Each nation has a GDP (Gross Domestic Product). The GDP grow each turn based on the power of your industry. You can support your GDP with:

• Keep peace. During peace times the GDP grows "normal". If you are in war with another nation[-s] your GDP grows only a half. Therefore sometimes I spend my monthly naval budget for peace at the political questions. The decission of plus or minus 30% political status.
  
• Sometimes you get some political questions. Choose every time the option "+1% GDP" (or something like this).
  
• If you want to be involved in a war do it with the wealthys nation. Their GDP growing will also limited to a half, so the effect affects your opponent too and you don't loose to much GDP in comparison to the wealthys nation.
  
• Try to raise your transport ships to 200% (max). Each % above 100% will also increase your GDP grow. Please note, each % below 100% will decrease your GDP grow.
  
• It was also mentioned that the number of controlled provinces is also a factor of growing your GDP.
  
• Hint: If possible try to start a war during a time with low GDP growing, then the effect of growing rate is reduced to 50% hasn't so much effect. half of 2% grow versus half of 10% grow. Because all nations in peace will get the normal grow rate.

Note: I reached with this politic:

The growing of your port capacity is related to the growing of your GDP.
Therefore follow my hints for GDP you will get also very big ports.

Note: At beginning of my new campain in 1890 I found two ports with more or less equal capacity. Now after 20 years and good growing GDP (see above) my port has 20 thousand tons more capacity than the port from the opponent.

What is the best gun of your ship? I cannot give you the only ONE answer.
I can give you only the advice: compare the different guns.
For example: You want a medium gun like 5'' or 6''.
Compare the technical parameter like:

• Accuracy
  
• Reloading time -> Cadence / How many shoots per minute
  
• Range / Which armor can be destroyed in distance x
  
• Weight
  
• State of research, e.g. a gun of type Mark III will be every time better like a Mark I.

I compare in the 1st step the accuracy and reloading time. The 2nd step is the weight.
What does it mean?
For my example: 5'' or 6''?

• Place one 5'' gun
  
• Place one 6'' gun
  
• Place a second 5'' gun and raise the caliber to 5.9''
  
• Now compare and decide.

Sometimes a 5.9'' (or any other x.9'') is much more better like the 6.0''. But it can also be happen, the 5.9'' weight is 30% above the 6'' gun. Than you have decide again.
Sometimes you cannot raise the gun to x.9'' then you have to choose the biggest possible value.

And last but not least: You must fit your weight limit of the ship.

What I like to choose (my experience till 1910):

• main artillery: take "the best" big available gun. I prefer a 9'' Mark 2 more like a 12'' Mark 1, because the accuracy and reloading time are better. And I raise the length to +20% = more accuracy, but longer reloading time. I prefer accuracy.
  And only 1 main caliber. I read anywhere: If you use less caliber your accuracy will increase, because the "spotters" (=the AI) can separate the miss hits and correct your shooting.
  
• second artillery: I prefer "more number of guns" +10% length for accuracy. If I have to decide one 5'' or two 2'' guns, I take the 2''. Therefore a lot of my ships got the 2.9'' gun. I don't know what comes after 1910, but for the start this gun is a "must have". Later in the game (Dreadnought) my ships got 2 types of deck gun second artillery (+1 casemate = 3 different calibers, or maybe only 2 different, if the casemate got the same caliber like one of my deck guns).
  Note: Later on in the campain the 2'' gun will be useless due to of the increased battle distance. Than choose maybe the next caliber 3.9'' or 4.9''. Like I wrote: Compare the parameter and what do you want to do with your ship. And whatch your battles (battle logs of the ships): Did you destroy all enemies at 10.000m or only at 500m - which gun got the most hits? (You and opponent). If you want to go into an infight than maybe the 2.9'' will be a good choice again.
  
• do not underestimate the second artillery (or small guns). A 2.9'' gun with one shoot each 7 seconds and 50% hit rate at 1.000m (Mark 3) can also ignite a fire at a battleship like a 13'' gun with one shot each 120 seconds and 5% hit rate (Mark 1). And I need the fires at the enemy ship. The damage repair troop (AI) must be busy all time! Otherwise each flooting is repaired after some seconds. Flootings are very good each time, they reduce the speed of the enemy. But fires keep the repair troops busy and destroy also the ship.
  In my current campain I got a bad design of battleship at the beginning from AI. The battleship had 3 big guns and only 3 small guns. My advice: If you are unsure, take a look to the last warship designs from II. World War for a good relation main guns versus second guns.