Sohnifke and Jones 



small to moderate waves at speeds up to 50 knots. 



Main foil cavitation was never observed during calm water 

 trials, even at 62 knots, indicating that section design met all ex- 

 pectations in this very important respect. No problems arose from 

 hydrodynamic interference between individual foil elements. How- 

 ever, both full scale trials and model tests at the National Physical 

 Laboratory showed that bow foil wake reduces main foil lift by ap- 

 proximately 10% . 



The Super -Ventilated Bow Foil 



Operating conditions for the bow foil are demanding. At foil- 

 borne speeds in rough seas the bow foil is subject to wide and rapid 

 changes of both immersion and angle -of -attack. The hydrofoil system 

 is wholly area- stabilized longitudinally and the lightly-loaded depth- 

 sensitive bow foil is the primary source of control, so that smooth 

 lift vs. immersion and lift vs. angle -of -attack characteristics are es- 

 sential. Sub -ca vita ting hydrofoil sections are prone to ventilation in 

 rough water and the resulting sharp losses in lift at the primary lon- 

 gitudinal control element cause an unacceptable diving tendency ™* . 

 Superventilated sections are therefore used for the bow foil, despite 

 their lower efficiency. In this case, occasional suppression of ven- 

 tilation gives sharp lift increases, but unlike the converse situation 

 with subcavitating sections this is an inherently safe effect. 



The BRAS D'OR bow foil is of diamond configuration (Fi- 

 gure 4) with a sub -ca vita ting centre strut and super -ventilated di- 

 hedral and anhedral elements. Tulin Two-Term lower surfaces ^ 

 were chosen for the super -ventilated sections (Figure 5) because 

 these appeared to offer the best compromise between hydrodynamic 

 efficiency and structural strength. Design incidence is nominally 5° 

 above zero lift (as established by model tests), and rake angle of the 

 unit is adjustable in flight to permit operation at optimum incidence 

 for the prevailing sea condition. 



Little information was available on the practical operation of 

 surface -piercing super-ventilated hydrofoils, so that extensive ex- 

 perimental development was necessary. Model size had to be as large 

 as practical to minimize scale effects : consequently, the bulk of the 

 work was done at quarter scale, taxing the limits of available towing 

 tank facilities. The same bow foil was also used as part of a complete 

 quarter scale manned model of the system. A great strength of the 

 development program lay in the ability to test the same model both in 

 the controlled environment of towing tanks and as a functional unit in 

 realistic seaways. 



296 



