One of my friends racing in that class was trying to explain it to me and made no sense. That article helped me understand the situation better but I definitely don't have a brain for the sciences!

Really surprised to see 3 outboard cats flip in one weekend. Thought they would be safer at the speed they run. Glad everyone is alright. Sorry but I would only race a V hull. It's crazy how many lives where lost in past 5 years in cats flipping. But I also give these guys credit to have the balls to do it!

These aerodynamic proximity effects can be tricky to lock down without running CFD. Certainly any vortex lift would need to act in a manner that enhances lift on the adjacent boat by virtue of the proximity of a counter-rotating vortex. Generally that would mean increasing the negative pressure on the deck side as I cannot quickly work out how the vortex interaction would act on the high pressure tunnel side. Whist counter-rotating vortex interaction may have a small role to play I suspect the endplate effect will be the predominate source of any lift enhancement. If the cats are running with a bow up attitude then high pressure air is leaking out from under the sponsons. This leakage may be mitigated by the mirror image flow in relative proximity (i.e. the high velocity air escaping from Cat number 1 is blowing against Cat number 2 thereby reducing the leakage, increasing the pressure and increasing the lift).

Sorry for the over-analysis

ND1

Think of an airplane wing, it has high pressure underneath it and creates low pressure over it. I believe what the story was saying in english (at least my interpretation) is that it increased the low amount of pressure on the top of the boat. With the same high pressure under the boat, and LESS pressure on top of the boat, the boat goes up and the further the bow goes up, the more pressure under it and the less pressure over it.

In the story it seems Steve David is solely attributing the initial pitching moment to the vortex interaction but given 2/3+ of the lift (of a wing in ground effect) is derived from the lower (high pressure) side the math doesn’t add up. The endplate effect is a much larger contributor. That said – it was only a short quote and he is certainly correct in that once that initial rotation occurs it is all over. That’s where vortex lift and drag really take over. Great this conversation is happening. Hopefully some of the lesser experienced open cockpit cat operators are reading as well : )

ND1