To earn doubloons, sail around and collect floating debris. Invest the treasures at different trading posts for more doubloons. Collect treasure from defeating ships and monsters if you have a ship with cannons. You can buy a ship in Dragon Cove (north-western island).
* Once you have cannons, a way to get lots of doubloons is by sinking sea creatures (around Dragon Cove) and pirates (between the Pirate Outpost and Golden Harbor). When they sink, they release a large amount of salvage. Sea creatures are generally easier to escape because they are slower and do not shoot back. But this takes practice, and groups of sea creatures or pirates may attack at the same time, and sometimes a mix of these.
You can get doubloons by sinking other ships, monsters and by selling cargo.
keep trading
the cannoneer is 20,000 doubloons.
you have to get doubloons by trading grain silk medicine and spice (go around all of the islands on skullduggery)
Earn 500,000 doubloons then get a loan at Golden Harbor at the bank .Or you can as many as Grains as you can then sell it at Pirate Outpost until you get 1,000,000 doubloons.
it costs 20,000 doubloons
It takes approximately 25-100 "days." Everytime you land is counted as a "day" in Skullduggery Island.
All you do there is to hire the Cargo Master, when you can afford him (salary 15,000 doubloons).
To get a large number of doubloons, sail around Dragon Cove (where you can sink sea monsters), or above Golden Harbor (where you will fight pirates). The accumulation of thousands of doubloons is a slow process, until you have large ships that can hold a lot of cargo.
"Cash" on Skullduggery Island is your doubloon count. You earn doubloons by salvage or by trading, and you can borrow some from the Golden Harbor bank. When you buy cargo, you spend your cash, and when you sell it, you make a profit. If your ship is sunk, you lose aprt of your doubloons and part of your cargo. You need doubloons to buy larger ships and hire skilled crewmen. (see related question)
You get a larger ship by having a lot of doubloons and then go to dragon cove and buy and new ship. Hint the cheapest ship is 3000 doubloons and the most expensive is more than 30000 doubloons.
When you have enough doubloons, from salvage or trade, you can buy a larger ship at the Shipbuilder on Dragon Cove.