originally incorporated twin skis with rectangular trailing edges for low resistance 

 reasons. We soon became aware of our mistake, however; for soon after the first 

 taxi run, and while taxiing over what may be very roughly described as four to six 

 inch wind waves, about 12 to 15 feet apart, we had a failure of the nose-mounted 

 airspeed boom. The dynamic characteristics of the Sea Dart structure were such that 

 these seeming insignificant waves produced a failure in an airpseed boom designed for 

 in excess of 50 g's. It is believed that the structural and hydrodynamic implications of 

 this incident might well be sufficient justification for test runs of full scale hardware in 

 this new N.A.C.A. facility, particularly in view of the trend toward greater airframe 

 slenderness and flexibility. 



It is also believed possible that hull bottom plating fatigue failures may be 

 partially attributed to the higher stresses resulting from resonance effects. It is known 

 that the pressure pulses imposed upon seaplane bottoms during high speed planing 

 over waves may exhibit a pressure-time history so steep as to approach a so-called 

 "step function" in character. Since a step-function theoretically contains all fre- 

 quencies, it would seem plausible that these very steep pressure pulses might, through 

 resonance effects, produce a plating stress in excess of that which would be produced 

 by the same peak pressure acting statically. It therefore seems that the new facility 

 would lend itself very nicely to an investigation of this type through runs of a segment 

 of actual seaplane bottom structure at high speed over waves and with pressure pick- 

 ups and strain gauges installed. 



One other area might be mentioned for which the new facility could supply 

 answers of interest to the high speed seaplane designer. Most of the impact theories 

 presently available are built around the concept of the single, isolated, design landing. 

 They are based on a mental image of a relatively large wave, and the detailed calcula- 

 tions therefore treat an impact on an unlimited expanse of water. The preceding dis- 

 cussion makes it clear that waves which are relatively small compared with the airplane 

 and having relatively steep slopes may be of considerable importance. It is therefore 

 suggested that a theory is needed to account for the finite expanse of the wave — an 

 allowance for breaking through the back side of the impacted wave. 



R. W. L. Gawn 



We should all be grateful to Mr. Parkinson for an excellent paper with a clear 

 account of many important developments in flying boats. It serves as a reminder that 

 there are some common hydrodynamic problems between the flying boat and the craft 

 known as fast patrol boats or in your country P.T. boats and it is instructive to consider 

 these and ascertain the extent to which experience matches up. 



The "Princess" flying boat is much the same all-up weight as the fast patrol boats 

 known as the BOLD Class. There is a notable difference in the hull in that the flying 

 boat is longer and narrower. In fact the length is 8.9 times the beam whereas the ratio 

 is only 5.0 in the fast patrol boat. 



An interesting point in common is that the running trim is controlled in both 

 craft. The elevators serve this purpose on the flying boat and operate in air. Transom 

 flaps are the counterpart on fast patrol boats and are noticeably smaller than the air- 

 craft's elevators, one reason being that they operate in water. The purpose is to reduce 

 the running trim primarily to make the fast patrol boat more seaworthy at the hump 

 speed by reducing slamming. It is very satisfactory in this respect and also leads to less 

 hull resistance. 



There has been controversy as to the relative merits of hard chine and round 

 bilge hull shapes for fast patrol boats. A boat has been built of each type in recent 

 years and trials carried out at sea and in addition there has been a series of comparative 

 tests on models. The latter indicated that the ship motions including accelerations are 

 very much the same, the hull resistance of the round bilge form is less up to a speed 

 length coefficient approaching 4 but is greater at higher speeds and the only other nota- 

 ble difference is that the hard chine form is drier in a seaway. The comprehensive sea 



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