O - Foils, Aft only--tests and Anaysis
INTERNATIONAL HYDROFOIL SOCIETY'S ALBUMS > O - Foils, Aft only--tests and Anaysis
After watching the videos of premature flight termination, here is what the team concluded as a possible explanation and solution to the problem.

In video 091353 at 18sec of 5:25 minutes can can observe the formation of a wingtip vortex. In the audio, Dwight comments about its formation. The vortex is visible as vaporized water (like steam) and the condition is known as cavitation. Cavitation can occur at low speeds at high angles of attack (AOA), or low AOA at high speeds >40 mph or so.

This, it is thought is the result of upward wing tip deflection due to structural flexibility limits being exceeded.

When the tip is deflected upwards, it twists without directional predictability. For the brief time that the vortex is seen it is twisting to increase the AOA. When lift is lost and there is no vapor trail, the twist is in reducing the AOA.

Static testing will demonstrate the foil's flexibility. If the main foil supports 300 lbs and the outboard section is 25% of the span, that section supports 75 lbs. If one pushes 75 lbs. upward uniformly across the outboard section it will deflect upward, I predict. There is no reason to believe that the AOA at the tip will remain constant as the foil is deformed upward. So as load is applied, you will experience dramatic variations in AOA, cavitation, and lift.

The solutions are:

1. Beef up the foil section with additional wraps of carbon fiber and resin (simplest solution). See fiber orientation in #3 below.
2. Rebuild foil with taper (narrow at the tips, wide at the root).
3. Rebuild the foil using upper fiber orientation with fibers crossing the line running from tip to tip at 45 degrees. The fibers will intersect at 90 degrees. The underside of the foil should have the fibers running from tip to tip.

The center section may deflect as well causing further fluctuations in total lift.

Boat designed, built and piloted by Dwight Filley, California
Videos By Jim Bixby, California
Written and posted by Ray Vellinga, 6/29/2013