Hydrofoil Motions in a Randonn Seaway 





^njJc of att,. ,le 



ftlly Wttti/ 



H I 





^^. < 



l-p*'^^^v j 



/thfo-i Lift", 



(a) 





Siw^uUte^ Lift. 

 (b) 



Figure (vii) 



Ventilation 



Cavitation is unlikely to occur on foil elements at the slower foilborne 

 speeds unless the foil angle of attack is very large (>10°). However, ventilation, 

 which has a similar effect, can occur at any speed when a foil is surface pierc- 

 ing or is close to the surface. 



The effects of ventilation have been simulated on the analog computer, but 

 this proved to be an extremely complex problem requiring a large number of 

 computing elements. 



The criterion for ventilation of a given foil element may be either the angle 

 of attack (a) or the lift coefficient (c^). a was used in our simulation. In 

 terms of a, at a given speed it was assumed that there exists a fixed value of 

 a, (a^, say) for which ventilation must occur if a >a^. Similarly, there exists 

 an a stop (a^), for which ventilation, if occurring, will stop when a<a^. It was 

 also assumed that if ventilation does occur, it will occur down to the first fully 

 submerged fence, also, if a^ < a < a^ and if ventilation is occurring, it will 

 cease if a fence goes through the water surface, either coming in or going out. 

 The amount of lift (or rolling moment) lost due to ventilation is a fixed, but con- 

 trollable fraction of the lift due to the affected portion of the foil which would 

 have been calculated if no ventilation were assumed. 



a is continuously available in the simulation, therefore, it is possible to 

 produce a circuit to subtract the correct amount of lift from the total for the 

 given foil element. This, however, is not straightforward, as a function of the 

 following form is required [Fig. (viii)]: 



639 



