770 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 7r,.10 



supported, either before or after entering the 

 shallow water, through a region of deep water. 

 The towhne tension for such an operation must 

 be approximated and the protuberances, with 

 their connections to the main hull, must be able 

 to withstand the forces and moments due to 

 waves and to the motion of the buoyed-up hull in 

 waves. No systematic design data are available 

 for use in .situations of this kind but the problem 

 is one readily and simply handled by a model 

 basin which possesses a model of the ship to 

 which the protuberances are to be attached. 



76.10 Vessels with Fat Hull Forms. Length 

 of hull, which is an asset when striving for speed 

 and propulsive efficiency, becomes somewhat of a 

 Uabihty when first cost, compactness, hull stiffness, 

 maneuverability, and habitability are important 

 factors. This is especially true for craft which are 

 small compared to the minimum sizes for berths, 

 lockers, passageways, and access for the crew. 

 Sailing yachts, tugs, small fishing craft, work 

 boats, and similar vessels are, more often than 

 not, fat and chubby. Icebreakers are larger 

 vessels in this category. Their displacement- 

 length quotients run up to 500 or more, with 

 0-diml fatness ratios of 17.5 and above. Their 

 length-beam ratios are invariably small, generally 

 less than 5 and sometimes as small as 3. 



Figs. 66. D and 66.1 indicate that separate 

 design lanes, or branches of regular lanes, are 

 needed for these craft. The two lanes of Fig. 

 66. A each have branches of this kind. They are 

 omitted from the latter figure partly to avoid 

 compUcation and partly because insufficient data 

 have been collected to locate them properly. In 

 general, data for existing craft in the fat and 

 chubby category are widely scattered. 



It is unfortunate that the Taylor Standard 

 Series was not extended to embody fat forms 

 having displacement-length quotients A/(0.010L)^ 

 in excess of 250, corresponding to a fatness ratio 

 V/(0.mLY of 8.77. Partly to fill this gap there 

 are given in Sec. 56.6 the results of an analysis 

 of miscellaneous model-test data on fat forms 

 made by R. F. P. Desel and J. T. Collins. 



76.11 Requirements and Design Notes for 

 Fishing Vessels. Fishing vessels of all types and 

 sizes are characterized by the requirement that 

 they must "keep the sea and work on it, all the 

 year around, and in all weathers" [Simpson, D. S., 

 SNAME, 1951, p. 561]. Large or small, they are 

 essentially robust, ocean-going craft, whether 

 they work five miles off a wintry, rock-bound 



coast or, like whale catchers, they travel from 

 Norway to the Antarctic and back again for 

 every whaUng season. 



It is pointed out by Ambrose Hunter ["The Art 

 of Trawler Planning," Ship and Boat Builder 

 and Naval Architect, London, Feb 1953, pp. 

 259-260] that fishing vessels in general: 



(I) Are similar to tugs in that they have to tow 

 fishing gear, nets, and trawls, yet have to possess 

 a high free-running speed 



(II) Are akin to lightships in that they spend 

 more time stopped at sea than any other type 



(III) Are hke sailing yachts in their resistance to 

 drifting, ease of handling, and extreme maneuver- 

 ability. 



Specific hydrodynamic requirements adapted 

 from D. S. Simpson [SNAME, 1951, p. 561] and 

 R. F. Symonds [SNAME, 1947, pp. 381, 384] in- 

 clude the following, together with appropriate 

 comments concerning design: 



(1) Adequate metacentric stability, both static 

 and dynamic, for all conditions of loading, includ- 

 ing the top weight of a coating of ice over every- 

 thing above water. This coating may amount to 

 20 tons or more. Metacentric stability is outside 

 the scope of this book; see PNA, 1939, Vol. 1, 

 Chap. Ill, pp. 99-137, and other standard 

 references. 



(2) Moderate change of trim between extreme 

 operating conditions, such as when entering and 

 leaving the fishing grounds 



(3) More than adequate or above-average wave- 

 going ability, at all speeds which can be main- 

 tained in the heaviest weather. These design 

 features are covered in Part 6 of Volume III. 



(4) Easy motion to provide a good working plat- 

 form in reasonably rough water, over a range of 

 speeds where fishing gear can be handled. The 

 design comments of (3) preceding apply here as 

 well. 



(5) Reasonably dry decks, accepting spray but 

 no breaking seas or solid water, over the range of 

 speeds mentioned in (4), so that fishing can 

 proceed even in bad weather. The design com- 

 ments of item (3) apply. 



(6) Extra-large freeing ports or freeing slots in 

 the bulwarks, screened to hold men, gear, and 

 fish but not water 



(7) Ability to slow down or to heave to while 

 retaining steering control, so that the vessel can 

 be held in any desired position in wind and waves 



