82-1 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 77.6 



(c) The CG at the planing speed rises above its 

 position when the craft is at rest, reckoned with 

 respect to the level of the undisturbed water 

 surface 



(d) The designed-speed range lies above T", 

 values of 2.5 or 3.0, F„ > 0.74 or 0.89. This 

 corresponds to the range given in (1) of Sec. 

 30.2, subject to the quaUfication given there. 



A fifth criterion for differentiating a semi- 

 planing from a full-planing type of boat is that 

 used at the Expermiental Towing Tank, Stevens 

 Institute of Technology. It is based on the fact 

 that the full-planing type is always built with 

 chines at the outer edges of the bottom. Here a 

 full-planing condition is defined as that in Avhich 

 the water breaks away cleanly from the chine for 

 the entire length of the boat. No water curls up 

 over the chine and wets either the sides or the 

 transom. 



A sixth criterion is that given in (3) of Sec. 30.2, 

 illustrated graphically in Fig. 30.B. This defines 

 the full-planing speed as that at which there is a 

 sharp reduction of the exponent n in the formula 

 Rr = kV: 



The length used for planing-craft design pur- 

 poses varies. It may be: 



(i) The chine length Lc , projected upon the 



baseplane 



(ii) The waterline length Lwl at normal load and 



trim when the craft is at rest 



(iii) The overall length Lqa ■ 



This matter is discussed further in subsequent 

 sections. 



77.6 Operating Requirements for Planing 

 Forms. There are a number of operating require- 

 ments peculiar to all planing forms which call 

 for consideration ahead of the particular require- 

 ments of the owner and operator. The former are 

 in addition to the normal requirements for speed, 

 maneuverabihty, good wavegoing behavior, and 

 adequate stability, the same as for any other 

 small craft. The items listed and discussed here 

 are adapted from those previously published by 

 C. R. Teller [Motor Boating, New York, Ann. 

 Show Number (Jan), 1952, pp. 66-68, 247-249]: 



(1) At less than planing speed, that is, when 

 running as a displacement-type boat, the planing 

 craft must be as seaworthy as the best displace- 

 ment boat. Otherwise it is no seagoing vessel. 



(2) The speed at which the craft passes the hump- 

 resistance region and begins to plane must be low. 



It should be no higher than the speed at which a 

 displacement-type hull of the same approximate 

 size begins to require an inordinate amount of 

 power. 



(3) To achieve successful wavegoing performance 

 a planing hull must plane at a speed so far below 

 the cruising range that in a seaway it will maintain 

 its planing characteristics. If the craft is involun- 

 tarily slowed to below that speed it should at 

 once regain its planing position upon an increase 

 in speed. This factor is of the most vital import- 

 ance. For example, if a planing hull must travel 

 at 14 kt to reach planing speed and if it has a 

 sustained cruising speed of only 16 kt, the adverse 

 effect of only a small or moderate sea Avill suffice 

 to slow the craft down to below the planing speed 

 of 14 kt. It will struggle along, alternately planing 

 and falling back below hump speed. Such a craft 

 is, in the words of Teller, "neither a successful 

 planing boat nor an honest displacement boat." 



(4) If the planing craft is deliberately throttled 

 to below planing speed, it must have as good 

 wavegoing performance as the best displacement- 

 type hull. OtherAvise, if the planing boat experi- 

 ences power-plant failure in a gale or has to be 

 slowed because of fog or any other cause, it is no 

 longer a seagoing vessel. 



(5) The planing hull must be as free from pound- 

 ing and slamming as the best displacement-type 

 hull of the same size, running at the same speed. 

 Otherwise it is no seagoing vessel. 



(6) Finally, the true planing craft is not a weight- 

 carrying vessel, in the sense of a cargo ship. Its 

 use should be restricted to carrying special 

 equipment, certain types of cargo in an emergency, 

 or so-called premium loads, where the expense 

 involved in transportation is justified by the 

 saving in time. An aircraft rescue boat is an 

 excellent example illustrating these features. 

 Indeed, the planing craft may well be regarded 

 more nearly as a seaplane or flying boat than as 

 an ordinary boat. If the fljdng boat is overloaded 

 it can not and will not take off from the water. 

 If the planing craft is overloaded it likewise will 

 not take off and plane. 



77.7 General Notes on the Powering of Small 

 Craft. The general subject of powering for motor- 

 boats and other small craft is touched upon here 

 briefly, as a preliminary discussion of quantitative 

 power estimates in Sees. 77.14 and 77.26. 



The designed or minimum speeds for which 

 motorboats are powered are usually those to be 



