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Dev Corner | Hydrodynamics

Briefing: Hydrodynamics
Written by: @Zwirbaum


Hello everyone!

Another week is upon us which means it is the time for another dev corner. Last week Thomas talked about what we are cooking with the Factions, while I will be talking about naval and naval-related changes. Even the most beautiful placeholder art will be gone eventually like tears in the rain. So strap in, and prepare for the deluge of the information that will be coming your way. Also, keep in mind that everything discussed here is in a relatively early stage, and as such is subject to change.

It is no secret that one of the most common sentiments across the Hearts of Iron IV player base is that the Navy seems to be rather hard to understand. Some elements are almost instantaneous in the effect (Supremacy), others take a long time (building the Navy) and some elements remain relatively hidden until the actual shooting starts (Supremacy Value of the Ships, Screening in Taskforces etc.). On top of that we are also having a fairly complex system of Naval Missions - where they work best when using them together, synergistically. However missions could be explained a bit better, and sometimes what is best to achieve your goal could be somewhat counterintuitive. (Giant Strike Force of Doom, sitting idle in the port somewhat projecting supremacy across the entire oceans without ever sailing out as one example). So how are we planning to address it?

Core Concept
Similarly to how last week we talked about high-level concepts for the Factions, I will try to do the same for the Naval Systems, but before that I will also list some of our goals that were the basis for what we are working on:

  • Updating and Reshaping Naval Gameplay, making it more strategic, giving you the opportunity for the counter-play if needed; a bit more predictable, and less ‘flip-floppy’
  • Updating Naval Missions so that they become more intuitive, with a much clearer purpose and use case
  • Encourage a more active use of fleets
  • Update and Communicate better to the Player some of the intricacies of the Naval Systems
  • Increasing the Importance of the Islands Control (in the Pacific) and Naval Logistics
  • Updating Carriers and address the interactions between land-based aircraft and naval taskforces

Update to the Naval Gameplay

Naval Dominance
First of all, and the most important of the changes is that we are introducing the concept of Naval Dominance. Naval Dominance is a sort of umbrella term for a couple of things. Similar to how ships had Supremacy Value, now they have Naval Dominance Value, which will be displayed on the Ship Card.

dc_hydrodynamics_001_marked.png

Mutsu has 509 Naval Dominance Value. We are also changing the old calculation, that was based mostly on Production Cost and Manpower, to have more things affecting the calculation, like Speed and Range, so for those who want to build Fast Battleships, increased dominance value may be the reward…

Next, we will want to talk about Naval Dominance - which is our way of indicating naval control of sea zones. Each Sea Zone, depending on the terrain type, has a certain threshold of dominance points you need to have before you can claim you ‘control’ it. And if you are at war, then similarly to the older system, you are also taking into account enemies' Dominance Value and the ratios between you and them. Also the ratio needed for ‘control’ now has been adjusted to require 66% instead of 50%+1.

Having control, or as we call it now, establishing Naval Dominance in a Sea Zone, provides you with certain advantages and bonuses. For instance, as you can see in the screenshot below - potentially reducing the amount of convoys needed for Trade and Supplies by up to 25% if you have secured the entire shipping route.
There are other benefits that I will not fully reveal yet, but amongst other things, there will be something to help you secure islands and potential naval invasion targets.

dc_hydrodynamics_002.png

In this example we can see that in order to claim ‘control’ over the Deep Oceans sea zone, you would need to accumulate at least 1000 points worth of Dominance, assuming nobody would contest you.

Dominance Gain
dc_hydrodynamics_003.png

This tooltip shows the information about the current amount of dominance accumulated in this sea zone, how long it will take to establish its full value, things that impact it, like airbases located on the islands in the seazone etc.

Dominance as opposed to the previous supremacy system now takes some time to establish, but it also doesn’t simply instantly disappear when ships engage in combat, or go to repair after a battle.

Naval Mission Updates
We will also be making the following changes to Naval Missions. We will divide current missions into 2 groups; Core Missions and Auxiliary Missions.

Core Missions - (PATROL, CONVOY RAIDING, CONVOY ESCORT, STRIKE FORCE)

Those missions are your primary way to interact with naval dominance. Each mission type will interact a bit differently. As it is right now, Patrol will be serving for Building Up Dominance, Convoy Raiding reducing Enemy Dominance, Convoy Escorts will provide a ‘protected’ value, which means enemy raiding won’t be able to reduce your dominance below that value, and Strike Force serving as a ‘Synergy Tool’ - and amplifying other missions. Hopefully this will provide a clear and relatively intuitive system on how to use the Naval Missions.

Auxiliary Missions - (NAVAL EXERCISE, MINELAYING, MINESWEEPING, NAVAL INVASION SUPPORT)

Those missions do not interact directly with naval dominance, however, they do benefit from it, like for example, being able to minelay or minesweep faster and more efficiently when operating within a region where you have established control and have naval dominance.

Naval Home Bases, Range & Supply
dc_hydrodynamics_004.png

This Dutch Fleet has set the port in Batavia to be their Home Base.

We are reintroducing the Home Base system for the Fleets. Each Fleet needs to have a Home Base. Any Naval Base that you have access to (Your own, Subject or Faction Members, or if you have secured Docking Rights) can be selected as a Home Base. So the question is; what does the Home Base do?

Naval Range
One of the changes that we are doing is that the ship's range is now projected from the Home Base instead of all Naval Bases.

dc_hydrodynamics_005.png

dc_hydrodynamics_006.png

As you can see depending on where Home Base is located, the range, and access to do the Naval Missions is quite different. A fleet with Königsberg set as Home Base does not have the range to do the missions in Norwegian Sea or Western Approaches Sea Zone.

Naval Supply
Previously, naval units would always draw the supplies from the Naval Bases closest to where the taskforces were operating, now - they will be drawing the supply from their selected Home Base.

dc_hydrodynamics_007.png

This fleet has a Home Base set in Honolulu - and is operating in the Micronesian Gap. Despite the port in Johnston Atoll being closer it draws the supply from Hawaii Naval Base Supply.

State Building Limit - Islands
In Götterdämmerung we introduced terrain-based limits for province-based buildings like Forts and Coastal Forts, so that you couldn’t build the Maginot Line everywhere. In a similar spirit, we will be introducing state-based building limits for the buildings. In this case we are now focusing on putting limits on the various Island categories, so that not every single tiniest of islands can have an airbase capable of storing and launching for missions 2000 planes every day. Right now those caps are based on the Island state categories (Tiny Island, Small Island, Large Island), and upon one concept we will talk about in the future.

dc_hydrodynamics_008.png

Marcus Island can now have at most a level two airbase and level three naval base. Those limits as all the numbers, stats and values are of course subject to change. Also there is totally nothing hidden under that Hearts of Iron IV logo.

Short Comment
Initially when I started writing this section, I was going to write how I envision things mentioned so far will change the naval gameplay, and how X will impact Y, however I think I am more interested in hearing what you, my dear readers, are thinking and your opinion on what you have read today.

Naval Invasions
We are doing some touch-ups to the naval invasions as well. In the current live version of the game, there is a global naval invasion capacity set by your technologies, doctrines and other modifiers, and then depending on how many divisions you assigned to the invasion, it would take a certain amount of time to plan that naval invasion. This system unfortunately had one issue, that in order to be ‘optimal’, it encouraged to spam 1-division naval invasions, as that technically allowed you to have a massive naval invasion planned just within a few days, at the small cost of carpal tunnel syndrome.

In the new system, there will be, depending on your technologies, doctrines etc. a certain amount of naval invasions you can plan at the same time, each being able to have a certain amount of divisions, and no matter what, always taking a specific amount of time to plan.

Also, for a country that hasn’t researched even the basic Transport technology, there will still be a possibility to launch a very limited naval invasion under the new system.

Appeal to my Lizard Brain
And last but not least, I’m going to tell you about one more thing - and that is that we are adding visual representation of control over the seas, visible on default map mode, which during a conflict should represent a gradual shift of control over the zones, giving the feeling of ‘naval frontlines’. Also this can serve as a kind of warning, that when your coastline sea zones start displaying your potential enemy colours.

dc_hydrodynamics_009.png

This is the current prototype of showing on the default map mode who has naval dominance. In this case Japan has the most dominance, and nobody is effectively contesting it, thus Japanese colours are displayed on the map.


Wrapping Up
So, to wrap things up, this is just a number of things we are doing for the Naval. I have not touched upon anything Carrier related, new equipment or new tools yet, or any UX/UI updates. I will return in due time to provide you with more information on all the things that are not-dry, in the meantime - here is a teaser of a thing that we may talk about in the future, with this beautiful placeholder art done by myself.

dc_hydrodynamics_010.png

Who will guess what this is?

This is my first dev corner, so I can only hope my writing is not too stiff. In time I hope it will get better.

Anyways, thanks for reading and until next time, farewell!

/Zwirbaum





Also, we have a survey for you to fill out when/if you have time regarding Naval Gameplay. Just keep in mind that this forum thread is for your feedback about the Developer Corner. If you have feedback about this specific survey we welcome your thoughts in a separate forum post, or in the HOI Discord!

EDIT 25/06/25 - Thank you to all participants for the Player Survey, this survey is now closed!
 
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You would need to set Hong-Kong as the new home base before sending the fleet there, if they have no range to operate out there, but switching Home Base can be done right now with CTRL-Right Click if my memory serves well, so it is a relatively painless 'operation'.
CTRL+Right Click does sound painless, but will this be apparent even to newer players? Maybe I'm missing the point of this change, but I can't yet see how all this Home Base stuff will actually enrich naval gameplay... Especially if this update aims to encourage more interesting naval operations with revamped missions and multiple fleets, then having to do (and even remembering to do!) Home Base setup all the time sounds a bit excessive and pointless.

Not to mention that key things like Supply and Range now seem to be tied to this Home Base stuff. It honestly feels like "change for the sake of change" - because the idea and game mechanics for logistics behid Naval Supply and Range don't seem to change... they're just linked to an artificial Home Base idea which feels kinda shoehorned in and softlocked behind an additional click.

Don't wanna sound too sceptical about this update (especially since I'm aware that this is just a basic concept intro, subject to change, etc.), but "what's the actual point?" is honestly my initial reaction about a lot of this - I mainly hope that Supremacy to Dominance isn't just some hollow rebranding! I'm still eager to hear more about the team's ideas in future Dev Corners, but none of the concept stuff outlined in this article honestly has any "wow" effect on me personally yet...

I'm hoping to be proven wrong though! ;)

EDIT: This comment instead perfectly shows the gaps in the current system, and I fear that the concept ideas in this Dev Corner article don't really go in that direction
 
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Naval guns did require seperate design studies and basically never shared caliber with land guns.
Pretty much the one exception is railroad artillery, which was often actually naval artillery attached to a train car.

Which does raise one of my annoyances: HOI4 treats a single 12" naval rifle on a flatbed car as more effective than a dozen of the exact same guns on a warship.
 
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Will there be techs to increase the naval base caps on small islands? Truk & Ulithi were both massive bases on atolls. Floating drydocks and dedicated repair ships enabled serious repairs near the frontline.

Also, how will the naval supply system interact with the forward replenishing project? Right now it's horrifically expensive and doesn't really do much of use. Will there be a comparable way to extend the range of submarines, say with something resembling the Type XIV?
U.S Navy Ice Cream Ships were a great morale booster during the war. Perhaps their use could cause a small org buff to the invading marines.
 
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Please expand the time it takes for naval exercises to reflect the time it took for the actual naval proving trials before the ships were commissioned. Also, base your build time on historical launch dates and not commission dates as is in game currently. For instance, the US had 7 of the 9 Brooklyn Class light Cruisers laid down prior to 1 Jan 1936. You don't include any of them as being partially built at that start date. Also, the dates you have for the launch of the 13 Farragut class destroyers (actually four of the Porter class and 9 of the Mahan class) are their historical commission dates. They were already undergoing proving trials (naval exercises) on 1 Jan, 1936.
 
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Something for he UI: when assigning convoy raiding show the trade routes of all your enemies with an associated flag. In the current version i find my self changing between intel screen and navy screen just to find tge trade routes.
 
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Really love the planned changes here.

Just one thing. I am not sure if i want to see this in the "default map mode".

dc_hydrodynamics_009.png



Maybe better in a navy map mode? Even a new one.


Oh, and btw, as you change much in the naval part of the game. Please also change the sorting of the "list of sunk ships", you can see at every own ship. The las sunk ships are always way down in the list, and you have to scroll to find the last kills. Why dont you make the list reverse, and show th latest killed ships at the top of the list?

And a last thing:
So often i oversee it when the enemy invades my coasts. Cant there be the option to get a message Window like "You have been seccessfully invaded", with a link to the province lost? This warning messages on the right side of the screen that warns you that the enemy tries to invade soon, dont help much. I oversee them reguarly, especially as the warning is so much time before, and the Invasion maybe never starts to happen.
 
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Really love the planned changes here.

Just one thing. I am not sure if i want to see this in the "default map mode".

dc_hydrodynamics_009.png



Maybe better in a navy map mode? Even a new one.


Oh, and btw, as you change much in the naval part of the game. Please also change the sorting of the "list of sunk ships", you can see at every own ship. The las sunk ships are always way down in the list, and you have to scroll to find the last kills. Why dont you make the list reverse, and show th latest killed ships at the top of the list?

And a last thing:
So often i oversee it when the enemy invades my coasts. Cant there be the option to get a message Window like "You have been seccessfully invaded", with a link to the province lost? This warning messages on the right side of the screen that warns you that the enemy tries to invade soon, dont help much. I oversee them reguarly, especially as the warning is so much time before, and the Invasion maybe never starts to happen.
I don't mind it. Maybe just a toggle.
 
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By removing two important exploits, waiting for England to momentarily lose supremacy, and being able to make 7 day invasions, as well as adding home bases, this will effectively make players like me or Hollow Being even less interested in the navy. The rest of us who do not care for the amount of micro needed, these exploits were a godsend because we can just learn these exploits and focus on game mechanics that we actually enjoy.
Sort of like how many people take a shortcut from understanding how division building works by just taking whatever is meta at the moment. And since we can use presets across games, that is a sharp departure from the recent efforts to ensuring quality of life for those who don't care about the system and those who do. (side note: playing the Soviet Union and Hungary who have limited need for naval invasions has never been more attractive.)

Alternatively, you can go the eu5 route and (please please please) allow us to automate it. Sure, I can play optimally if I want to, but I can also just let the Ai handle it if I don't care too much. Actually, feel free to make everything harder but please allow us to automate some of it. I am willing to do even more micro for research and politics at the cost of having to do less micro with navy and air.
 
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Will invasions still require an arbitrary Supremacy Dominance threshold to proceed? I always felt annoyed being blocked from executing invasions I knew for a fact would be actually uncontested. Much like the mentioned problem of strike groups influencing supremacy without ever sortieing: if it's so dangerous, the enemy can prove it by sinking my ships!

I'm honestly wondering if we need invasion planning time at all. Paradrops don't have it, land operations don't have it either and can accumulate full planning bonus long before the player decides where they actually want to conduct an attack.
 
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I await your next corner before I really sink into you. The navy is hoi iv's worst area. I can defeat any navy with just spamming CLs, DDs and SSs. Lack of Carrier dominance in the pacific and the op of battleships exist in game, although both can be easily defeated with torpedoes on CLs, DDs and SSs in game. Not in real life. Want to get it right? Fix Airpower. Fix torpedoes. Fix range engagement rules. And while you are at it, teleporting aircraft instead of using convoys and carriers to move aircraft from island to island would also help balance the game.
 
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After the navy system was reworked a while after launch, I found myself just not using the navy system because it required so much work and upkeep and investment, all for things that could be annihilated in a few in game weeks if you weren't paying attention or the other side had too many destroyers or whatever. I'm optimistic that this will make it less of a slog
 
Naval Home Bases, Range & Supply
dc_hydrodynamics_004.png

This Dutch Fleet has set the port in Batavia to be their Home Base.
We are reintroducing the Home Base system for the Fleets. Each Fleet needs to have a Home Base. Any Naval Base that you have access to (Your own, Subject or Faction Members, or if you have secured Docking Rights) can be selected as a Home Base. So the question is; what does the Home Base do?

Naval Range
One of the changes that we are doing is that the ship's range is now projected from the Home Base instead of all Naval Bases
I don't care who you send, I'm not using the "strike fleet" mission. When I've tried it, my capitals are too late to make a meaningful contribution, when the AI does it, their capitals are too late to make a meaningful contribution if there's even a fight left to contribute to after my cruisers have mulched their destroyers, and generally I get far better results by just making fast capitals and having them act as part of the patrol.

I think I get that the hope is that players will build more destroyers and forwards deploy them at bases that can support them while leaving the capitals at home to strike fleet, but what I foresee is even harsher neglect of destroyers in favor of submarines, cruisers, and capitals, which can operate far from good ports anyways because actually using those forwards deployed destroyers as intended will require significantly more micromanagement.

I really think the home base thing is a terrible idea that will only make your players want even less to do with navy because you're adding another layer of unpleasant and obtuse micromanagement and essentially bringing one of the worst parts of air gameplay to the sea.
 
Please expand the time it takes for naval exercises to reflect the time it took for the actual naval proving trials before the ships were commissioned. Also, base your build time on historical launch dates and not commission dates as is in game currently. For instance, the US had 7 of the 9 Brooklyn Class light Cruisers laid down prior to 1 Jan 1936. You don't include any of them as being partially built at that start date. Also, the dates you have for the launch of the 13 Farragut class destroyers (actually four of the Porter class and 9 of the Mahan class) are their historical commission dates. They were already undergoing proving trials (naval exercises) on 1 Jan, 1936.
Whilst I'm all for making ship production times more practical, the launch date is technically not the completion date, just when the hull is deemed watertight enough to leave the drydock/slipway to clear space. Look at Graf Zeppelin for example, the was launched in 1938 but the hull was never fully completed. The partially finished ship is just moved to a regular dock with heavy cranes and equipment to do the rest after launch.
 
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By removing two important exploits, waiting for England to momentarily lose supremacy, and being able to make 7 day invasions, as well as adding home bases, this will effectively make players like me or Hollow Being even less interested in the navy. The rest of us who do not care for the amount of micro needed, these exploits were a godsend because we can just learn these exploits and focus on game mechanics that we actually enjoy.
Sort of like how many people take a shortcut from understanding how division building works by just taking whatever is meta at the moment. And since we can use presets across games, that is a sharp departure from the recent efforts to ensuring quality of life for those who don't care about the system and those who do. (side note: playing the Soviet Union and Hungary who have limited need for naval invasions has never been more attractive.)

Alternatively, you can go the eu5 route and (please please please) allow us to automate it. Sure, I can play optimally if I want to, but I can also just let the Ai handle it if I don't care too much. Actually, feel free to make everything harder but please allow us to automate some of it. I am willing to do even more micro for research and politics at the cost of having to do less micro with navy and air.
The bigger invasions are less micro. There just need to be alerts on the invader side for when the invasion is about to begin, and when it starts, so players feel free to focus on other things. A top bar notification and a unique alert sound would also be useful.

It also sounds like you aren't interested in the navy at all, and are just finding ways to bypass it.

Will invasions still require an arbitrary Supremacy Dominance threshold to proceed? I always felt annoyed being blocked from executing invasions I knew for a fact would be actually uncontested.
The problem is task forces on the strike force mission don't attack unescorted invasions. You'd need a much larger escort if the enemy could attack your invasion flotilla. They are changing the naval supremacy from forces in port, so they do have to go out and fight more to affect naval dominance.
 
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Is there any plan to do anything with naval theaters beyond their current scope? For example, when I play the UK, I usually create a separate theater for the Pacific, while having most of my navy in the Atlantic and Mediterranean. Would there be multiple "home bases" for the different theaters, where they can only draw supply from the theater they are in? I imagine this could potentially have room for cheese to get a larger supply zone, but perhaps it could have a cooldown between changing a home base, I am not sure.

Also, could you guys take a look at how ships in reserve are assigned to a task force that has automatic reinforcement? There seem to be some bugs around ship icons that represent needs for automatic reinforcement (the same ones as the icons you can apply to a ship when you build it, like the binoculars, plane, shield, etc.). When using automatic reinforcement, especially with cruisers I have noticed, using the Star symbol (indicating a ship, say a destroyer, that has any type of icon can reinforce the fleet until it has reached the amount of that type that you want, say 0/10 ships... etc.) causes some issues sometimes with reinforcements, and I have noticed fleets glitching their composition by trying to bring in another ship, but it immediately goes back into the reserve fleet on the next game tick, and so forth. Often, it is hard to get the ships correctly labeled and brought into the fleet, because of bugs like this, as if the automatic reinforcement request is not recognizing the ships as valid.

I also imagine you are looking into naval combat engagement rules, but as you may know, there can be a bit of an issue with the current rules. For example, having a strong strike fleet that you want to absolutely pummel the enemy with can be detrimental to your fuel supply if you set them to always engage, as they might go targeting a single submarine that is raiding convoys, which is certainly inefficient. It would be nice to have a setting that is like, "only approach combats that are worth our time/suited to our specialty/justify me burning gas to go get their ass" and similarly for ships that you want to do those small things. But it sounds like some of that is hopefully going to be redone with the ship missions already.

Overall, as a navy enjoyer, I would like to say THANK YOU for putting time into reworking this lovely part of the game, and the changes look exciting!

(edit: typo)
 
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How will the launching of naval invasions tie together with the new dominance system? Will you require dominance to invade a location? Will you require dominance along the entire path of the invasion? Will this be an end to extremely long distance naval invasions from Japan into Italy in 1936?
 
I think the colored sea zones will be a nice touch.
 
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I again bring up the need for a general fight everything naval mission. Don't force me to choose between fighting convoys or enemy fleets.

As for the currently OP subs, I think the issue is that not only are they too powerful in terms of stats and combat, subs also also the cheapest/quickest method of building a powerful navy from scratch in terms of production and research. For minor nations, subs are the only way to compete with the established majors.

Paradox needs to decide what should be possible for minor nations in terms of navy. I was able to single handedly defeat the allied navies with subs as Axis Yugoslavia and defeat Japan as New Zealand.

With a 1936 start, should a minor nation be able to build a powerful enough navy to 1v1 Britain by 1943?
If yes, how?
Currently, OP subs are the only way.
 
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