This is a very simple glitch for some items. although it's a glitch and sometimes actually uses your resources for the dupe. Best used with no/little cash. Warning, most items do not work but after short tests there are some. In order to lock the duped items into existence you must leave them on the ship, close the app, shut it down, then restart for best performance. The bug may or may not delete something. This has not been confirmed as I can't find anything missing after numerous tests but something feels like it's no longer there. To preform this glitch you must have one of the item, and a spot for TWO of the item. Place the first item down. Delete it and place it in a spot where it WILL NOT intersect the other one that may be duped in. Hit undo once otherwise you may lose the dupe and a part of your ship. Leave the dupe if it spawns and the original there and close out of the game and turn it off. Restart the app and have fun with your two of whatever item you duped.
Fast Cash
Using the Submarine battle (Under Siege). Grab one or more SH-60K helicopters and send them to any ship other than Myoko. Bombard Myoko untill her defeat and wait for the SH-60K to finish then repeat. With a proper fleet this can be completed at 2min per victory.
Grinding for Beginners
First, take all the weapons and boilers of the initial battleship. Then, buy the cruiser body and put the boilers and weapons on it. This way you can easily farm destroyer levels, maybe even dual destroyers. You can do this with the battleship body for greater results. Even without armour, the bigger your ship is, the stronger it is, and since hull parts are free you can easily make a strong ship by making it with as many hull parts as possible.