg. deg. deg. deg. ~ draught), with

nobi ge keels 8-25 4-35 12-25 6-8 21-2 I3'9 41-1 41-I “Revenge” (deepx

draught), wit

bilge keels . 6~6 4-24 8-6 6-4 II;55 10-8 .14-85 14-85 Among the conclusions reached by Mr R. E. Froude in the case of a ship rolling in a uniform swell were:

However non-uniform initially, the rolling ultimately falls into the uniform forced oscillation; it does so the sooner, caeteris paribus, the higher the resistance, and with the few er “ cycles " or alterations of amplitude of rolling, the more nearly synchronous the swell with the ship. The amplitude of the ultimate uniform rolling is an approximate mean of the alternate maxima and minima of the precedent non-uniform rollin . If the rolling starts from zero, the maximum amplitude falls short of twice the ultimate uniform amplitude, the more so the higher the resistance and the more synchronous the swell; and in a synchronous swell the maximum amplitude cannot exceed the ultimate uniform amplitude, unless so initially.

In two papers by Captain and Professor Kriloff of St Petersburg, it does

read before the I.N.A. in 1896 and 1898, the whole motion of the ship, including pitching and rolling, is dealt with; every variation which can reasonably be conceived is taken into account in these pa ers.

A method for reducing. rolling of ships in a sea-Way byithe use of water-chambers was devised by the writer in 1874 in Connexion with the design of the “ inflexible, ” which was expected to be a bad roller. It consists in fitting one or more tanks across the ship of such shape that when filled to a suitable height with water the motion of the water from side to side as the vessel rolls is such as to retard the rolling. Let fig. 30 represent a series of transverse sections of a ship fitted with a water-chamber, in various positions in rolling from port to starboard; and sup ose the water to move so as to be most effective in quelling rolling. Let G represent the centre of gravity of the ship including the water in the chamber, g the centre of gravity of the water in the chamber, and B the centre of buoyancy of the ship; and let the arrows over the sections indicate the direction in which the-ship is rolling at the instant considered. In position No. I suppose the ship to have reached the extreme heel to port and to be on the point of commencing the return roll, then g should have reached the middle line on its way down towards the port side and the righting couple will be that due to the angle of heel, supposing the water to be a fixed weight amidships. "ln the position No. 2 the ship. has performed part of the roll back towards the upright; the water will have moved farther down the incline, so that g will be some distance from the middle line on the port side as shown, and therefore G will also have moved out from the middle line on the port side; hence the righting couple will be less than what would correspond to the angle of heeli the water were a fixed weight amidships. In position No. 3 the ship has just reached the upright and will be moving with the maximum angular velocity; the water will have moved still farther down the incline, and g will be at a greater distance from the middle line on the port side, and therefore G will have moved farther out from the middle line, whereas B will have returned to the middle line; so that the weight of the ship and the upward pressure of the water will form a couple tending to retard the ship's rotation, although she is for the moment in the upright position. In the position No. 4 the ship is heeling over to starboard and the centre of gravity of the water is returning towards the middle line; but it and G are still on the an nm 1..-

"' "ab, ¢'“ "uv ¢"» ¢» on

an '*'“

" nu n-2 QL * »-W

I

FIG. 30. A

port side, and the righting couple is therefore greater than that corresponding to the angle of heel of the ship and a fixed centre of

gravity amidships. In the position No. 5 the ship has momentarily