Navy is important part of your nation in Victoria 3 if you want be major power. Good navy you will use when you perform naval invasions or you could use them to protect your ships in case you are “attacked” by a more powerful state and not lose the convoys that carry your valuable resources.
Because in Victoria 3 works following formula 1 naval base = 1 flotilla. So you need to build naval bases to get more flotillas. These naval bases you must build on seaside provinces and is good to build them slowly with shipyards along because naval bases consume their outputs so you don’t overpay.
Is good to watch your prices of inputs for naval bases to be sure you don’t overpay much. In order not to pay a lot so really proceed more slowly. It will be better than having problems with the price and losing money unnecessarily.
Building naval bases also increase “maximum declared interests” so you will probably build them with purpose of increasing the declared intrests. Having a lot of naval bases increase your military wages so don’t build much of them. So build only as many as you need to have a strong navy. There’s no point in being too strong. It’ll cost you a lot.
Which ships is good to build?
Naval bases also have production methods in which you could change performance of your ships. Is good to increase this production methods by time to have stronger navy. But for me isn’t important to focus on increasing navy power in early phases. More important for you will be research on economy or society.
Ship class increase “maximum declared interests” and stats of your navy flotille. Basically you have 0,5 per level but with increasing this production method you could have more. Second production method called convoy escorts could helps with morale loss. And the last production method are convoy raiders which increase convoy raiders efficiency.
To have strong navy you must increase ship class as much as you need. Also you need to upgrade convoy escorts to optimize morale loss and if you need to fight with someone which is more stronger than you then you need to optimize convoy raiders.
Last thing when you building navy are admirals. This admirals you must recruit and assign them to group of naval bases. Admirals also have own traits/stats which affects stats of fleets which they are responsible. So it’s good to choose carefully so that you do not hire an admiral who will then lose everything or worsen the performance of your ships.
First steps with navy could be 20 fleets which allows you basic invasions and basic raids against countries that are weak. But if you are important naval power so definitely not stay at 20 fleets.