Transportation on Water 



347 



and buoyancy, together with all necessary strength. In the 



submarine boat ballast and air tanks are employed. The air 



tanks are subdivided into such 



small compartments that m-^ 



jury to one or several does not 



render the boat unseaworthy. 



Lifeboats and rafts have been 



built which have shown re- 

 markable stability in difficult 



conditions. A life raft, for 



instance, has been made with 



many air compartments, and 



under test has upheld a load 



of 5000 pounds placed on one 



side without tipping danger- 

 ously. The raft has been cut 



into two parts and each part 



has carried safely a full load 



of passengers. 



The problem is especially 



interesting in connection with 



the sailboat (Fig. 119). A 



sailboat (Fig. 120) must have enough stability to resist the 



heeling force of the wind upon the sail. Ballast or a keel 



weighted with iron or lead are among the means employed to 



make such a boat stable. 

 The mast of a sailboat acts 

 like a lever when the wind 

 tips the boat. The longer 

 the force arm of the lever 

 (pages 254-5), the less the 

 force of the wind required to 

 tip the boat . But as the boat 



FIG. 120. Stability of a sailboat, it f , i MI 



M = the center of mass ; B = the center ' ' 6lS OV6r > tne WinQ S P 111S 



of buoyancy. over the top of the sail and 



FIQ. 119. The sailboat. 



