Hi everyone, its Friday which means its time for another development diary. This one is all about our underwater friends: The Submarines!The role of the submarine in Hearts of Iron IV is the same as it was historically: To hunt down enemy shipping and not have to challenge a superior enemy fleet directly to hurt them. Its a particularly attractive choice as Germany who will likely be facing Britain and the massive Royal Navy.Each nation can research 4 different submarine models, each more powerful and harder to catch.It is also possible to create variants of these using field experience where you can improve their torpedo attack, stealth or engine speed.Submarines, like the rest of your ships are affected by doctrines. Here is a screenshot showing the submarine focus branch in the Trade Interdiction doctrine.It improves the ability of submarines to detect other ships and their organization, but most importantly it improves their coordination while convoy raiding. Whenever a submarine in the fleet detects and enemy convoy this coordination value will control how quickly other submarines can gather and help in the attack (and also get away when cavalry arrives) so it is important for someone focusing on getting the most out of their submarine force.As mentioned earlier the goal of the submarine is primarily to attack your enemy's economy by preying on convoy routes. For example:- Attacking convoys along an enemy trade route will deny them foreign resources.- A nation split across the globe like Britain must convoy home resources to their capital area from many places in its empire to use them, making them vulnerable to submarine raiding.- When receiving lend lease, convoys carrying equipment can be sunk and will be lost forever.- When deploying troops on foreign theaters of war a nation must ferry supply and reinforcement equipment to them. By taking out these convoys their troops will be weaker and easier to defeat.- Troops being transported or during a planned invasion may be attacked leading to them being wiped out or severely weakened. You always want to be very careful and guard your transport routes before sending troops somewhere in war time.Each convoying route will have a visible efficiency shown in a tooltip indicating on average how much goes through due to attacks and other lack of convoys. Any convoy attacks are shown with indicators on the map that you can check out after battles with a report and will give you a hint on where the enemy is concentrating its raiders. Its advisable to put ships on convoy escort missions to help defend against submarines and other threats.Most ships are not capable of effective sub hunting, especially not capital ships so having destroyers and light cruisers with a fleet is important for defense and being able to deal with the submarines. Submarines may also be spotted by aircraft so keeping your air superiority in areas will help as well.In the naval battle above German submarines have managed to sink a large amount of convoys and are about to slip away into the night from the Royal Navy (who have arrived in time to save the last 5). Note that they have an indicator on them that tell you that the enemy does not know their exact location and can't attack them.This was the last diary before Christmas and we will be returning again after much needed rest with diaries on January 8th!