screen door on a submarine

A screen door on a submarine would be absurd as it would allow the vessel to fill with water.

screen door on a submarine (plural screen doors on a submarine or screen doors on submarines)

1. (idiomatic) Something that is useless or absurd.
   • 1995, The Economist‎^[1], volume 336, numbers 7921-7925, Economist Newspaper Limited:

     But realists put the final number of jobs saved through privatisation at no more than 800. "It's about as useful to us as a screen door on a submarine," says an angry mayor's office in Sacramento .

   • 2005 July, Rich Beattie, “Trigger Happy”, in Boating‎^[2], volume 78, number 7, →ISSN, page 123:

     I doubt anybody would give you static about unloaded and stored long guns anywhere in U.S. waters, and few places would about handguns (although unloaded guns are as useful as a screen door on a submarine).”

   • 2016, Rocky Heckman, Designing Platform Independent Mobile Apps and Services‎^[3], John Wiley & Sons, →ISBN:

     If you don't know what security vulnerabilities look like at the code level, then having the source code is just as useful as a screen door on a submarine from a security perspective.