how come large warships only have 1 hp and small ones have 6hp?

Research better hulls. The "cargo" hull should only be used for utility ships.

Cargo hulls, which are available from the beginning of the game, only have 1 hit point.  They are not intended to be used as combat ships, but instead are usually used as freighters, constructors and space miners.  Tiny, small, medium, large and huge hulls are the combat platforms and have more than one hitpoint.  You start the game with tiny hulls.  You must research "interstellar construction" or its equivalent to get small hulls.  You must research Advanced Hulls and then Medium Hulls to build medium ships.  You must then research Superior Hulls and then large hulls for large hulled ships.  Finally, you must research Master Hull Building and then huge hulls to build huge hulled ships. 

Don't underestimate the Cargo hull.  It has something few other ships have at the very start- space.  You can load one up with a ton of cannons to jump-start your military progress, attach a bunch of engines to get absurd speed for a scout/explorer, or put a popgun on your speed design to make an exceptional early-game interceptor.  Were it not for the cargo hull, the DL campaign would have been a lot tougher for me.

My usual use for cargo hulls is explorers (lots of engines, lots of sensors) which can provide an edge early on - later I discard the engines and change them to semi-stationary monitoring posts. For ZYWs, cargo hulls are also invaluable, as they are cheap to build and can be upgraded to spore ships once they reach their intended target.

If you find yourself being bullied, slap a ton of weapons on a cargo hull and let it stay in orbit - you'll get a boost to your military score which will affect diplomatic relations favourably. As long as you don't intend to really fight with them.

OTOH, cargo hulls for constructors are a waste of space - a small hull is sufficient and often cheaper and quicker to build.

But never, ever, fight a war with cargo hulls. It's throwing free experience points / upgrades at enemy ships. Of course, at the start of the game... cargo hull, lots of engines, ATT 1 - against weaponless constructors, bases, scouts, colony ships... can be worth it. Just don't expect that strategy to last longer than the first 15 - 20 turns.

You forgot one of the best targets for this - asteroid mines. If you can't take them, blow them up to force the enemy to rebuild them. Or at least deny him the production output.

Would the cargo hull strategy still work well with the Arcean Super ability? As long as you get the initiative in every fight.

Still, in all cases the cargo hull idea can be as good as using small ships. The 12 HP you get with a small ship is practically identical to the 1 HP for a cargo hull once you opponents attack values get into the 20's. Let alone the 8 HP for a tiny. Those extra HP will help with your military rating but not give you too much in the way of surviving a real battle.  That said, i personally still don't use cargo hulls for anything military except for transports.

There are Hit point upgrades available that can boost a cargo hull. A cargo hull scout ship with survey gear will pick up a few 4 point hit point upgrades in the early survey phase. The big problem with cargo hulls is the upkeep once you mount any kind of weapon system, since math is not my strong point I will let those who understand the point cost thinggy to explane this. I have found that in the late game cargo-survey cruisers that have been just zipping here and there across the galexy can be as powerful as any medium hull I can build at that instant, the medium survey- cruiser would just be cheaper to support. If I was to put a story line about a game these venerable old ships would lend to some good side storys. So I keep them out of the scrap yard more out of some part of my own Navy past than any game use they would still possess .It was a sad day for me when I saw my old Submarine listed in the navy scrap yard list of ships to be broken down. This is why I always like to upgrade all ships including the old cargo hulls.

You actually get 8 hp for a small and 5 hp for a tiny-I assume you're speaking from memory with both Xinathium Hull Plating and most if not all techs that provide an hp boost applied...in which case smalls and tinies are of course no longer worthwhile.

That would make a perfect Glass Cannon....

A better example might be ; an empty cargo ship {exxon-valdez} mounting 4  5inch rapid fire auto guns and 4 heavy machine guns vs an old ww2 pt boat {McCales Navy} who would win?Theres a lot of space to be used on a cargo, but theres also a lot of dead space giving no support to the hull.

Prior to the game rule where ships get to fire back even if they get destroyed on the first round, it was pretty common game cheese to have a cargo hull with enough firepower to take out an enemy in one shot. You would never get fire at and so would not need HP. Again this works only when you initiate the battle. This then became the Arcean's super ability.

Yes I guess i was thinking about later in the game when a player would have higher HP fropm techs, anonolies etc. Still, makes no difference to my point that haveing 1 HP or 20, isn't going to make a difference in many cases once the game gets going. 

Cargo hulls are normally intended to be for the utility class ships - they're for the ships that don't fight. However, as the others have mentioned, they can be incredibly useful at the beginning of the game because they can hold a lot of stuff before you've researched the larger attack hulls.

Also, an actual warship of any type (tiny, small, medium, large, huge) will only have 1 hp if you upgrade it as well (for the duration of the time it takes for the upgrade to be complete). So it's a good idea to make sure no enemies are nearby before you do it.

Of course, the AI LOVES to upgrade all their ships no matter where they are. I often see AI upgrades that take over 150 turns to complete. Even if you are at war with them, they will upgrade their ships right next to your fleet, and you can wipe them out with ease. I don't understand why the AI would be allowed to do something so ridiculous all the time like that, because whatever they upgraded it to is going to be obselete 150 turns later.

After reading this, I'm beginning to suspect that the AI is running my state government.

I'm a little surprised that no one has mentioned the fact that you can take the +hitpoint modules that some races can research, slap them on a cargo hull, and have the strongest ship in the game- long before any of the larger hulls are available.

In my current game one of my cargo-hulled survey ships got an extra 12 hp from anomalies, plus experience upgrades.  I upgraded it to a combat ship and a few skirmishes later it's up to 26 hp -- with vast room for components it's a fleet unto itself against the AI's small hulls.  Watching it plow through the defenders on its way to an AI homeworld reminds me of bugs splattering against a car windshield