8f)l 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 77.40 



(1) If the hull is built of wood, it inevitably 

 absorbs a certain amount of moisture as soon as 

 it is launched, no matter how well it is protected 

 by paints and similar coatings. This is called 

 soakage. The moisture can be absorbed above the 

 waterline from rain and spray, as well as below 

 that line, from routine immersion. It adds 

 directly to the weight of the boat as built. 



(2) Woods of the same kind and grade vary 

 rather widely in weight, even if of the same 

 moisture content. A boat is more likely to be 

 buUt of heavy pieces than light ones, unless a 

 deliberate selection is made. Wooden boats con- 

 structed to the same drawings, by the same 

 builder, have been known to vary plus and minus 

 10 per cent in weight in the course of a building 

 program extending over a year or more. 



(3) For a design that is different from one for 

 which adequate weight data are available, or 

 for one that is novel, no naval architect can 

 estimate accurately, in advance, the weight of the 

 various parts, nor can he list all the parts that the 

 builder (or he) will put into the boat by the time 

 it is finished 



(4) The owner, operator, crew, and others are 

 certain to add weights which were never con- 

 templated or allowed for in the design. When the 

 performance of the boat suffers as a result, the 

 designer is usually the one who has to take the 

 initial blame. 



(5) New technologic developments bring into 

 being certain items of equipment, such as radar, 

 which may not have been in existence or even 

 thought of when the design was completed. It is 

 for this reason that numerous combatant vessels 

 have been designed with margins to take care of 

 increases in weight throughout most of the life 

 of the vessel. 



The element of soakage in a wooden hull needs 

 much more emphasis than has been given to it 

 in the past. It is perhaps safe to say that many 

 small-boat designers and builders have an utterly 

 unrealistic attitude concerning the amount 

 (weight) of moisture which a wooden hull can and 

 does absorb when afloat and exposed to the 

 elements. It has long been recognized that even 

 the best of paint and enamel coverings will not 

 prevent this absorption. It is perhaps not so weU 

 known that a plastic coating on one surface only 

 will not prevent absorption through the other. 

 The astounding weight reductions recorded by 

 drying and weighing a small sailboat hull, where 

 the weight ranged from 375 lb wet to 275 lb dry, 



are vividly presented by C. C. Walcutt [Yachting, 

 May 1955, pp. 64-65, 108-110]. 



The number of motorboat designs of the past 

 for which the total weights have been under- 

 estimated is no less than appaUing. The finished 

 weight has been known to exceed the weight 

 estimate by 10 per cent or more, and the speed 

 to be 10 per cent below the predicted value, solely 

 because of overweight. Despite the time, labor, 

 and expense involved, correct motorboat design 

 procedure calls for the making of a detailed weight 

 estimate. This is possible as soon as the design 

 of the hull arrangements and hull structure is 

 essentially complete, the list of fittings and equip- 

 ment is made, and decisions have been rendered 

 as to just what gear the boat is to carry. 



77.40 Self-Propelled Tests for Models with 

 Djmamic Lift. An exception to the procedure set 

 down in Sec. 1.5 of the Introduction to Volume I 

 is justified here to emphasise the necessity for 

 the self-propulsion of models of craft in which the 

 forces or moments generated by the propulsion 

 devices are likely to be large compared to those 

 generated by buoyancy or dynamic lift. The 

 various velocity and pressure fields and forces set 

 up by the propulsion devices are of such magnitude 

 in proportion to the other forces acting that they 

 can not be neglected in an endeavor to determine 

 the resistance and running attitude of the full- 

 scale craft. For high-speed vessels this running 

 attitude is most important because the slope 

 drag is large in proportion to the hydrodynamic 

 drag. 



A never-to-be-forgotten example of the in- 

 adequacy of the towed model test alone is that 

 of the sea sled described in Sec. 30.13. Indeed, 

 the lift force produced by the surface propellers 

 at the stern of a craft of this type may be so 

 necessary to reduce the trim and the overall 

 resistance that unless the propellers are running 

 fast enough to produce this force the craft may 

 become nearly inoperable. It may, in effect, have 

 only two speeds, full speed or stop, with an in- 

 abiUty to run satisfactorily or efficiently when 

 slowed down. 



It was for many decades the practice, when 

 stepping up the observed resistance of models of 

 full-planing craft, to ignore the friction resistance 

 entirely. The total observed resistance was 

 multipUed by the cube of the scale ratio, with the 

 usual allowance for water density, to predict the 

 resistance of the prototype. This was on the basis 

 that the wetted surface diminished appreciably 



