[Puster Bill]

Thanks for all the replies!

After reading them, and giving it some careful thought, I think what is happening is that the increased drag from the dive planes going hard up slows the boat down to the point where the dive planes can't support her.

I should have mentioned that I'm using GWX, which of course means that the deeper you go, the more negatively bouyant you are.

I had forgotten about the Thresher, which was a true hydrodynamic "stall" situation. This isn't exactly the same. It's not like you loose lift because of flow seperation on the diving planes, it just that the extra drag of going hard arise on the planes slows you down enough that you can't go up. And I want to reiterate that this is at slow speeds, 1 to 3 knots at most. You can even see a precursor of it at higher speeds: Try tooling around at, say, 6 knots straight and level. Then order surface. You'll slow down by a knot or so as the plane operators go hard up on the planes. It doesn't matter if you are travelling fast enough, but if you are deep and slow, it could be a problem.

Oh, and in theory it would be possible to "loop" a submarine on dive planes, given enough power and a strong enough hull (because you would need an awful lot of vertical 'room' to do it). A U-boat couldn't, because of lack of power and lack of depth capability, but I would expect an ultra deep diving nuclear boat to be able to do so, especially if it was a "short" boat like a Skipjack class (which isn't to say the Skipjacks *COULD* do it, just that their hull form would be better suited for it than a longer, skinner sub like a 688 boat).