So what you want is Liners able to fend off capital class ships? Sorry bud, but nowhere in the history of the world have civilian vessels or vehicals been superior to military equipment. Why should the future be different?
From what you're saying, Wobbly, all you want out of this is a cheep and over-muscled battleship. Liners arn't that. Liners are exactly what the name says; Liners. Sure, they carry important cargo but that's dosn't justify giving them battleship class weapons.
Again, these are CIVILIAN SHIPS, not pirate or military, CIVILIAN. Civillians don't get automatic weapons, rocket launchers, moarters, or even true sniper rifles do they? Nope. All of that is for the military. Can civilians get mounted machine guns, battleship turrets, or cruise missiles? Nope.
And yes, Spa and Cruise make their own weapons. They're called "Transport Turrets!" "OMFG! NO WAI!", right? Well listen up, bub, 'cause they do in fact have their own weapons. Get the class 7's, and watch the NPC pirates dissolve under your fire. Just go out and try it. Or, alternatly, buy some Adv.Debilitator turrets or Chain Gun Purple Goddess turrets and rip appart those pirates with streams of energy.
Give them both Gunboat shields, fairly decent armor, and have most of the turret slots be transport turret slots. For the lux liner, give it a pair or two of gunboat slots, and let the prison liner get 5 or 6 plus a little boost in armor. Of course, the cargo hold isn't as big, but thats the price you pay for better weapons.
Face it Wobbly, what Luxury Liner captain is going to stick around and fight? They might disturb the passangers, they would never fly on that line again, and then that business would loose money. More often then not, a Lux Liner captain will kill a few pirates and hit the cruise button all the while screaming for backup. NOT return fire untill all the pirates are gone. That's just f-ing stupid on the captains part.
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