Sunday 28 January 2018

Foiling high speed stalls

Team Cool Runnings (CR4) experienced a high speed stall when foiling in the Solent in late October 2017.

We've been studying what could have caused this as it happened at a top speed of about 34 knots in relatively calm/flat waters of Southampton river.

This picture shows the sudden speed loss as the cavitation forms around the foil and quickly develops into a massive speed brake as the volume of the air bubble increases.

The picture (courtesy of Mario Caponnetto) below shows quite well one of the root of the causes. As
the cat accelerated she also flew very high and with that the drift/leeway increased drastically causing a very high angle of attack on the lower leeward end of the v shaped foil carving through the water. In CFD simulations you can see in red the area which is most prone for starting cavitation. As we reached higher speeds, quite suddenly in a gust, the rake settings were to high and coupled with the our wind proof trampoline*it lead to a bow up high ride. This increases the leeway dramatically as ever less surface of the asymmetric shaft side of the foil is supporting the high side forces of the rig. This is even made worse by the shallow apparent wind angles these boats endure downwind ( less than 30-40 degr AWA).

In layman terms we went to fast flying to high. The foil spat the dummy at us when there was hardly any lateral area left for either uplift or stop us from skipping sideways. Lesson low = fly save

*  Windproof trampoline assist our endplate effect of our Aero skirt and minimises  losses and tip vortex around the bottom of the mainsail.