A girl has used one to "make a wish" at the bridge to the right of Main Street. You can only get it by crossing under the bridge right to left.
You will earn many more once you leave Fort Ridley, trading and collecting salvage.
There are no cheats on Poptropica, so you will have to tediously accumulate your doubloons through salvage and trading. This should take upwards of 30 "days" on the island, which are measured by trips between the individual islands. The process becomes much faster once you master the trading route (clockwise) and buy larger ships.
How To Get More Doubloons
(see related question below)
You earn doubloons on Skullduggery Island in two ways.
The first is by "salvage" when you come across random floating items in the sea. These are more numerous around sunken ships. Collecting them, however, is not without peril because sea monsters and pirate ships are more likely to attack the longer you spend away from an island. (see below for sinking sea creatures)
The second, which is ultimately more important, is by trading among the islands. You buy one of the four types of cargo with your doubloons, then carry them to another island to sell at a higher price. *It is important to get the next larger ship (at Dragon Cove) whenever you can afford it. Larger ships are faster, stronger, hold more cargo, and have more cannons for attack or defense. With larger ships, the amount of profit gets substantially larger. You can borrow money from the Golden Harbor Bank, but only an amount equal to your current worth, and it must be paid back with interest. (The only action taken to enforce its repayment is that an overdue loan prevents you from buying a larger ship.)
The Circular Trading Route
Travel clockwise around the islands. Except for Parrot Port, one kind of cargo is always cheaper in a port. (The profit on goods from Parrot Port is comparatively small.) By stocking up at one of the four islands, you can make a substantial profit by selling at the island just clockwise from it :
Sea Monsters and Pirates
Sinking a sea monster or pirate is a good way to get a LOT of doubloons, but it is risky unless you are very good at it. You will lose some of your doubloons if you are sunk. Generally, the sea creatures attack at the left side and above Dragon Cove. So once you have 2 or more cannon, you can hunt them there. Note that smaller ships will fill the cargo completely with just one creature. Beware of multiple attacks by creatures and ships.
Once you have enough doubloons, hire the skilled crewmembers who improve your ship's performance, notably the cargo master (increases space) and the shipwright (repairs damage).
Doubloons add up faster than you think when you have larger ships. You should pay off any loans you have before you get 500,000 doubloons, so that when you get that amount, you can borrow another half-million from the bank.
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The way to get doubloons while sailing on Skullduggery Island is by salvage and trading. Salvage is just found in the water, and can be sold on any island. Trading involves buying a cargo cheaply on one island and selling it for more on another.
It is important to buy a larger ship whenever you can afford it. Larger ships collect more salvage, move faster, and have more cannons.
Trading The Cargo
Silk is bought cheaply at Dragon Cove and sold for much more at Bouffant Bay.
Medicine is bought cheaply at Bouffant Bay and sold for much more at Parrot Port or Golden Harbor.
Grain is bought cheaply at Golden Harbor and sold for much more at the Pirate Outpost.
Spice is bought cheaply at the Pirate Outpost and sold for much more at Dragon Cove.
(The "clockwise trading route" takes you to each island in order, and maximizes your profits.)
Sea Monsters and Pirates
Sinking a sea monster or pirate is a good way to get a LOT of doubloons, but it is risky unless you are very good at it. You will lose some of your doubloons if you are sunk. Generally, the sea creatures attack at the left side and above Dragon Cove. So once you have 2 or more cannon, you can hunt them there. Note that smaller ships will fill their cargo completely with just one creature. Beware of multiple attacks by creatures and ships.
Doubloons are collected by salvaging cargo from the sea, or by trading among the five trading islands. There are no shortcuts to obtain larger numbers of doubloons.
It is important to buy the next larger ship (at Dragon Cove) whenever you can afford it. Larger ships collect more salvage, move faster, and have more cannons for attack and defense.
(see related question)
Save up. As you get larger ships, 10,000 doubloons becomes pocket change.
Once you have at least 1500 doubloons, go to the Golden Harbor Bank and borrow another 1500, and buy your first ship, the Jimmy Rigger, at the shipbuilder at Dragon Cove. Once you have increased your cargo space and have a cannon, you can tangle with the sea creatures above Dragon Cove, which award considerable salvage when they sink, and even more when you have a bigger ship.
If you have trouble fighting creatures and pirates, carry cargoes in the clockwise circular trading route :
Silk from Dragon Cove to Bouffant Bay
Medicine from Bouffant Bay to Parrot Port
Grain from Golden Harbor to the Pirate Outpost
Spice from the Pirate Outpost to Dragon Cove
And don't forget to pay back your loans. Otherwise you can't buy larger ships.
Cross the covered bridge to the right of the Fort Ridley village, and go down to the river on the right side. Cross to the left and you will find the first doubloon.
There is one doubloon under the covered bridge, where the girl has made a wish with it. Go down to the river and cross right to left.
You can reach the doubloon from the right side of the bridge. Cross over, then go down to the river and cross to the left. The doubloon will be in the sand there.
The doubloon found under the bridge is used to buy the chicken feed in the general store.
Cross the bridge, then go down to the water. You can cross the river to the left, and you will find a doubloon there.
Cross the bridge, then go down to the water. You can cross the river to the left, and you will find a doubloon there.
The girl who made a wish with a doubloon is on the right side of the bridge. The doubloon is in the water under the right side. The woman whose husband is missing at sea is on the beach to the right of the governor's house (far right of the island).
Just right of main street at Fort Ridley. Go left under the far side to find a doubloon.
you cross the bridge on the right (after you go in the island) go down, and go left. get the doubloon. Sorry i can't describe it very much just go to Youtube put in Skullduggery Island cheats and you find it.
Apparently at the covered bridge over the river (or estuary). If you look under the right side of the bridge, you will find the doubloon the girl used.
You go east and you go on the hill go left. Go down the rocks go down then go left Then there's the doubloon.
There is a doubloon under the bridge -- the girl made a wish with it. Go down the right side and cross the river to the left.
Get the chicken feed by finding the doubloon under the bridge to the right of Main Street (cross over and then go left under the bridge to get it).
You find your first doubloon under the covered bridge. Go down the right side and cross to the left. Other doubloons are acquired in the sailing stage of the game. (see related question)