"like a screen door on submarine"-what does it mean?

Imagine you're on a submarine and you have a normal watertight door and then screendoor in same doorway as you would on your home...like an inside door and an outside door. Now imagine you are submerged (underwater) and the captain asks you to open the watertight door but keep the screendoor closed...how useful do you think the screendoor would be at preventing the submarine from sinking?

I don't know if you're referring to the song, ..... if so ... AMAZING SONG! If not ... CHECK OUT THE SONG!! Its called Screen door by Rich Mullens

Anyway ... on top of being the generic explanation of "useless" ... a screen door on a submarine would also be a form of rather unnecessary extravagance. yea, it wouldn't work, so it would be useless ... but you would also have to have some sort of re enforcement BEHIND it ... so the screen door would be something extra you don't need. If you use this for an analogy ... perhaps in the work place, you wouldn't complete a task i full only to add the same, yet watered down, solution. This would be completing twice the work in vain.

bottom line ... don't over do/think things. Do what you need to to get it done, then leave it be!

CHECK OUT THE SONG!

Everyone agrees that it's useless, but the intent is even a bit deeper: That someone went through the trouble of installing a "useless door," is really more of the point. When you see that someone's work did not contribute to the goal or solve the problem that's when the saying becomes clear.

Simple as this "WORTHLESS"

What good is a screen door on one of those?

Doors on subs are water tight, locking, steel ect.

It's useless it isn't going to keep the water out.

It's full of holes.

It doesn't hold water.

You're a lier.

the phrase basically means "its useless" or it cant work.

put "As useless as" in front of your quotation and you will see

Worse than useless. It will doom you to failure.

useless