6 - Design of hydrofoil sections

Summary

Criteria for the design of blade sections may be selected to include:

1. i. Minimum thickness and chord to meet strength requirements;

2. ii. Sufficient camber to generate the design lift;

3. iii. Distribution of thickness and camber to yield the least negative pressure coefficient to avoid or mitigate cavitation;

4. iv. Thickness- and loading-pressure distributions to avoid boundary layer separation with least chord to yield minimum drag consistent with requirements i. and iii.;

5. v. Leading and trailing edges to satisfy strength and manufacturing requirements.

The first part of this chapter follows from linearized theories developed by aerodynamicists more than 50 years ago, placing emphasis on the use of existing camber and thickness distributions yielding least negative minimum pressure coefficients, C_pmin at ideal angle of attack. At nonideal angles (which always occur in operation in the spatially and temporally varying hull wake flows) we are required to seek sections having greatest tolerance to angle deviations and at the same time having negative minimum pressure coefficients exceeding the level that indicates occurence of cavitation. This tolerance depends critically upon the leading edge radius and the forebody shape as well as upon the extent of the flat part of the pressure distribution. Thus we are led to the more recent findings of researchers who have developed profiles having greater tolerance to angle of attack. When cavitation is unavoidable the latest approach is to use blunter leading edges to generate shorter, more stable cavities thereby avoiding “cloud” cavitation which causes highly deleterious erosion or pitting of the blades.