Sec. 76.22 



Huli Beom-. p-Extreme Beam 



DESIGN OF SPECTAL-PURPOSF. CRAFT 



Outside Phrtiol Beam 



789 



starboard Hull Construction Centerplane'" 

 Direction of Motion for Both DioQrQms»- 



/\pproximate Direction of Motion 

 /of Ends of Crests of 

 Reflected Waves 



^ — Crests of 



-Diverqinq Waves 



^..^a^rr777777 7//////\'k/// /^77777Z7^;;^i:^ 



\ Starboard Hull | Approximate. "xDirection 



Rudder of Motion of End^\of '" 



Crests of DiveroinQ ^-^ Waves 



Fig. 76.0 Definition ajjd Design Sketches fob 

 Catamarans 



Thus instead of the inside crests of the Velo.x 

 system from the port hull traveling across toward 

 the stern of the starboard hull they are reflected 

 on the construction centerplane of the catamaran 

 and travel back toward the stern of the port hull, 

 whose bow genei'ated them. 



Considering the small projected area of each 

 hull against which a stern-wave crest could push 

 it is perhaps wise, if possible, to have this re- 

 flected crest just clear the stern. Assuming that, 

 as illustrated in Fig. lO.B, the crest Unes diverge 

 at an angle of about 20 deg to the construction 

 centerplane of each hull, Fig. 76.0 indicates 

 that the spread between the hulls should be at 

 least the waterline length times the natural 

 tangent of 20 deg, or about ^.Z^ALwl ■ 



The waterline beam of each hull, port plus 

 starboard, depends greatly upon the amount of 

 weight that must be carried on a given length, 

 and upon the permissible draft. It may vary 

 from about O-OGLrt^ on a sailing catamaran de- 

 signed to reach a Taylor quotient T, of 3 or 3.5, 

 to 0.12LjfrL , 0.l5LwL , or more on a craft for more 

 utiUtarian purposes. 



If the catamaran is sail-propelled, the lee hull 

 is immersed more deeply when underway than 

 when at rest, and the weather hull less deeply. 

 Despite the seemingly large spread between the 

 hulls, the craft heels somewhat to leeward under 



sail. If poorly handled it may even capsize. This 

 means that the burden of preventing leeway falls 

 on the leeward hull. Like the flying proa or the 

 sailing canoe of Oceania, described and illustrated 

 in Sec. 24.21 and Fig. 24.M, the leeward hull 

 should therefore be flat (or nearly so) on the 

 outside and cambered in planform on the inside. 

 This means that whatever speeding up of water 

 occurs in the venturi section between the two 

 hulls is an advantage. It increases the magnitude 

 of the —Ap's on the windward side of the lee 

 hull, which rides deeper in the water than the 

 windward hull. On the other hand, the clear 

 space between hulls must not be too small, else a 

 blocking effect takes place there. With a cata- 

 maran assembly of the type shown in Fig. 76.0, 

 the clear space between them should be not less 

 than 3 or 3.5 times the maximum waterline beam 

 of each hull. 



If the catamaran is mechanically propelled the 

 planforms of the hulls are apparently not too 

 important provided the speed-length quotient T, 

 is not too large. Each hull may be symmetrical 

 about its own centerplane, or each may be flat 

 on the inside, as best suits other features. 



Because of the ever-present difficulty of obtain- 

 ing sufficient usable and protected volume within 

 the hulls of a catamaran, these huUs may be 

 flared rather sharply above the waterline, especial- 

 ly on their insides. 



Bracing the two hulls against twisting in waves 

 is a structural problem but determining the loads 

 and forces involved is one of hydrodynamics. 

 Unfortunately no method has yet been devised 

 for calculating their values. 



Catamaran hulls with flat bottoms, if properly 

 shaped, can be reUed upon to produce some dy- 

 namic lift, as in a planing craft. However, the 

 aspect ratio is usually too small to permit the 

 bottom of such a hull to act as an efficient planing 

 surface. 



It is sometimes reported that the "tunnel" 

 formed by the inboard surfaces of two catamaran 

 huUs and the under surface of a large, horizontal 

 deck structure joining them has a special shape. 

 It is intended that the blocking effect of the air 

 trapped in this tunnel will provide a -f-Ap under 

 the deck and lift the assembly partly out of the 

 water. Something of this kind might take place 

 at values of T^ = 5 or more, but even then it is 

 most uncertain. 



G. H. Duggan designed and built an unusual 

 form of saihng yacht, the Dominion of 1898, in 



