What mods are you using?
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I made a quick mission to see what happens when I 'chase' a merchant. I used a Medium Modern Composite type with a max. speed of 17 kn. I set it at 15 kn. in the mission and chased after it with some desultory shelling.
with RFB 2.0:
15.5, 13.5, 10.5, 10.5, 11.0 kn.
with ISP:
12.0, 6.0, 7.0, 9.0, 9.0, 7.5 kn.
Now I hasten to say these figures don't prove much. I only did two tests and the 'target' ship speeds are simply estimated by distance covered in 3 minutes; that is from 0 to 3, 3 to 6, 6 to 9, etc., etc. So, it does not take into account the curvature of the path, and in fact, the ship was not taking a straight course, but trying to steer away, while I was having to shoot with a rear mount gun, and not directly behind it.
This is not that easy to test in an objective fashion, so I think I will have to just compare turning speed and radius.
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In any case, I don't think a full solution to this is possible without being able to recode the AI.