Schmitke and Jones 



tics by direct observation of depth of immersion. Figure 9 shows the 

 steady state lift coefficient (based on horizontally projected immersed 

 area) of the bow foil unit over the foilborne speed range. The full scale 

 lift coefficient falls within limits established by quarter scale model 

 tests at NPL and LUMF except at low speeds. For these, leading 

 edge ventilation extended only partially down the span of the dihedral 

 foils at quarter scale but was complete at full scale, giving lower lift 

 but more stable flow. 



The inhibiting effect of the outboard intersections was clear 

 at quarter scale. The dihedral foils ventilated from the mid-back 

 spoilers under most conditions, but the intersections, acting as fences, 

 prevented the initiation of leading edge ventilation until they emerged. 

 It almost invariably occurred on one foil at a time since the associat- 

 ed loss of lift caused the intersections to re -immerse, inhibiting the 

 second dihedral more strongly. At full scale, initial establishment of 

 leading edge ventilation on the dihedrals still appeared to be associated 

 with emergence of the intersections, but invariably occurred simulta- 

 neously on both dihedral foils. Effects were less clear but full scale 

 ventilation certainly occurred more readily and more strongly than 

 indicated even by LUMF quarter scale tests at correctly scaled 

 Froude and cavitation numbers. This was due at least in part to the 

 fact that full scale trials were seldom held under really calm condi- 

 tions, the practical limit for "calm" water being set at waves 3 feet 

 in height. Occasionally, during take-off in exceptionaly smooth seas, 

 leading edge ventilation was delayed until ship speed approached 40 

 knots ; during this interim period, pitch angles of up to 9° were ob- 

 served. This situation was easily overcome by increasing speed or 

 bow foil incidence until leading edge ventilation occurred. 



Bow foil rake angle optimization trials showed that a strong 

 and persistent ventilated cavity was achieved in calm water at the 

 design rake angle setting (0°). In rough water optimum rake angle 

 varied with heading to the sea ; suitable flow and ship motion charac- 

 teristics were generally achieved in head, beam and following State 5 

 seas at rake angles of -1°, 0° and 1 l/2° respectively. 



In short, steep seas, flow re -attachment sometimes occurr- 

 ed on the bow foil dihedrals during deep immersion at the face of 

 larger waves. The resulting discontinuous increase in lift gave added 

 impetus to bow up pitch motion. As depth of immersion decreased at 

 the rear wave slope, ventilated flow was re-established, accompanied 

 by a sudden decrease in lift. Oscillograph records of vertical accele- 

 ration during these periods exhibit a sharp positive and negative spike 

 followed by a return to normal acceleration levels as the ship encout- 

 ered waves of more typical size and normal ventilated flow was re- 



298 



