Sorry if this question has already been asked.

Attack (target lock): Discard this card and spend your target lock to perform this attack.

After you perform this attack, the defender receives 1 stress token if its hull value is 4 or lower.

I was recently at a story tournament and was informed that Hull Value NEVER decreases or increases. Damage is either added or removed. I believe the guys at the event and am not questioning them, but I was under the impression if a ship takes on damage it lowers the remaining hull vale.

My question is where do I find this in the rules?

From the rule book, page 12 (emphasis mine):

When the number of Damage cards dealt to a ship equals or exceeds its hull value (yellow number), that ship is destroyed (see “Destroying Ships” on page 16).

It says that the number of Damage cards exceeds the hull value, not that the hull value reaches 0!

Everyone says "I have X hull left" because, even though it's not technically correct, it's a lot easier than saying "I can take 3 more damage cards before dying".

Also, from the FAQ:

When determining whether the defender receives a stress token from Flechette Torpedoes, the defender’s starting hull value (including any equipped Hull Upgrade card) is used, not the defender’s remaining hull points.

Hull value is never lowered. The rules state that a ship is destroyed when the number of damage cards is greater than or equal to the hull value.

So basically that yellow number just states how many cards it can have until it dies, but it is never reduced.

Edit:

Thank you

The only things that change the hull value are the Hull Upgrade and Combat Retrofit, which increase it, and thus increase the number of damage cards needed to kill the ship.