How do you imagine it UI-wise and fluff (logic?)-wise?
I think it's OP because:
-On galleon when you're a good pilot you have always on of your sides pointing at enemy. The galleon would be almost invincible.
-Similar with the junker, spire (front = kind of side?), mobula (?) and squid.
Fluff-wise I think it's simply silly. Only thing I can imagine are additional armour plates (which can be somehow set up and down in matter of seconds) which....er... somehow decrease damage of the weapon. I don't know how, actually.
Game-wise it may be considered if it's more like "Damage taken to the (side of?) the ship is halved and no gun (at the side) can be shot". Then it could be somehow balanced tool allowing to tank-up the hard moment. If we do this I'd also add time effect such is in hydrogene - hydrogene works around 5 seconds after turning it off - I'd say we could make it even 10 in this case (or 5 to activate and 5 to deactivate).
This is a Pilot Tool, so UI-wise, it's like any other Pilot Tool. It takes up a slot to use it for one side only. It's not like I can take this one tool and say "oh, they're on the right, so i'm gonna make my right side tanky", then as they go around you, changing that. No, it's more like "okay, they're behind me, but my only shield item is for my right side. Oh well, gotta take the damage." So you would have to take 3 Directional Armor items to shield all the normal sides, but that doesn't do anything for attacks above/below. There are 6 sides to any ship, after all.
The reason I made it weaken damage is 1) it needs SOME sort of downside, and 2) armor can get in the way of the weapons working properly, thus reducing their effectiveness. Beyond that, I don't need to apply any logic for how it works. I really don't. Why? Because hitting a boxy thing on my ship with a mallet shouldn't magically fix my balloon or hull. As that doesn't make sense, nothing else really has to make absolute sense either.
Working for a few seconds after it's turned off is fine. But Hydrogen lasts for THREE seconds, not five.
Balance wise, the Galleon that only has 1 side facing the enemy all the time, thus making it an easy choice which item to bring, also makes it so while it can tank damage, it's not dishing out nearly as much damage as normal. There are also multiple other sides that a faster ship like a Squid or Goldfish could get to, thus getting around the armor. Perhaps the effect only works on ARMOR and has no effect on damage to Hull.
The damage reduction and weapon reduction can both obviously be scaled to whatever is balanced. I simply threw out numbers for example purposes.