Since the beginning I was very discontent with the ship combat rules of Spelljammer: My players just bolted five light catapults and 5 medium ballistas onto their whaleship and were able to annihilate entire fleets of standard ships using this modification. The scary deathspiders were not as scary anymore due to their limited range of 2 hexes thanks to their heavy ballistas.
Since I am too lazy to think up how to remedy the weird ship combat, I instead decided to work on a conversion for Spelljammer into Full Thrust. Utilizing this system I have already got a well working ship combat system and do not need to worry about any imbalances.
This is what I have come up with - we just had one ship combat so far, so I really cannot comment whether it is working well yet:
As a rule of thumb, the players should not be allowed to design entire ships, as this will only allow them to exploit any loopholes in the system. If a situation should arise, where the players are needed to construct a ship anyway, be sure to increase the price by 50 to 100 percent, since it is most likely a unique ship and there are no existing blue prints or the necessary facilities to build it without an additional cost.
Ships do not need to pay for drives or FTL drives, because they need to buy a Spelljamming helm instead, which will determine the thrust rating. After the design process, the ships point value is multiplied by 500 to determine its value in goldpieces. Any remaining weaponspace is divided by 2 for escorts, by 3 for cruisers and by 5 for capital ships and rounded up. This number indicates how much space can be used to retrofit any weapons. However, you should not allow the PCs to buy weapons out of the book, instead let someone at a spaceport give them an offer - for example building another subminition pod / greek fire projector onto the ship for 1500 goldpieces or exchanging an existing weapon and bolting another weapon onto the deck in its place. Weapon replacement works by selling the current weapon for half the price and installing the new weapon for the full price.
Repairing a system costs half its price (1000 goldpieces per hullpoint for example) or a quarter of its price if a repaircrew is on the ship. Repaircrews cannot be repaired though, as a failed threshold roll indicates their death.
Forge: 100.000 + 10.000
Crown of the Stars: 1.000.000
Nonmagical helm: 100.000
Minor lifejammer: 5.000
Major lifejammer: 25.000
|Lanceship||15||7||43||21500||3||1 firecontrolsystem, 1 B battery|
|Doombat||5||4||18||9000||1||1 firecontrolsystem, 1 C battery, 1 submunition pack, modified hull (+2 hullpoints)|
|Manowar, IEN design||60||30||302||151000||3||3 firecontrolsystems, 3 B batteries, 1 minelayer, 1 cloaking device, 1 screensystem|
|Manowar, standard||60||30||182||91000||4||3 firecontrolsystems, 3 B batteries, 1 minelayer, 1 screensystem|
|Wasp||18||9||51||25500||2||1 firecontrolsystem, 1 pulsetorpedo|
|Dragonfly||10||5||25||12500||2||1 firecontrolsystem, 1 C battery|
|Whaleship||90||45||265||132500||7||3 firecontrolsystems, 1 minelayer, 3 screensystems, 5 needlebeams|
|Galleon||40||20||128||64000||3||3 firecontrolsystems, 2 B batteries, 1 C battery, 1 screensystem|
|Nautiloid||35||17||112||56000||2||2 firecontrolsystems, 3 B batteries, 1 B battery (front arc only), 1 minelayer|
|Deathspider||100||50||333||166500||4||3 firecontrolsystems, 1 A battery, 4 pulsetorpedos, 1 minelayer, 2 screensystems|
|Triop||45||22||130||65000||3||3 firecontrolsystems, 4 C batteries, 2 minelayers, + 2 thrust rating|
|Clipper||65||32||220||110000||1||3 firecontrolsystems, 6 AA mega batteries|
|Squidship||45||22||146||73000||3||3 firecontrolsystems, 1 A battery, 2 B batteries, 1 screensystem|
|Dragonship||45||22||133||66500||3||3 firecontrolsystems, 2 B batteries, 1 screensystem|
|Hammership||60||30||186||93000||4||3 firecontrolsystems, 2 A batteries, 1 pulsetorpedo, 1 screensystem|
|Tradesman||25||12||72||36000||3||2 firecontrolsystems, 1 A battery, 1 B battery|
|Quad of Thay||63||31||261||130500||1||3 firecontrolsystems, 10 needlebeams, 3 screensystems|
|Mindspider||40||20||108||54000||5||2 firecontrolsystems, 2 B batteries, 1 minelayer|
|Mantis||60||30||221||110500||3||3 firecontrolsystems, 1 A battery, 2 B batteries, 2 screensystems, 1 minesweeper|
|Battle Dolphin||70||35||208||104000||5||3 firecontrolsystems, 2 B batteries, 2 screensystems|
|Gnomish Welk||40||20||99||49500||3||3 firecontrolsystems, 1 B battery, 1 minelayer|
|Cuttle Command||100||50||371||160500||3||3 firecontrolsystems, 2 A batteries, 4 pulsetorpedos, 1 minelayer, 3 screensystems|
|Great Bombard||50||25||165||82500||2||3 firecontrolsystems, 2 C batteries, 1 wavegun, 1 screensystem|
|Unity Ship||30||15||87||43500||3||2 firecontrolsystems, 3 B batteries|
|Illithid Dreadnought||100||50||312||156000||6||3 firecontrolsystems, 2 A batteries, 4 B batteries, 2 screensystems|
|Octopus||70||35||221||110500||4||3 firecontrolsystems, 2 A batteries, 2 pulsetorpedos, 1 screensystem|
|Armada||100||50||360||180000||1||3 firecontrolsystems, 8 fighterbays, additional launching capability|
|Thoric Tradesman||30||30||83||41500||3||2 firecontrolsystems, 1 B battery, 1 AA mega battery, +1 thrust rating, modified hull|
|Crystalship||300||150||858||429000||18||5 firecontrolsystems, 2 B batteries, 4 C batteries, 1 pulsetorpedo, 3 screensystems, 1 novacannon, 5 needlebeams, 10 submunitiopacks|