It seems to me that in order to create dynamic lift, a boat has to actually be in the water. If the boat is not in contact with the water, you won't get any lift from it (obviously). So, even a fully planing boat displaces a bit of water and so has some component of static lift, so planing cannot be an absolute condition, but rather an arbitrary decision about whether enough of the lift is dynamic.

I think it's quite telling that most of the hits for "semi-planing" that Google turns up are boat ads. This suggests a phrase coined by builders not scientists, and I assume is intended to mean a boat that sits somewhere in between planing-as-in-skiff and displacement-as-in-car-ferry. i.e. there is a significant component of dynamic lift, but not so much that you'd call it planing.

As for cats "semi-planing": I've never really believed it. I find it hard to believe that something that shape is going to produce any significant fraction of the 2.5kN (like 255kg) of lift needed to keep my 2-up Blade afloat. Calling cats semi-planing strikes me as an attempt at explaining why cats go as fast as they do by people who don't understand what Froude's Law actually means. But this is just speculation: I only made it to one lecture on fluid dynamics , and it all seemed much too difficult <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Paul