This is a guess because Naval warfare in Imp1 has been a black box since the beginning:
Naval Blockades: Heavier Better
Naval Interdiction: Faster Better.

So if I am trying to starve an opponent: I use a heavy squadron for blockade and Raiders for grabbing enemy ships (sailing patrol).
Having an Admiral helps your odds. Also, you seem to win more battles but lose more ships if you play with a more aggressive posture.

Oh, so you use two separate fleets, one for blockading and another for patrolling? That's interesting. I've always just sent everything in to blockade so they can fight together. I should test that and see if it helps the interception rate.

The only thing I'm 100% sure of is that fleets do move at the speed of the slowest ship. You can definitely see this when you try to move around Ships of the Line with Raiders, then compare the speed if you move just the Raiders.

No clue if the heavy/light warships have different blockade/interdiction rates. I always just lump everything into one fleet, because they do fight together and it is MUCH better to ensure you have the best possible battle outcomes with the combined fleet than any theoretical interdiction bonus you would get with split fleets.

I honestly think the light warships are just there for a lower cost alternative if all you care about is quickly moving around the map and blockading without naval combat. Heavy ships sacrifice speed but they're much better at naval combat.

The manual mentions that range is the most important stat when it comes to naval combat, and I think that's true. It's why the Raiders are much better in combat than Ships of the Line, even though the SotL has much better stats in everything EXCEPT range, The introduction of Raiders is kind of the only point in the game where light warships eclipse heavy warships for pure battle power. Once you hit ironclads, I never bother with building new light warships because the speed difference between them shrinks and the slightly higher speed of light ships isn't worth forgoing heavy ships any more.