Sec. 77.24 



PRELIMINARY DESIGN OF A MOl'ORBOAT 



81 



the boat will operate most of the time in sheltered 

 harbors or rivers. In addition, it improves the 

 steering when planing. 



A tentative keel Kne is next drawn in the 

 elevation. It is so placed with reference to the 

 chine as to give a rise-of-floor angle amidships 

 in the range of 14 to 18 deg. At the stern the 

 range is from 1 to 4 deg. If constant rise-of-floor 

 angle aft is desired, with no twist in the afterbody 

 bottom, the angle may be of the order of 6 to 

 9 deg. The lines of two motorboats of good 

 performance published by L. Lord ["Naval Archi- 

 tecture of Planing Hulls," 1946], in Fig. 41 on 

 page 89 and Fig. 43 on page 92, have constant 

 rise-of-floor angles of 5.8 deg and 9.2 deg, respec- 

 tively. Those of one planing boat whose Unes are 

 pubUshed by D. D. Beach [The Rudder, Jan 

 1954, p. 38] average about 8.3 deg. 



The sections are next sketched in. For the 

 ABC tender, sections of inverted-bell shape are 

 suitable forward, fairing into slightly convex 

 sections in the run. As previously mentioned, 

 these sections give volume forward but still 

 allow the water and spray to break off cleanly at 

 the chine. They mil probably result in slightly 

 greater resistance in smooth water, but should 

 give improved wavegoing performance. Sections 

 with slightly convex bottom segments are favored 

 aft, as they provide more hull volume and allow 

 lower mounting of the engines. 



When the sections are sketched in, a tentative 

 check is made of the volume of the hull up to the 

 designed waterline at rest. When this was first 

 done for the ABC tender the volume was found 

 to be smaller than that corresponding to a weight 

 of 19,000 lb of salt water. It would have been 

 necessary to increase the draft about 0.5 ft to 

 support the estimated boat weight. This was 

 unacceptable as it lowered the chine too far and 

 it altered the wavegoing characteristics intended 

 for the forward sections. 



Increasing the volume required the following 

 changes: 



(1) The chine submergence at the AP was 

 increased to 0.0275Lwl or 0.962 ft . The revised 

 chine crossed the DWL between Sta. 3 and 

 Sta. 4. It was still a straight line from Sta. 7 to 

 the stern at Sta. 10. 



(2) The depth of the keel below the DWL was 

 increased shghtly 



(3) The chine forward remained at the same 

 position mth respect to the DWL. 



The net effect was to decrease the risc-of-floor 

 angles in the run of the boat. Every effort was 

 made to keep the buttock lines straight in the 

 run, especially abaft Sta. 7. 



The sections were then redrawn with the same 

 general characteristics, and the new hull volume 

 was calculated. The volume below the DWL was 

 found to be sUghtly more than needed, but a 

 uniform decrease in draft of about 0.21 ft along 

 the whole length gave the correct volume. A 

 lifting of the whole boat in this manner is usually 

 found acceptable, because all the changes in the 

 hull characteristics are favorable. The revised 

 chine height at the FP is +0.073 IL^t and at 

 the AP it is —0.0211Lwl ■ The relocated chine 

 crosses the new designed waterhne between Stas. 

 4 and 5. 



Next, the fore-and-aft position of the center of 

 buoyancy CB is determined, and a check is 

 made with available data such as those presented 

 at the end of Sec. 77.19 to determine whether 

 the CB lies mthin a range of CG position found 

 acceptable for good planing-boat design. If the 

 CB is not less than 0.55Z/nr£ or more than 0.65L„.i, 

 from the FP, the CG can probably be placed 

 within that range so that the craft will float at 

 the draft and in the attitude desired when at rest. 

 If the CB is as far forward as 0.45LH^i from the 

 FP, or as far aft as 0.70L^z, , the hull will have 

 to be reshaped to correct this condition. The 

 craft is liable to porpoise if the at-rest CB — and 

 the CG — are too far aft. 



After making the necessary shifts in the section 

 and other hues, and checking the volumes and 

 CG positions, the lines are faired and drawn, 

 including the abovewater body to the main-deck 

 edge. The final result for the ABC planing-form 

 tender is shown in Fig. 77. K. The section-area 

 or A/ Ax curve is drawn in Fig. 77. L, in the usual 

 1:4 box. This is supplemented by the B/Bwx 

 curve, drawn so that the half-beam Bwx/2 is 

 equal to one-fourth the waterline length L^l ■ 



Fig. 77.L Dimensionless Wateelhste-Opfset and 



Section-Area Curves for Planing Tender 



OF ABC Ship 



