You won't survive the deadly world of Assassin's Creed: Black Flag Resynced unless your ship is upgraded and capable of taking on the deadliest foes in the Caribbean. To accomplish this, you will need to scour the game for Ultimate Plans, as they are required to achieve a lot of the final upgrades for various parts of the Jackdaw.

However, there are a couple of upgrades not tied directly to the Jackdaw that need these schematics. One of these is the Harpoon Strength Ultimate Plan, which does exactly what it says on the parchment. A critical upgrade, this plan increases the damage of our hunting harpoons, which means we can catch some of the tougher fish needed to improve Kenway's gear.

Here is where you can find it and how to implement this Ultimate Plan.

Harpoon Strength Ultimate Plan

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Unlike many of the other Ultimate Plans, this one is actually tied to the Treasure Maps and not the various underwater shipwrecks you'd reach with a Diving Bell. Because of this, we can actually get this plan quite early in our adventure without being locked out because we haven't progressed far enough into the main campaign.

To start, we will need to locate the actual Treasure Map first. This can be found on the island of Mariguana at coordinates 848,517. You can find this island on the eastern side of the map, just north of the Great Inagua. Once you reach this island, you will need to head to coordinates 847,514, which is roughly in the center of this location.

Specifically, you are looking for a small cave system that is covered by vines you can enter by crouch walking. When you enter the cave, look for a small path on the right that will lead you to an area with both a Secret chest on a small mound of rocks and a skeleton shackled to a post. Interact with the skeleton to obtain the Andreas Island Treasure Map.

Now that we have the Treasure Map, head over to Andreas Island, which is at coordinates 543,700. Located southwest of Nassau, when you arrive on the island, make your way to the southernmost part of the island at 544,693. Look for a small plot of land with three palm trees that's barely detached from the main island. On the other side of the palm trees, you will find a spot that prompts you to dig up the chest.

Doing so will trigger a brief cutscene that ends with you earning the Harpoon Strength Ultimate Plan.

How To Unlock The Harpoon Strength Ultimate Plan

With the Ultimate Plan in hand, head back to the island of Great Inagua and approach the Harbourmaster's stand on the docks. Make sure your Harbourmaster is fully upgraded, as the Ultimate Plan tier of upgrades will not be available until you've fully restored this building. You can do this by talking to the man with the boat icon right outside the Harbourmaster and spending a few thousand Reales to improve this vendor.

Once the vendor is completely upgraded, talk to the merchant and make your way to the menu that lists all the components of the Jackdaw you can improve. It will cost you 4,000 Reales and the Harpoon Strength Ultimate Plan to fully upgrade this part of the ship. There is no other way to achieve Harpoon III, so be ready to hunt down this plan and spend a bit of money.

All that said, I do think upgrading your Harpoon Strength is worth all the effort. Hunting fish is a critical part of reaching the highest level of Edward's gear, as many of them need hides or skulls from whales or tougher sharks. Given you have a limited number of Harpoons, ensuring you can actually even kill this animal before it flees is vital to your success.

While it's not something you need to buy right away, once you start venturing south, it is definitely worth picking up as soon as possible.