Sec. 68.4 



ABOVEWATER-FORM LAYOUT 



547 



running at its designed speed when it needs all 

 its reserve buoyancy. The answer to this is that 

 one never knows what a ship may have to do 

 during its lifetime, nor what kind of rough 

 handling and severe treatment it may receive 

 when trying to do its best. 



A logical and practical specification, supple- 

 mentng that of floodability and range of trans- 

 verse metacentric stability, calls for a minimum 

 reserve-buoyancy volume, expressed as a percent- 

 age of the displacement volume below the designed 

 waterplane. A minimum of 25 per cent for a 

 new vessel is a reasonable and not particularly 

 exacting requirement. Reserve-buoyancy ratios 

 for a number of submarines, of the vintage of 

 1901 through 1918, are given by E. Dodero 

 [Ann. Rep., Rome Model Basin, 1941, Vol. X, 

 pp. 95-107]. These vary from 0.112 through 

 0.467, averaging 0.252. The watertight closures 

 of a surface ship are by no means as secure as 

 are those of a submarine, so these values represent 

 some sort of minimum for the average surface 

 vessel. For the latter, a maximum of 0.35 or 

 more is not too much. This, with freeboard and 

 other requirements, should insure that hatches 

 and other vulnerable hull openings are reasonably 

 out of reach of the destructive action of solid, 

 green water [Goodall, F. C, "Whaleback Stea- 

 mers," INA, 1892, pp. 192-193]. 



Even though the hull may be extended farther 

 upward than reserve-buoyancy requirements de- 

 mand, such as the deck of a ferryboat which 

 must match the top of a landing platform at 

 various stages of the tide, it may not be necessary 

 to build an intact or watertight hull all the way up. 



One item taken care of in floodability calcula- 

 tions but often overlooked on small vessels for 

 which these calculations are not made is the matter 

 of the fore-and-aft position of the reserve buoyancy. 

 If a craft is bilged and flooded aft it does little 

 good to have a great volume of reserve buoyancy 

 forward. On many fishing vessels, especially tuna 

 clippers, the freeboard aft is deliberately low to 

 facilitate getting fish in over the side. It has to 

 be recognized that the safety of these craft is 

 equally jeopardized by the absence of adequate 

 reserve buoyancy there [Hanson, H. C, "The 

 Tuna Clipper of the Pacific," SNAME, Spring 

 Meet'g., 1954, p. 6]. 



68.3 Freeboard and Sheer for Protected 

 Waters. Any craft which produces bow and 

 stern wave crests of appreciable height when 

 underway in protected waters, such as a free- 



running tug, needs a certain amount of freeboard 

 regardless of that embodied in the reserve-buoy- 

 ancy requirements. If it may be called upon during 

 its lifetime to operate when trimmed heavily by 

 the bow or the stern it needs still more. The 

 position and shape of the sheer line with reference 

 to the designed waterline is then controlled largely 

 by the wave-profile, trim, and reserve-buoyancy 

 requirements. 



The craft may have a curved sheer line, as 

 for a seagoing vessel, or it may have a straight 

 weather-deck line if the freeboard everywhere 

 exceeds the minimum. However, there is an 

 optical illusion involved in looking at any vessel 

 with a straight deck line throughout its length 

 which makes it appear that the hull is hogged 

 slightly. When appearance is a consideration, it 

 is desirable to incorporate some curvature in the 

 deck or hull line at the side, with the bow normally 

 higher than the stern and the stern higher than 

 some position near or slightly abaft amidships. 



If there are practical reasons for straight deck 

 lines throughout a considerable part of the length, 

 it is possible to retain the sheered appearance by 

 adding sheer only at the bow, or in the forebody. 

 By the clever and artistic use of curved bulwark 

 lines, combined with the slight depression in the 

 deck edge at the side due to camber, or even of 

 curved painted lines on the hull, it is possible to 

 avoid entirely the appearance of hogging while 

 holding a perfectly straight deck line at the 

 centerplane. 



"It was generally found best for appearance' sake to 

 fLx the lowest point of the freeboard about one-fifth to 

 one-seventh of the ship's length abaft amidships; and to 

 give rather quicker curvature aft so as to prevent the 

 tangent to the sheer line falling below the horizontal when 

 the ship had the maximum trim by the stern" [Narbeth, 

 J. H., INA, 1942, pp. 144-145]. 



68.4 Freeboard and Sheer for General Service. 



Concerning freeboard requirements for general 

 service, it may be well at this point to review 

 the object of freeboard, and its functions. These 

 were well expressed some seventy years ago in 

 the following terms: 



"Perhaps the most important of these are: to limit 

 the ship's load; to provide a reserve of buoyancy, both 

 as a margin against leakage and as lifting power in a 

 sea way; to assist in securing a sufficient range of stabiUty; 

 to provide a suitable height of working platform, and to 

 protect the vessel from deck damage" [West, H. H., 

 INA, 1883, Vol. 24, p. 205]. 



