The concept of the windknife, is to give the builder the ability to produce, accurate, durable hydrofoils of varying sizes, quickly without the need for expensive tooling each time a new size is required.
Extruded Aluminium hydrofoils have been used successfully on Dinghies such as the Laser Pico, 29er, 59er and Mega Byte for over 10 years.
The most important part of an aerofoil is the nose section, this is where most of the work is done and hence where most of the accuracy is required.
It isnoticable that the front section of a long thin foil is similar to that of a short fat foil. Below is a NACA0009 section and a NACA0012 section both of similar thickness super imposed, it can be seen that it is really only the rear sections that differ. Therefore it is valid to use the same nose section for the construction of different sized foils.
The windknife uses a common nose section which can be compressed or expanded.
The nose of the windknife foil is a NACA0009 section, this is a good middle of the road section. Equally good for centreboards and rudders.
The principle application for these foils is for dinghy and beach catamaran rudders and boards. For rudders these tend to be designed to fit standard rudder stocks which are either 20mm or 25mm thick.
Centreboards are often thicker, up to 35mm, and the smallest boats such as the optimist might go down to 15mm thick. Therefore the windknife has been designed to make foils between 13mm-35mm thick.
In general the most important paramater is the area in the water (wetted area). It can be seen that a FF27 makes a good daggerboard and a FF14 makes a good rudder.
Examples of common sizes are shown below.