136 



STABILITY OF LIFEBOATS. 



The open boats, Plate 80, have a continually increasing righting moment 

 until they reach such an angle that the rail goes under (30 degrees for the metallic 

 and 32 degrees for the wooden) when the boat founders. The curves are normal 

 and quite what one would expect, though it is of interest to note the very appre- 

 ciable increase in stability which is caused by the seating of a portion of the pas- 

 sengers (15 in the metallic and 16 in the wooden) on the bottom of the boat in- 

 stead of on the thwarts and seats provided for them. It is noticeable also that the 

 wooden boat has less stability than the metallic though of the same external form. 

 This was caused partly by the center of gravity of the boat itself being somewhat 

 higher and partly by the fact that the thwart and seats were about one inch higher 

 above the keel than in the metallic boat. The construction plans show no reason 

 why the thwarts and seats should not be lowered appreciably and it would seem 

 desirable to have them as low as possible. 



For example, the wooden boat from the cross curves has an uncorrected right- 

 ing arm, when loaded with 50 people and inclined at 30 degrees, of 0.90 foot. The 

 distance of the center of gravity above the assumed axis of inclination is 1 1 inches, 

 or 0.92 foot. 



The correction for the righting arm is then 0.92 X sin 30 degrees = 0.46 

 foot, and the corrected righting arm is 0.90 — 0.46 = 0.44 foot. 



Now if the thwarts were lowered 6 inches the resultant center of gravity 

 would be lowered 0.35 foot in which case the correction would be 0.57 X sin 30 

 degrees == 0.28 foot, and the corrected righting arm is 0.90 — 0.28 = 0.62 foot, 

 which is an increase in statical stability of about 40 per cent. 



Probably the curves of dynamical stability on Plate 82 give the most com- 

 prehensive comparison of the merits of the different types from the stability view- 

 point as they give the work done in foot-tons to incline the boats to the various an- 

 gles. These were derived by integrating the curves of righting moments using 

 one of the Institute's integraphs for the work. The following table is derived 

 from these curves. 



Table of Dynamical Stability — Load Condition. 



