SAILING CRAFT 85 



timber, with the inevitable result of loosening 

 the whole. To meet these strains longitudinal 

 strength must be supplied. The keel supplies 

 much of it, so does the planking (or skin) to a 

 lesser degree ; but not enough ; and the ribs, by 

 themselves, are for transverse stiffening only. 

 Four means are therefore employed to hold 

 the parts together lengthwise keelsons, shelf- 

 pieces, fillings, and some form of truss. 



The keelson is an inverted keel inside the 

 vessel. The floors, which are the timbers 

 uniting the two sides of the frame (or ribs), 

 are given a middle seating on the keel. The 

 keelson is then placed over them, exactly in 

 line with the keel, when bolts as long as the 

 thickness of all three are used to unite the 

 whole in one solid backbone, and this back- 

 bone with the ribs. Side or ' sister ' keelsons 

 were used in the Navy on either side of the 

 mainmast for a distance equal to about a third 

 of the length of the keelson. But they were 

 little used in merchant vessels, and their 

 longitudinal resistance was only partial and 

 incidental. Shelf-pieces and waterways were 

 adapted from French models by Sir Robert 

 Seppings, who became chief constructor to 

 the Navy some years after Trafalgar. They are 

 thick timbers running continuously under and 



