[NO BUG] Upgrading trireme to frigate uses up an entire iron mine?

[opopanax666]

I have 4 iron mines in my territory. I had a dozen triremes. I got to the point where I could build frigates, but decided to upgrade my triremes. So sent them to friendly waters, and upgraded 1, then the 2nd, 3rd and 4th. When trying to upgrade the 5th, I noticed the line "need 1 iron" all of a sudden was red, so I couldn't upgrade
I looked at the top bar to my iron resources and it says "4 total - 4 used" So, it uses up an entire iron mine to upgrade a ship?
Please, tell me this is a bug, and not a feature...

 

[lixoman100]

I have 4 iron mines in my territory. I had a dozen triremes. I got to the point where I could build frigates, but decided to upgrade my triremes. So sent them to friendly waters, and upgraded 1, then the 2nd, 3rd and 4th. When trying to upgrade the 5th, I noticed the line "need 1 iron" all of a sudden was red, so I couldn't upgrade
I looked at the top bar to my iron resources and it says "4 total - 4 used" So, it uses up an entire iron mine to upgrade a ship?
Please, tell me this is a bug, and not a feature...

It is not a bug.

Triremes require no strategic resources. Frigates require iron (1 iron resource in use for each frigate you have). The resource is in use as long as the frigate exists. Same for all other strategic resources like horses, oil and coal.

 

[opopanax666]

So, even if I built the frigates, I would have "lost" the iron? Makes sense, I guess

Thanks for the reply!

 

[lixoman100]

So, even if I built the frigates, I would have "lost" the iron? Makes sense, I guess

Thanks for the reply!

You are correct. Just like you "lose" horses when making a Knight or oil when you make a tank.

And you are welcome