TRANSACTIONS OF SECTION G. — PRESIDENTIAL ADDRESS, 4G9 



Section G.— ENGINEERING. 



President of the Section. — Professor J. H. Biles, LL.D., D.Sc, 



M.Inst. C.E. 



THURSDAY, AUGUST 31. 



The President delivered the following Address : — 



During recent years accidents have happened to ships and they have mysteriously 

 disappeared. The complete disappearance without leaving any trace has led to the 

 assumption that the vessel has capsized. The circumstances of such cases obviously 

 preclude the existence of any direct evidence. The only subjects of investigation can 

 be (1) the condition of the ship prior to the accident, and (2) the probability that such 

 a condition could be one which in any known possible circumstances could lead to dis- 

 aster. The first is determinable by evidence in any particular case. The second 

 involves a consideration of the whole question of the behaviour of ships at sea. 

 What is the effect upon any given ship of a known series of waves ? What waves is 

 a ship likely to meet ? 



This subject has occupied the attention of scientific engineers, and it may be said 

 to have been considered a solved problem. We have thought that if a ship has a 

 certain metacentric height and a certain range of positive stability she is quite safe 

 from the action of a series of waves of any kind which we know to exist. If, however, 

 a known ship (and perhaps more than one) has these safety- ensuring qualities and 

 mysteriously disappears, it may be desirable to review the grounds of our belief to see 

 whether any known possible combination of circumstances may cause disaster. 



Let us then first briefly review the grounds of our belief. Fifty years ago Mr. Wm. 

 Froude showed that the large angles occasionally reached in rolling are not due to a 

 Bingle wave- impulse, but are the cumulative effect of the operation of successive waves. 

 The period T of a small double oscillation of a ship in water free from wave dis- 

 turbance and resistance is 2ir / , where k is the radius of gyration and h is the 



metacentric height (i.e., the height of the metacentre above the centre of gravity). 



/2 it I 

 , where I is the length of the wave from crest to 



crest and g is the acceleration due to gravity. The line of action of the resultant of the 

 supporting pressures acting on a ship in undisturbed water is the vertical through the 

 centre of gravity of the volume of the water displaced by the ship. In wave- water it is 

 in the normal to the effective wave-slope (which is approximately the wave- surface). 

 The oscillation of this normal as the waves pass causes a varying couple tending to 

 incline the vessel. If the vessel is very quickly inclined by this couple she will place her- 

 self in or near the normal and the inclining couple will be of zero value. If, however, 

 her movements are very slow, the normal may make one or more oscillations before any 

 appreciable effect is produced on the vessel. The tendency to incline in one direction 

 caused by the normal acting on one side of the vertical is checked by the rapid oscilla- 

 tion of the normal to the other side of the vertical. It is, therefore, evident that the 



