You get the first upgrade for free through the story mode in Black Flag. This goes hand-in-hand with the heavy shot upgrade, which increases the power of the broadside cannons. More cannons means more shots fired, which leads to additional damage. Instead, you're adding more cannons to both sides of the ship. When you upgrade the broadside cannons, you're not making them stronger. The reason for this is because the broadside cannons on either side of the Jackdaw are the most powerful close range attack in the ship's arsenal. Likewise, you want to angle your attack so that the Jackdaw's broadside cannons do most of the work. The best strategy when it comes to taking down an enemy ship is to avoid their broadside cannons once you're within range. It takes a considerable amount of money and metal to upgrade the hull armor, but you should get the intermediate hull upgrade before moving on to anything else, then come back to the reinforced upgrade as soon as you can.

The stronger your hull armor, the more attacks the Jackdaw can withstand before it sinks to the bottom of the ocean.

This should be your first priority when it comes to upgrading the Jackdaw. It doesn't matter how powerful your ship is if the armor surrounding the hull is less than stellar.

Weapons assassins creed black flag