Yep, the Grassraper might be of some use in jump point defense/assaults, but it is too slow and vulnerable to missiles to do well anywhere else.
1. Design a Standard Military Engine instead of the Gunboat one you are using and then stick in about 10 of them - or until you get to at least a minimum speed of 4000.
I disagree. It looks like the propulsion technology is lagging badly. The Grassraper appears to be using 2nd generation technology on the engines, and 3rd or 4th generation technology everywhere else. To improve the design I'd suggest either:
a) Make the ship smaller. Dump the shield unit, dump an armor layer or three, downsize the reactor to match the laser, downgrade the beam fire control unit so it isn't tracking faster than the main weapon. In short, try to shrink the destroyer down to a 1,000 ton FAC built around a laser.
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b) Research Ion propulsion, and then follow Beersatron's advice with military engines. With Ion tech your ships could be moving 4,000 km/s with only 33% engine mass, or they could travel 6,000 km/s with 50% of the ship mass devoted to engines.
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c) Go for a juggernaut fleet. You seem to have the tracking speed and defensive tech for it. The juggernaut theory is that speed doesn't matter if the defenses are strong enough or the attack range long enough. Just drive straight for the objective, crush anything in your way, and ignore anyone faster than you who isn't in the way. Defense is also easy, just sit on the jump points and squash anyone trying to move through you. For this to work you need to put anti-missile defenses on the ships, and add multiple shield units. Also, swap out the Gun Boat engine for military engines to improve fuel efficiency. The Grassraper could become a decent jump point defense craft if you added additional lasers, resized to beam-fire control, and resized the reactor, but a primary jugernaught offensive ship will need to be armed with either long range missiles or multiple point defense weapons such as banks of turreted lasers or rail guns.