How about this design then?
Vanguard class Gunboat 10,500 tons 254 Crew 1934.6 BP TCS 210 TH 2100 EM 0
10000 km/s Armour 1-42 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 5 PPV 8
Maint Life 1.1 Years MSP 576 AFR 176% IFR 2.4% 1YR 480 5YR 7201 Max Repair 525 MSP
Intended Deployment Time: 1.4 months Spare Berths 0
"Pyre" 1050 EP Ion Drive (2) Power 1050 Fuel Use 141.8% Signature 1050 Exp 17%
Fuel Capacity 3,500,000 Litres Range 42.3 billion km (48 days at full power)
25cm Railgun V4/C4 (1x4) Range 200,000km TS: 10000 km/s Power 15-4 RM 4 ROF 20 5 5 5 5 4 3 2 2 2 2
Fire Control S12 128-12000 (1) Max Range: 256,000 km TS: 12000 km/s 96 92 88 84 80 77 73 69 65 61
Gas-Cooled Fast Reactor Technology PB-1 (2) Total Power Output 4.5 Armour 0 Exp 5%
This design is classed as a Military Vessel for maintenance purposes
I'd say trade space used for fuel storage for more weapons. Again, you are not planing on building a patrol ship, are you? You want a ship designed to kill those fast FACs and for that a few billion km are enough. Support them with a tanker until the fighting starts and you can get it anywhere you want.
Also: your ship has a speed of about 10000 km/s. Your beam fire control has a tracking speed of 12000. Your BFC is faster than your weapon - I assume your fire control tracking tech is 4000km/s, which means you can trim the FC down to 2x tracking size (resulting in a 8 HS BFC with 8000 tracking speed). 8000km/s tracking speed is actually sufficient to hit the FACs plenty of times. Less tonnage, more speed.
Judging by the numbers, I guess the engine is 50HS big with a 1. 75 modifier. I could come up with a design like this:
Half the size class Cruiser 4,900 tons 137 Crew 1099.45 BP TCS 98 TH 787.5 EM 0
10714 km/s Armour 1-25 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 2 PPV 8
Maint Life 2.1 Years MSP 280 AFR 96% IFR 1.3% 1YR 85 5YR 1271 Max Repair 154 MSP
Intended Deployment Time: 6 months Spare Berths 0
Military ID-105 x1.75 Drive (10) Power 105 Fuel Use 269.41% Signature 78.75 Exp 17% <== Same size / power / fuel use of ONE of your engines
Fuel Capacity 1,000,000 Litres Range 13.6 billion km (14 days at full power)
25cm C4 Soft X-ray Laser (1) Range 256,000km TS: 10714 km/s Power 16-4 RM 6 ROF 20 16 16 16 16 16 16 13 12 10 9
Laser Fire Control S08 128-8000 (1) Max Range: 256,000 km TS: 8000 km/s 96 92 88 84 80 77 73 69 65 61
Stellarator Fusion Reactor P6 (1) Total Power Output 6 Armour 0 Exp 5%
This design is classed as a Military Vessel for maintenance purposes
The laser is just a placeholder for a 8HS weapon of your choice. Depending on your armor tech, you'll have to account for more space used by armor though.
Or:
New Class class Cruiser 9,700 tons 280 Crew 2128.3 BP TCS 194 TH 1575 EM 0
10824 km/s Armour 1-40 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 4 PPV 24
Maint Life 2.44 Years MSP 617 AFR 167% IFR 2.3% 1YR 143 5YR 2141 Max Repair 154 MSP
Intended Deployment Time: 6 months Spare Berths 0
Military ID-105 x1.75 Drive (20) Power 105 Fuel Use 269.41% Signature 78.75 Exp 17%
Fuel Capacity 2,000,000 Litres Range 13.8 billion km (14 days at full power)
25cm C4 Soft X-ray Laser (3) Range 256,000km TS: 10824 km/s Power 16-4 RM 6 ROF 20 16 16 16 16 16 16 13 12 10 9
Laser Fire Control S08 128-8000 (1) Max Range: 256,000 km TS: 8000 km/s 96 92 88 84 80 77 73 69 65 61
Stellarator Fusion Reactor P6 (2) Total Power Output 12 Armour 0 Exp 5%
This design is classed as a Military Vessel for maintenance purposes
I am kind of uncertain, why you'll want such big ships though. The more tonnage you add, the more engines you need. . . which results in more tonnage again and less range. Thus adding more fuel storage requires yet again more engines. At some point you get diminishing return for any engine added.
If you want a beam cruiser to use against other enemies other than the spoiler race, your tech simply isnt quite up to the task yet (you have paper thin armor and too little armament to justify such an expensive ship).
Since you are going down the route of very specialized ships anyway, you can design a ship to fight the enemy's mothership - it is very slow and has even shorter weapon range than its FACs => remove engines from your designs and trade it for weaponry; make an even smaller BFC (S02 128-2000),etc. Just make sure the mothership doesnt have any FACs still in its hangars (i. e. damage its shields to provoke it into launching them) before you close in on it with your slower mothershipkillers. . .
Hint: 'Follow at 120k km' order against the FACs and 'Follow 7k km' against the carrier. YMMVDONT PANIC! heh. . As Jumpp wrote, unless you lead them right to Sol, they usually stay in the system you spotted them. They WILL follow you through jumppoints though, if they see you using it after an engagement.
I had one of their ships in a system right next to Sol (they destroyed my very first ship launched =( ) and I came back 20 years(!) later to kill it. Treat it like a big bad bear, dont bother it and it should leave you alone unless you are unlucky enough for it to move towards you.
If you are fighting the swarm the best defence is usually long range, fast missiles. A lot of long range, fast missiles. 20-30 per salvo per target. You should be able to design a missile that goes 15k or so.
Why is that better than having a faster beam ship to engage the swarm? It's a bit like David vs Goliath - you can throw thousands of soldiers at Goliath or use a single guy who is just faster and smarter than him. With sufficient speed and range the only factor that could spoil the plan is the mothership's shield regeneration rate.
