This is an out-of-date guide, the new one is here: https://forum.cosmoteer.net/d/7783-cosmoteer-ultimate-ship-design-and-part-guide
Note: This is a Battle guide, for all you people who care about looks more than usability, this guide is for the rest of us.
Note: • DOTS = IMPORTANT! (Mostly for noobs)
Note: Picture credit goes to "Cawfie", @Jagdgeschwader-51 (AKA "Necrobite" from Discord,) and me. (The boring ones...)
Thing 1!
•Don't put reactors in clumps!! (or in long lines, definitely not those!) They explode violently when destroyed! If you ship seems to be exploding a lot, or it seems too easy to destroy, this is probably your problem.

(picture by "Cawfie")
Thing 2!
•Don't use missile storage. Missile storage is only good for 3 missiles and takes up a 2/3 area, while a missile factory produces infinite missiles, and takes up a 3/3 space. And they are both explosive.
Note: missile storage can be useful in some circumstances, although I am not the expert on this I've been told it can be good for short fly-bys and a reload when running.


(Again, pictures by "Cawfie")
Thing 3!
•Don't use ammo storage. It's not much different from the missile storage, but it is better seeing as it holds more ammo then the factory does, whereas missile storage holds the same amount as the factory.
But still putting a factory wherever your storage would go is better for cost efficiency and crew needs...
Note: Apparently ammo storage can also be useful in much the same way as missile storage. As people in "peoples tips" have been telling me for a while now. (shocker! 0neye doesn't know everything! 😮)

(At this point I'll just make it a note...)
Thing 4! (suggested by Doge420GoodGuy )
"Don't stick gun "turrets" out of shields, they also have a hitbox! 💢 "

Thing 5! (suggested by jbox1 )
"Don’t rely solely on forwards thrusters, remember your reverse engines!

Thing 6!
Don't use too much armor! I see a lot of people building huge ships, and FILLING the extra space with armor! Don't do that! It will make your ship WAY too slow, and more expensive! I'm not saying to not to use armor, quite the opposite. I'm just saying don't bury your CR in too-big of a block of armor, after your weapons are destroyed it will be of no use.

Thing 7! (suggested by GhostTaker )
"Don't clutter your ship with un-necessary equipment! (Or block corridors.)"

Thing 8! (Suggested by @Jagdgeschwader-51 AKA: Necrobite.)
•"Don't block PD, allow them to use their full angle for maximum efficiency!"
Thing 9!
•Don't fill your ship with corridors! (Or use them as space-fillers. Pun intended.) They are weak and can be penetrated by cannon shells, they cost money, and they just add more space for your crew to have to walk through.
If your ship looks more like this,

then like this,

Try getting rid of those corridors. (And all the other things wrong with it.)
Thing 10!
Don't use too many electro bolts! Electro bolts are always good, but usually more than 6 is not needed. Electro bolts are really useful vs shields, and are great in any combination. But like with anything there is a point when adding more becomes wasteful.
So do add EB, just not an excessive amount.
Thing 11!
Try not to use bunks. Bunks give you 2 crew for half the cost of a crew quarters, (not including the 1000 credits of crew) while the crew quarters gives you 6 crew. So it is saving you 250 credits, and a 1x2 space by using a crew quarters instead of 3 bunks.

Thing 12!
•Don't fill in every available door space with a door! First of all, there is the fire risk, fire spreads through doors (and corridor)! Second of all, doors cost 200 credits! That is almost as much as an armor piece, or as much as 2 corridors! Added up, doors can be one of the main things making your ship expensive.
Most of the doors in this example ship will never be used, and are just taking up space.

While there are many fewer doors on this ship, it is just as efficient.

Thing 14!
Don't trust the power bar when building rails! The power bar will try to tell you that you can only power 6 accelerators with 1 reactor. You can actually power up to 12 (with a really efficient setup) with 1 reactor. A good module is 2 crew quarters, 1 reactor, and 10-12 accelerators (plus FE) depending on how you make it.
An example module. (with full railgun)

Thing 1!
•Do protect your reactors and other explosive stuff with internal armor!! It WILL save your ship someday! If your ship is easy to destroy, this may be your main problem.

Feel free to make it a pain in the ass to get to your reactors!

Thing 2! (suggested by jbox1 )
•Do place shield projectors behind a layer of armor (or even another shield!)

Thing 3! (again, suggested by jbox1 )
Do use internal thrusters!Putting your thrusters inside your ship will save space for weapons, shields, or PD on the outside. It will also protect them from harm, so you don't turn into a sitting duck.

Thing 4!
•Do Keep under or at the "recomended bar" for power! Any extra and you will have more power than you are using, (wasting money.) If your weapons / shields etc. aren't getting enough power, try moving your reactors closer!


Thing 5! (suggested by Equalizer )
•"Do be aware how long a ship takes to 'power up'. A ship that is reliant on power storage can be vulnerable a long time in a multiplier game if the game is set to have them start empty."
0neye's note: You can also test this by turning all your weapons on autofire to see if your ship can power them efficiently. If it can't, try moving your reactors closer ("Hey! that's the tip above!") to the things that need power the most. Only add reactors as a last resort.

Thing 6! (suggested by "Cawfie" from Discord)
"Do play, learn, experiment. All of these are just tips that all have their 'But' and 'Except when's. The best advice I can give to you is, Have fun!"
Thing 7!
Do use Engine rooms! They provide a huge boost to your thrusters, even though it is 3x3 it is not that hard to fit into your ship, standard thrusters, large thrusters, and huge thrusters work great with this as they are easy to put in around the engine room, and when boosted provide a substantial benefit. Small thrusters (with their recent nerf) are not such a good choice as their already small thrust is not going to increase as much with a 75% bonus.
Important note: engine rooms are NOT thrusters! I see a lot of people putting them inside their ship where they either only boost 1 thruster, or none at all. Engine rooms only boost thrusters they are RIGHT next to.
Some examples of ways to fit thrusters around engine rooms.
( SpaceCats )

( Mine. slower then Veranias, but also takes less power, meaning you can put 2 of them together with 1 reactor.)

Thing 8!
•Do Put your ammo factories next to your cannons! (or missiles) Ammo factories don't explode as violently as reactors. So putting however many you need (1/2 standard, 1/1 large) right next to your cannons, they will get constant ammo, letting you DESTROY your enemies without hesitation!


Thing 9! (suggested by Ultranova )
•Do Use fire extinguishers! Pretty self-explanatory. (Unless you don't believe that fire exists.)

Thing 10!
•Do "Bury" your Ions behind layers of armor. This will increase its lifetime by blocking shots with the armor.
Like so.

(Note that the lasers are only powered for the picture, they are only to stop ions from cutting through the hole.)
Thing 11!
Do use POWER storage! (But not too much.) Unlike ammo storage, power storage does not explode, and it holds a substantial amount of power for its size. Also, although you should put your reactors close to what you want to power, they are incredibly explosive. So you may want to move them back just a bit and put in a power storage.
An example of a way you can put storage next to the power-hungry things.

Thing 12!
•Do use compartments! Compartments are great because the crew almost never get confused and walk halfway across the ship to get a battery for something 40 blocks away... They are also great for reducing your chain reactions by separating parts of your ship so the explosions are too far away, and fire does not spread between them. Another benefit of compartments is that they are much easier to copy and paste.
A good size for compartments varies based on how big you can make it only using 1 reactor. Now if this means using a lot of storage, that's fine. But also think about if it would be better as 2 sections instead of one, or vice-versa. AS I said beforeyou can test this by turning all your weapons on autofire to see if your compartment can handle that. If it can't you might need to take away something, the choice is up to you.


Thing 13!
•Do (Try to) spread your ship / weapons out! If all your weapons are packed close together they will all be hit by a volley of cannons or lasers. If you spread your weapons out then the same volley will only hit a few of your weapons. (Sometimes you have to pack your weapons close together because of space or cost.)


Thing 14!
Do Put your power for rails in between them, (if they are 2 squares apart) putting your power in between them won't do much besides save you space. This is kind of a niche tip because it is not that important but I needed something for railguns in here... ;-;

(For listing opinions or big blocks of info separately!)
Hi,
I think @Oneye 's ships are of the most interesting & efficient - I have learned so much from him - he is also the only player I requested ships to use in competitive MP games.
But of course, I disagree with a lot of rules here - especially the "don'ts"
• You can clump reactors in small or large ships (maybe not so much in medium ones).
Just do not build long "reactor-chains" going all through your ship!
• You can use storage if you want to have a flyby ship that reloads while in the loop.
• Heavy armored ships are a choice for many players lately and they do score wins!
0neye's note: this is not what I meant, so I edited the tip to be more specific.
• The thruster rules are almost completely different since 0.14.0!
• Compartments I think are a matter of taste - they have some advantages but also disadvantages.
+ can be protected by armor well; fire does not spread; good for "spread out" ship designs and thus against splash damage.
- every compartment needs its own power source, fire extinguisher
AND Crew; sometimes get separated from control rooms early.
(Personally, I never built ships divided by compartments so far.)
The riskiest design choice from 0neye certainly is: Ships having too few control rooms!
Although this is also not always true I used to say:
One control room is never enough!
Universal rules:
Cosmoteer is certainly already complex enough to always build a counter to any ship!
So if you think something is overpowered - think again! (all current miss-balances considered.)
Allround ships are the hardest to build - but still worth trying!
And lastly, try to be creative and bend rules as much as possible!
(credits to @Dalas120 who excels in the latter!)
Personal no go: Control room placed in reactors "explosion/fire-radius".
regards
•Protect the control rooms and be sure to have enough of them on large ships that they aren't a vulnerability.
•Don't put ammo, missile factories, power plants far away from the things they supply. Crew are really slow, inefficient and costly at moving things any further than about 2 tiles.
•Do put turning thrusters as far from the center of the ships as possible so they have more leverage (faster turn with less thrust). This, of course, can conflict with ensuring they aren't really vulnerable.
•Do be aware how long a ship takes to 'power up'. A ship that is reliant on power storage can be vulnerable a long time in a multiplier game if the game is set to have them start empty.
•Design ships with a role/combat tactic in mind. A ship that tries to do everything will almost always do nothing well.
•Expendable parts (crew compartments cough) can be used as pseudo armor.
•Be aware of where the doors are. An important door that is missing can greatly weaken shields or slow cannon rate of fire. It is also easy to have unnecessary doors.
•Don't rely solely on compartments. This game is a work in progress and anything could change.
Don't be a slave to the guidelines. There are situations where almost any of them can be wrong.
1) Sometimes when not building flat, boring walls you have to clump reactors to adequately serve curling shields.
2) I've never found a use case for missile storage, but I also haven't tried to dense pack missile launchers. They may be useful for the same reason as ammo storage.
3) I have found ammo storage useful to split up long hauling tasks. Used in this way they aren't explosive because they're always nearly empty. One storage can be used to feed ammo from multiple factories farther back that can't be forward due to door placement constraints or by using storages one broadsides worth of factories can serve guns on both sides of a broadsider.
4) The gun barrels aren't the problem, the turrets are. If it's not in thick black outline on interior view it doesn't have a hitbox. All cases of this not related to the semicircular parts of cannons derive from a longstanding bug that was finally fixed relatively recently.
5) Focusing on forwards thrust is how you counter some designs.
6) Yeah, space-filling armor is pretty much pointless.
7) The most recent change relating to compartments was to make them not break irretrievably when the ship is repaired in bounty mode. If they do stop working it's an excuse to design more ships.
8) Sometimes you need to block corridors to fit things, and what equipment is unnecessary depends on the purpose of the ship.
1) Except on small ships, where there simply isn't room for internal armor.
2) Except when your primary concern is missiles, which will destroy stuff in front of shields through them.
3) Except on small ships.
4) Except that the recommended energy bar assumes you aren't being hit by electro bolts and assumes you have no low energy use compartments (like cannon compartments or missile compartments) that have a reactor generating power you can't use.
They're generally right, but if you never try violating them you'll never learn their limits.
-Tip: When using laser weapons, DON"T PUT REACTORS behind them. Most of the laser weapons have low health, or require so much armor to protect them, you might as well get another ship.
-Tip 2: Are your reactors being continuously used when a power storage is right in front of it? Make a doorway that is only connected to the reactor and storage. Make sure the storage has a door to a walkway, and only the storage.
-Tip 3: If your Heavy Cannons are running out of ammo a lot and you already have an ammo factory connected to it, Connect another and add some extra crew. It will load twice as fast and it almost never stops firing!!
Tip. When filling empty spaces with corridors, divide the spaces into sections using armor. This can help contain fires, delay your ship from being cut in half and overall, provides greater internal support as well as a sort of frame.
For another "guide" visit Octos!
And for beginners look here, and here.