During the tests of Hydrofoil 3 (see Summer Project 2010) we observed that the aspect ratio of the front foils was too low. The consequence was that the margin for error was minimal: a precise angle of attack had to be maintained. The foil had to be immersed almost to the first strut in order to provide satisfactory performance. Roll stability was also critically low. From the video the difficult handling of the craft is apparent.
To enhance performance we decided to extend the existing foils. The molds were CNC milled foam covered with Oracal. The skins consist of three layers (+45/-45/0°) of 200g/m^2 hybrid carbon/glass fabric and a spar of T700SC (100mm wide 300g/m^2 UD folded in four). First the skins were laminated and cured under vacuum, then the upper half was aligned on the existing foil, the spars connected using several pultruded 4mm diameter rods and plenty of carbon tows. Before closing the molds, additionally to the usual microballons, carbon snippets mixed with resin were liberally applied on the joint. To complete the foils only fences and endcaps had to be applied.
Since the plan was to extend the front beam, the front support structure was also reinforced. The front beam was extended from 1.76 m to over 2.9 m.
The strong curvature of the free surface behind the front foils, due to downwash, was clearly observed on several occasions. It is visible particularly well in the above right picture.
The adopted measures proved to be a complete success. The boat was trimmed to fly in the 12 to 13 knot range. Flying for longer periods of time was no longer a problem and the stern lifted nicely out of the water. For some strange, to be investigated, reason the 15A would often not attain the high revolutions it normally did. If it did, however speeds over 14 kn were regularly recorded. As the trim of the craft worsened with increasing speed, the result usually was a nosedive. A particularly spectacular one is depicted below
The last picture compares the gps log of the Summer Project 2010 and Winter Project 2011, the maximum speed figures are most likely due to incorrect fixes of the gps. Very interesting to note is the average speed when in movement: it passed from 4.0kn to 6.9kn. It increased by 172.5%! Indeed crashes with the modified foils were much less frequent even at the beginning of the setup phase.
Here is some video footage of the project