It gets thrown around a lot, but something really bugs me! When people start saying that their rudders are Cavitating. Now unless someone has skipped on the 30 knot barrier altogether and is rapidly approaching the world water speed record in a Moth this just isn’t right.
Cavitation is basically the low pressure area on a foil getting so low, that the water vaporises itself and turns to steam.
What is actually happening on the moth is Ventilation. This is where air gets sucked down the low pressure side of the foil from the water surface – the rudder loses steerage and you have to back off to reattach the flow – this happens a lot on the 0010 section foils (earlier Prowlers, I think they’ve changed section now) and especially in cold water.
Fences go a long way to solving this as they stop the air getting too far down the blade but they are draggy and a lot of hassle.
I did have a weird ventilation experience a while ago when foiling particularly high and the leeward rudder tip broke out – I saved the crash and carried on with a very draggy rudder for a few seconds until the air was gone and normal service resumed. Curious I had another shot at it and watched the rudder this time and you could visibly see the bubbles on the foil and the pocket getting smaller until they were all scrubbed off.
You don’t see that every day – Anyway, vague science lesson over – I’m off to the Tiger Trophy in the morning with a good bunch of about 8 other Mothies. Will be fun!