Canadian Hydrofoil Program. Hydrodynamics and Simulation 



on good hullborne performance and seakeeping. 



The Sub -Ca vita tin g Main Foil 



In addition to providing adequate heave damping through high 

 — — , the main foil unit must provide roll stiffness without undue 

 heave stiffness. The BRAS D'OR unit (Figure 2) accomplishes this 

 by use of anhedral and dihedral surface -piercing elements at either 

 end of a fully- submerged main foil unit. The latter serves as the pri- 

 mary lifting element and, combined with the dihedrals, provides the 

 required characteristics with high efficiency. The upper anhedral 

 panels are highly cambered and twisted to develop high lift at take- 

 off. The anhedral tips are incidence-controlled to augment roll sta- 

 bility at low foilborne speeds and improve turning performance. 



The theoretical tools required for hydrodynamic design of a 

 subcavitating foil system have been obtained by adding free surface 

 corrections to methods borrowed from subsonic aerodynamics. The 

 prevention of cavitation is the chief hydrodynamic constraint in sec- 

 tion design, and cavitati on-free operation above 40 knots necessi- 

 tates a departure from conventional low speed aerofoils to delayed- 

 cavitation sections such as illustrated in Figure 3. The design of the 

 particular sections used in BRAS D'OR is described in v ' . They 

 are highly efficient and provide an approximately uniform pressure 

 distribution when operating over a wide range of angle -of -attack in 

 close proximity to the free surface. The practical upper limits of the 

 delayed -cavitation regime are approximately 60 knots in calm 

 water and 50 knots in rough seas - the design speeds for BRAS 

 D'OR. 



Full scale trials showed that in general the BRAS D'OR main 

 foil unit has successfully met its design requirements for high effi- 

 ciency, low !*|~ ' and high ? . The only significant hydrodynamic 

 problems encountered were associated with emergence of the anhe- 

 dral-dihedral foil intersections at about 45 knots, which resulted in 

 lateral jerkiness even in calm water. There were two specific pro- 

 blems. The first was caused by intermittent ventilation of the dihe- 

 dral foils and anhedral tips and was countered by installing addition- 

 al anti -ventilation fences. The second was fundamental to the main 

 foil geometry and was more difficult to solve. It was due to increas- 

 ed roll and heave stiffness below the intersections, when both the 

 anhedral tips and the dihedral foils become surface-piercing. It was 

 alleviated by reducing the mean angle -of -attack of the anhedral tips 

 by 2°. This increased main foil immersion and delayed the emerg- 

 ence of the anhedral-dihedral intersections to approximately 50 

 knots. The net result was a significant increase in riding comfort in 



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