Hydroptere Rocket targets 80 knots!

I think they meant that qualitatively, cavitation presents a hurdle to performance that is analogous to the hurdle that sonic speed presents to aircraft performance. In both cases, there's a fluid speed that one tries to stay below if one can, or else accept the drag and go for shapes that are optimized for flight above the barrier speed. Above the barrier speed, there are flow phenomena that don't occur below that speed.

Like subsonic flight, subcavitating operation drives the section shapes to roof-top designs, and supercavitating operation drives the section shape to thin wedge-shaped sections that are not unlike supersonic sections. One thing I've not seen in most high speed foil designs is sweep, but that should help as well, and for many of the same reasons.

 

This idea is looking dangerously like a modern glider in ground effect. The long slender fuse, the lower wing in ground effect and the upper wing providing the lift/thrust. In fact if you broke the starboard wing off a glider threw it on the port tip and inverted the tail plane there you have it! All a bit tongue in cheek here but none the less they are looking like modern sailplanes in components.

Not surprising I guess as the designer is an aeronautical chappie.

 

We experimented adding sweep to our high speed (175kts) rudders. It is a good way of increasing air flow down the trialing edge, which was our driver to do it, but at very high speed you must be careful how the sweep works with the pivot point, otherwise it gets unstable in light load conditions. Less of a problem with a fixed foil, but adding sweep requires more structure, and for these foils slenderness is key, so whether the payoff is worth it would depend on how critical the trailing edge air flow is. At marginal supercavitating speeds (<80kts or so) it may be beneficial, but at higher speeds there is enough suction in any case. We used a concave trailing edge to encourage suction, which worked effectively as an alternative to sweep at lower speeds.

 

Lurch, I am not quite sure about the effectiveness of ground effect. The beam wing's chord is rather narrow with respect to height above the water surface so that ground effect will be minimal and definitely with respect to what the hydrofoils can generate more easily. Proof is that the hor stabilizer of the "sailplane" which should preferably be out of ground effect is at the same height as the beam wing. This hor stabilizer is also surprisingly small. The low hor wing on lee has an aspect ratio that would generate significant ground effect lift, and disproportionately more so as the craft would roll to lee.

 

I guess none of us will know for sure until the designer reveals more of his ethos behind the design. When I fly and watch others flying there is very much a ground effect present especially on the flare and just after wheels off before the climb out, this is pretty much one wing chord width from the surface. It would make sense to use this free source of lift on the horizontal port wing. The foils would be loaded less through this use so reducing the foil area...

Who knows - just speculating?

 

The proportions are there to see and are thus not speculations. There is ground effect at one chord height but that does not make it a ground effect machine by design. The difficulty with ground effect at low skimming height is the non linear nature of L/D with respect to the distance to the surface. With foils in the water this is probably not a very useful complication. The low foil to lee is more intriguing, though.