BUOYANCY AND STABILITY OF TROOP TRANSPORTS. 153 



with rather small stiffness. According to the best practice they are in the lightest 

 condition given almost zero stability, that is, a very small positive metacentric 

 height, in which case they may be expected to have a metacentric height in the loaded 

 condition sufficient for safety under peace conditions and yet so small as to give easy 

 movements in a seaway.* German designers of such vessels have gone further in 

 direction of reducing the stability,! and most of the ex-German ships used as troop 

 transports during the war have a negative metacentric height in the light condition 

 unless they are given a considerable amount of ballast. 



Ballast may be solid or liquid. Solid ballast usually consists in pig-iron, scrap- 

 iron, cement, gravel, or sand and, if permanently on board the ship, is referred to as 

 "fixed" ballast. Liquid ballast ordinarily consists of water, fresh or salt. The 

 quantity of ballast should be determined by a careful calculation so as to secure 

 the desired metacentric height under various conditions of loading, but when a 

 ship is taken over in an emergency for immediate use, there is no time for elaborate 

 calculations and the amount of ballast required must be estimated preliminarily, 

 based on the behavior of the ship in commercial service and on a comparison with 

 other ships. Later, when experience has been gained at sea, the ballast may be ad- 

 justed more accurately, and here the period of roll affords a valuable guide. This 

 procedure was followed in most of the ex-German ships used as troop transports. 



On the whole, water ballast is preferable to solid ballast, since it can be more 

 readily taken on board and again discharged when not needed, and does not in gen- 

 eral occupy useful space in the hold. Solid ballast should not be used except when 

 the tanks available for water ballast are of insufficient capacity for this pur- 

 pose. In many ships, however, the amount of ballast required in the light con- 

 dition is much greater than it is possible or practicable to carry in the tanks, and it 

 becomes necessary, in order to avoid shipping or unshipping of ballast on each voy- 

 age, to carry permanently a considerable weight of solid ballast, which in the full- 

 load condition is a useless deadweight. One reason why tanks of sufficient capa- 

 city may not be available for the purpose of ballasting, even where the tank space is 

 very large, is that when the troops are on board most of the tanks are used for 

 fresh water and it may not be possible at the port of debarkation to obtain fresh 

 water in sufficient quantity or of satisfactory quality. There is then no choice but 

 either to leave the tanks empty on the return voyage or to fill them with salt water, 

 but the latter course is clearly objectionable, as it necessitates washing out of the 

 tanks at the port of debarkation, for which operation there may be no time or op- 

 portunity. Hence it is often preferred to leave most of the tanks empty on the re- 

 turn voyage and to carry fixed ballast instead. 



The need of fixed ballast would, of course, disappear if the ships were designed 

 so as to have sufficient stiffness under all conditions, and this solution is to be pre- 

 ferred in vessels designed and built exclusively for' troop transport, where safety 

 under war conditions is the first consideration. In passenger steamers, designed for 



*L. Peskett, Institute of Naval Architects, 1914, p. 191. 

 fP. Driessen, Schiffbau, 1909, p. 269. 



