148 BUOYANCY AND STABILITY OF TROOP TRANSPORTS. 



Each suction pipe, where it enters the compartment from which it drains, 

 should be provided with a screw-down, non-return or other suitable valve, which 

 can be worked from the bulkhead deck. These valves should be kept closed except 

 when the pumps are in use. 



Where ventilation trunks, etc., pass through watertight bulkheads below the 

 bulkhead deck, valves operated from above that deck should, in all cases, be fitted 

 at the bulkheads. No such trunks should be allowed to pierce the bulkheads at a low 

 level. 



The precautions here enumerated were carried out as far as practicable in the 

 transports operated by the United States Navy in accordance with general orders to 

 that effect issued by the Bureau of Construction and Repair. 



STABILITY. 



The stability of a troop transport should be such that under the above stipu- 

 lated conditions of flooding there will remain a certain positive metacentric height, 

 while the list, if any, must not be greater than that the boats can be lowered on both 

 sides. In order to ensure this result an investigation of the stability must be made. 

 This presents no difficulty where information is at hand concerning the lines and 

 the distribution of weights and where an inclining experiment has been made, but 

 where such information is not available and where time is limited, approximate or 

 crude methods must be adopted. We shall here describe the procedure followed by 

 the Bureau of Construction and Repair during the war in such cases, as occurred 

 quite frequently when merchant vessels in an emergency were taken over by the 

 Government and used as troop transports. 



I. Combined Inclining and Rolling Experiment. — The first point to consider 

 was the metacentric height under various conditions of loading when the ship is in- 

 tact. A fair idea of the form of the hull was obtained by lifting offsets of the 

 bottom while the ship was in dock or, when this could not be done, by taking inter- 

 nal measurements. When practicable, an inclining experiment was made, and at the 

 same time the period of roll was determined. A combined inclining and rolling ex- 

 periment gives the value of the coefficient K in the formula : — 



y, KB 



VGM 



Where T is the time of a single swing in seconds^ B is the beam and GM the meta- 

 centric height, both in feet. 



This formula affords a ready means of estimating the metacentric height at 

 any time even when at sea, by simply observing the period of roll. 



Table II gives the results of such experiments for a few ships. 



