I'm no pro at naval battles, but I can supply a little info. Best thing, though, is to run some custom battles until you get a feel for it.
Brigs, sloops, 5th and 6th rates all have a range of 500 which is the greatest cannon range for naval units. (Bomb ketches have a range of 750 for their mortars, and I believe rocket ships have a similar range.)
The best thing to do is just play to the strengths of your units. With brigs vs. 6th rates, your ships are slightly faster than theirs, though they have a little more firepower; so you need to try to outmaneuver them. I usually try to get a few volleys of chainshot in them to increase your speed and maneuverability advantage by a bit more. Try to stay in front and avoid the broadsides. If you can separate your opponent and apply local superiority, you have the advantage, just like in land battles. Try to attack from upwind.
I usually only use grapeshot when I have them on the run.
Control your units individually once you close to range. It takes quite a bit of micromanagement and you have to keep track of the whole field, but that's what the pause button is for.
sloops vs indiamen
Sloops have the range advantage. Stay at the edge of the sloops' range, where the Indiamen can't return fire, and pound them into submission.
If you're controlling the Indiamen, try to close the distance, but I think you would still be at a disadvantage. Indiamen are pretty weak.6th rates vs 5th rates
If you're the 6th rate, run.
If you're the 5th rate, charge.
Although, either of these can beat a 4th rate (400 range) due to range advantage.
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