[2021] I play co op mostly...

Grats on the Yugumo! She's a nice ship~

So, economics in this game can be a little weird. I believe I should explain a few things to try and help out.

Firstly! Service Costs!
Higher Tiers have higher service costs. In a way, this helps to de-incentivize playing only the highest tier over and over and over. That being said, some power-users buy the Permacamos and just play high tier all the time, but it's part of the design concept.

Also worth knowing, higher tier Premiums don't have as much Service Cost pressure, which also having extra credit earnings. Tier 10 "Special Ships" are kinda like Premiums, but have no bonus credit earning. Instead they just have super low Service Costs (so that's where the extra credits really come from)

Secondly! Bonus Money!
Most people are not excited when they are put up against higher tier ships. The good news is that higher tier ships still eat torpedoes just fine, and they get lit on fire just fine too. You might have trouble Pen'ing them with AP sometimes, but it's not as big an issue as folks might think.

Why do I say this? Your Tier 9 Yugumo will actually get LESS credits/xp for attacking Tier 7 and 8 ships, compared to attacking a Tier 9. Only Tier 10's will give bonus credits when you do damage to them.

Lastly! Co-Op!
Worth knowing that Co-Op has reduced rewards. Why? Because it's a very basic mode where bots charge forward so you can maul them~ It's fun, but it's also very easy, so the rewards are reduced because of that.

There is a special Service Cost reduction in Co-Op. It chops 25% off the basic amount! However, higher tier ships like your Yugumo and whichever Tier 10 you might get someday will definitely feel the pressure, even with that 25% being reduced.

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So, how can we make it easier to grind credits? Better yet, how is it easier to do it for FREE?

1) Play middle-ish tier ships.
Tiers 7-8 are the sweet spot if you're using Tech Tree ships, especially if you don't have permacamos.

2) Get Premiums from doing in-game activities!
Every month or two there's another free premium. Novorossiysk just happened, and Repulse is available for in-game play alone as well. More will happen in the future, so you'll get some free Premiums along the way.

3) Try Operations!
They're playing against the AI but with a bit of story-feel to them. The rewards are better than Co-Op, though they take more time to complete. Still, better to have 1 Service Cost payment over 15 minutes instead of 3~

4) Random Battles pay more!
You mentioned this might not be your thing, which I totally understand~ However, if you want a more PvE experience, trying playing Tier 3-4 during the off hours. When it's not Primetime, you can find games with just a few players on each side. Mostly it's just bots that are sailing around and being shootable~ Kill some bots and get some stuff <3

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I'm going to link a video that talks about the Economics of World of Warships. The How It Works series is REALLY good at giving a lot of information in a easy-to-watch package:

In case you don't see a video above, here's a link you can click: https://www.youtube.com/watch?v=0_bXHAqLkKc

The change was that the Matchmaker is capped at 3 Destroyers and 3 Battleships if nobody has waited over 25 seconds in the Queue.

So you can see 4 Destroyers or 4 Battleships at most if someone was waiting longer than the 25 seconds when the match happened.

If I recall correctly, 2 is the cap for Co-Op.

5 is no longer possible with the change to Matchmaking for Co-Op that dropped in the recent patch.

4 DDs can still occur if the Queue time exceeds 25 seconds for even a single player that is in your match.