[edit: clarification]
Useful for ships under attack from the air. A kit like this for lifeboats could be useful when abandoning ship at war.
[added] It's made of inflatable rubber. In order to submerge, water-filled plastic bags are hoisted over the structure.-- pashute, May 28 2017 The magic of a cereal box. https://en.wikipedi...ng_powder_submarine [RayfordSteele, May 30 2017] Submerging is easy; coming back up is the hard part.-- pocmloc, May 28 2017 Ships are typically designed with significant positive buoyancy. This is so that they don't sink.
To submerge, the vessel would need slight negative buoyancy.
This would place the keel deep below the water surface. The hull would need to be substantially stronger than a conventional design to avoid it being crushed by the water pressure, presuming that the entire interior of the vessel is free-vented to atmosphere.
The "cover" is going to have to be a rigid metal structure. It will be heavy, and change the hull's metacentric height. This is generally considered unhelpful ...-- 8th of 7, May 28 2017 It's made of inflatable rubber. In order to submerge, water filled plastic bags are hoisted over the structure.-- pashute, May 28 2017 I'm not sure I'm clear on this. So you're basically covering the entire vessel with a big water balloon?
If that's what you're talking about the water balloon would sit right below the surface at best with a bump in the middle where the ship is.
You'd need to have some way to keep the water located above the ship from flowing over the sides down into the lower portion of the bag otherwise you'd just be wrapping the ship in rubber, which would not only give the attacker a clearer, larger target to shoot at but they'd mock your engineering aptitude before they killed you.
I've taken the liberty of assuming the design has a flooded outrigger portion to keep the ship upright as it sunk because if you just have a big rubber thing full of water above deck the vessel would just flip over. Again, humorous for the attacker to see you roll over and play dead as they begin their attack profile, but not much good to keep you from being made dead for reals.
Designing a ship that floats on the water is easy, designing a ship that submerges is easy, designing a ship that submerges then comes back up, not so easy.
If I'm totally misunderstanding your design, never mind.-- doctorremulac3, May 28 2017 Perhaps Draping sea colored camouflage over side so it looks like the sea has a pimple or a permanent wave where the ship used to be. Would make it much harder to aim at something vital.
Or tow a dummy ship or lifeboat, so there two targets. One could appear partly submerged. The floating solar still many lifeboats contain could take on a decoy function.-- popbottle, May 28 2017 You're all thinking too unambitiously. Consider a combination of a submarine aircraft carrier, and a floating dry dock. The watertight hangar section of the submarine would be large enough to fully enclose the ship. The door would be shut and sealed and then the vessel could submerge, containing the ship within it.-- pocmloc, May 28 2017 If a ship was torpedoed, then that emergency airbag superstructure might be a pleasant alternative to Davy Jones' locker.-- wjt, May 30 2017 Oh this is too easy. Just fill the ballast area full of baking powder.
Really though, to design a ship that can handle being under water, start with a submarine and go from there.-- RayfordSteele, May 30 2017 + @amonium_lactate
:-) @simon ch.
+!! @william t. (emergency airbag)-- pashute, Jun 10 2017 random, halfbakery