Black pennant meaning ...

[Oldgateboatdriver]

The "pirate" flag is not, repeat, not an attack flag for submarines (how would you fly them underwater?). It is flown upon return home to indicate a succesful mission, not merely return from patrol. Success may be having sunk ennemy vessels (war time) or fulfilling a specific tasked mission (such as carrying out a surveillance mission on a new potential ennemy warship in peace time).

 

[dimsum]

The "pirate" flag is not, repeat, not an attack flag for submarines (how would you fly them underwater?).

Where there's a will, there's a way

 

[brihard]

The "pirate" flag is not, repeat, not an attack flag for submarines (how would you fly them underwater?).

Moistly?

 

[Underway]

You can't fly them underwater, physics. You can swim them though. Unless someone designs a supercavitating flag... there's an idea!