In CoW it never makes sense to research both cruisers and battleships. The new research tree didn't change much about this - the distinction between cruisers having better air defense while battleships being better in other aspects isn't enough to motivate players to put that enormous research effort into it. But in reality, all major fleets (Japan, USA, GB, France, Germany, Russia and Italy) in fact had both. While countries with smaller fleets restricted themselves to cruisers. The combination "only battleships and destroyers, no cruisers", which is the most frequent in CoW, didn't exist.
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My favourite solution to model this - if easy to implement:
* Cruisers level 6 and 7 can see submarines within their line of sight. This would simulate the development of better sensors for submarine detection during the 1940s. Of course cruisers were not the only ships that were equipped with sonar etc., but as much as I know the primary ones.
* So cruisers don't become too powerful, reduce their HP from 50 to 45 (not unrealistic compared to battleships - a battleship was not only taller, but also had stronger armour).
* Increase progression in research costs and duration for cruisers (i.e. reduce them for the lower levels and increase them for the high levels).
* Increase level 5 cruisers range from 50km to 60km (otherwise the gap between level 5 and level 6 cruisers would become too drastic).
* Since the above would nerf subs, increase attack and defense values of subs in the higher levels.
Result would be:
If you want high firepower on the sea, you have to research battleships (cruisers would be a bit less useful for that, then).
If you want to secure your battleships from both naval bombers and subs, you also have to research cruisers.
And if you can content yourself with a fleet without big firepower but safe from subs and air, going for cruisers and not having battleships at all would be the option.
All in all this would open more strategic options that might make sense depending on the situation - would make the game more interesting and realistic.
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Simpler but only second best solution:
* New dependency in the research tree: To be able to research battleship level 4, you must first research cruisers level 3.
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My favourite solution to model this - if easy to implement:
* Cruisers level 6 and 7 can see submarines within their line of sight. This would simulate the development of better sensors for submarine detection during the 1940s. Of course cruisers were not the only ships that were equipped with sonar etc., but as much as I know the primary ones.
* So cruisers don't become too powerful, reduce their HP from 50 to 45 (not unrealistic compared to battleships - a battleship was not only taller, but also had stronger armour).
* Increase progression in research costs and duration for cruisers (i.e. reduce them for the lower levels and increase them for the high levels).
* Increase level 5 cruisers range from 50km to 60km (otherwise the gap between level 5 and level 6 cruisers would become too drastic).
* Since the above would nerf subs, increase attack and defense values of subs in the higher levels.
Result would be:
If you want high firepower on the sea, you have to research battleships (cruisers would be a bit less useful for that, then).
If you want to secure your battleships from both naval bombers and subs, you also have to research cruisers.
And if you can content yourself with a fleet without big firepower but safe from subs and air, going for cruisers and not having battleships at all would be the option.
All in all this would open more strategic options that might make sense depending on the situation - would make the game more interesting and realistic.
----------------------------------
Simpler but only second best solution:
* New dependency in the research tree: To be able to research battleship level 4, you must first research cruisers level 3.
