Ok I was just out sailing trying to see how long I could go without sinking (or getting bored), and i ended up sinking 11 hunters and 3 warshhips using a war frigate with a medium level crew (only one pirate using overpowered fury):
1 Red Dervish
4 Scorned Sirens
2 Cutter Sharks
2 Century Hawks
1 Flying Strom
It took about an hour and a half and I had to go between tortuga, ravens cove, devils anvil, cuba and ravens cove again in case I needed a quick getaway.
I ended up porting because A: it would take a long time to repair again, B: I was and had to go and C: i was bored.
How long have you gone?