In which game do you want to add the new ship?
PoTC has its ships defined in "PROGRAM\Ships\Ships_init.c". So do AoP and CoAS, though the format of the file is different.
In PoTC, ship descriptions are found in "RESOURCE\INI\TEXTS\ENGLISH\ShipModels_descriptions.txt". Translated versions can be found in "RESOURCE\INI\TEXTS\SPANISH" and "RESOURCE\INI\TEXTS\RUSSIAN". If you choose to put your descriptions into the version in subfolder "SPANISH", please also translate them into English and put them into the "ENGLISH" version. The Russian translators can then translate it.
Again in PoTC, edit "PROGRAM\InternalSettings.h". Go to the bottom and change ENABLE_CHEATMODE to 1.
In "Ships_init.c" are several lines which control the periods in which this ship can appear. For example, for "FastGalleon1":
Code:
refShip.period.0 = 1.0; //
refShip.period.1 = 0.9; //
refShip.period.2 = 0.4; //
refShip.period.3 = 0.0; //
refShip.period.4 = 0.0; //
refShip.period.5 = 0.0; //
This limits the ship to appearing in "Early Explorers", "Spanish Main" and "Golden Age of Piracy" periods.
Start a new game and choose the "FreePlay" storyline. You can change the start date, so make sure it is during the time that your ship is allowed to appear. Then click on the ship icon to choose your ship. With Cheatmode enabled, you can pick any ship which is valid for your chosen date. (Without Cheatmode, you're limited to ships of tier 5 or smaller.) Assuming you have added a valid entry for your ship in "Ships_init.c", you should be able to pick your ship. It will have a blank icon instead of a small picture of the ship; we'll deal with that later.
Start the game. If it crashes when you leave the deck and go to the port, there's probably something wrong with the model. Check "system.log". Otherwise, you should then be able to board your ship and go to sea. Go to worldmap and sail to open sea, somewhere well away from any island so that no land icon appears on the worldmap. Return to sailing mode. Your ship should be the only thing on the sea. Save game. Now, if you change the model or texture, you can load that save and it should show the changes. If anything goes wrong, it has to be your ship because there's nothing else there - again, check "system.log". Even if the game doesn't crash or the ship doesn't look strange, check "system.log" anyway and look for "Bad rope" messages.
Once the ship is finished, I'll tell you how to make an icon for it. (Or you can upload all your files, then I'll make the icon.)
Meanwhile, tell me what you've put in the 'refShip.BigPicTexName' line in your ship's entry in "Ships_init.c", and which ship it was cloned from. Just to have something other than a blank icon, it should be possible to give your ship the same icon as the original.