Slitherine

Whenever I move a submerged submarine and it happens to end its turn within sight an enemy naval unit (often a destroyer which is part of the fleet screen) the destroyer immediately attacks the submerged submarine with gunfire, meaning the sub must be on the surface. The destroyers often move closer and attack without use of sonar: their attack does not use the depth charge animations nor is there the sonar ping animation and sound.

Needless to say, my submarines are routinely butchered.

The manual says this about submarines:

Submarines are naval vessels capable of moving and attacking while being submerged. In this state only enemy ships equipped with the Sonar Pulse ability can effectively reveal and attack a submarine.

But clearly, my subs are not moving submerged, but on the surface. So how is this actually working in the game? Are submarines surfacing at the start of their move and ending the turn on the surface? If so, this really limits their utility. May as well just set them in one spot, submerged, and leave them, for all the good they do you. I know the AI subs move in unseen because they often attack me by surprise and I don't find them until I do a sonar search - or happen to run a surface ship right into the hex they are in.

Clarification from the developers or beta testers would be much appreciated.

UPDATE:
I think I answered my own question. I looked closely at the icons that show up when you go to move a submarine and the third ring of hexes has a symbol on it of a submarine riding waves - pretty clear now that it means that the first two hexes of travel are submerged, and the last one is completed on the surface. Actually this makes perfect sense because WW2 diesel-electric subs could travel faster on the surface than submerged and the third ring of hexes shows maximum distance attained if you end up on the surface. So this explains a lot about how I have been abusing my subs by not noticing this symbol for surfacing. Hopefully my hard-learned lesson will be of use to others who might have wondered the same thing. Game again working as designed (WAD.) Son of a gun.

Nice one, now I finally understand the rather poor performance from my subs aswell .

I think I answered my own question. I looked closely at the icons that show up when you go to move a submarine and the third ring of hexes has a symbol on it of a submarine riding waves - pretty clear now that it means that the first two hexes of travel are submerged, and the last one is completed on the surface

Spot on