106 THE EVOLUTION OF THE LIGHTSHIP. 



Beginning in the middle of the last century, elaborate series of experiments 

 with ships and models were taken up in England and France, and served to estab- 

 lish the true theories of ship design. An invaluable series of these experiments was 

 that conducted on various naval vessels in England by the Admiralty in further- 

 ance of the theory of stability, as brought forward by the distinguished naval scien- 

 tist Froude in 1861. These experiments, extending over a period of ten years or 

 more, proved to a great extent the relation of the form of hull, metacentric height, 

 inertia, and rolling keels, to the movement of a vessel. The importance of these 

 relations was gradually recognized, and they were made the basis of design for new 

 ships. 



These relations were still further emphasized by another series, which was 

 apparently the first conducted on the lightship itself. They took place in 1888 on 

 the Nouveau Dyck stationed oif the French coast near Dunkerque. The Nouveau 

 Dyck was a converted fishing vessel, heavily ballasted, and it rolled and plunged so 

 violently that it was practically uninhabitable, to say nothing of its poor qualities 

 as a light platform. An exhaustive study of the causes of this excessive move- 

 ment was then taken up. Many observations were made on the period of waves at 

 the station, and it was found to be fairly constant. Then, observations on the 

 period of oscillation of this vessel, which had a metacentric height of nearly 4 

 feet, showed that it synchronized with that of the wave. Various trials proved 

 that the vessel's period could be increased most by raising and stowing the ballast 

 at the sides of the ship. By this means the metacentric height was reduced to 2^/^ 

 feet and the period doubled. The synchronization was thus stopped, and the vessel 

 so improved in its performance that it was held in service. 



Repeated demonstrations of this nature so established the necessity of con- 

 sidering these principles that lightships are rarely built now without preliminary 

 investigation along the lines indicated. Individual engineers in different countries 

 have often selected certain of these principles and featured them in their design. 

 Bilge keels and inertia ballast figure prominently in the French lightship Le 

 Sandettie, built in 1902. Some 16 per cent of the 341 tons displacement is in 

 ballast, so placed in a deep keel that it adds greatly to the inertia and radius of 

 gyration of the vessel, and, together with deep bilge keels, gives it a long period 

 of roll. Le Sandettie has been made the subject of many interesting articles by 

 its designers, and complete descriptions of the vessel may be found in the publica- 

 tions of many of the scientific societies and nearly all of the maritime journals is- 

 sued at the time of its completion. 



The form of a number of recent U. S. lightvessels is such that the 

 wedges of immersion and emersion in transverse rolling approach equality, while 

 the metacentric height has been reduced to a minimum of 12 inches, and the move- 

 ment of the vessel thereby greatly steadied. Bilge keels have been fitted, without 

 being featured, but inertia ballast has not been generally used. 



The ultimate object of design is, of course, to include the use of each element 

 to the extent best calculated to control both rolling and pitching. For this pur- 



