Especially when your only able to do a max speed of 25 and the enemy FLEET after your 6 (a$$) is coming at you at speed 30 and u dont even have a chance to charge your weapons.
Pirate ships can always do speed 31, even those that can't, if you are willing to double your engines for a few moments.
A quick run in at medium speed with engines doubled and a really charged up front shield allows you to get a point blank overloaded alpha strike on the armed freighter or merchantman, while you take no internals yourself. (The mother freighter has usually very few weapons, so can often be ignored, if you want, until end-game.) Then break off, hit repair on your engines, and maybe turn off the engine doubling until you are ready for another run.
Bonk gives some good ideas; I usually use the hostages (which have no marines and are thus easily captured) to draw some fire for a salvo. (they only survive one salvo, but the nearest ship to them will turn off to engage them, which often buys you some seperation.)
Don't forget the cargo pods have spare parts on them, for beaming over after you've used a few of yours.
Also, different ships draw different AI. You might want to experiment a bit. I played for 6 hours in a LR I've come to like, and never faced a single convoy except for one ambush mission. (Where the convoy was the one ambushed; so I had another pirate helping me.)
-Herr Burt