Congratulations, the Jugg can be a challenge as one gets the handle on their sailing.
Also note that although you were only level 13 sailing, your level 21 cannons which did most of the work disabling the Jugg.
Im really curious as to why the Juggernaut is thought to be so rare, or that difficult.
I see it all the time and often have to avoid it in attacking other ships, and have often had to sink it just to make it safe to battle the other ships around it.
Did it with a light sloop once to prove it could be done.
It has alot of firepower, avoid it, and its just another piece of driftwood in a big pond.
I guess its just my... been playing the game for a year and a half savvy. hehe
Also as far as the plunder, you cant count on it. I've received that 1 crate from the Juggernauts, Colossus's, Behemoth's, etc. And i've gotten royals off of Bulwarks.
Its all random, the higher levels are of course programmed to usually give the higher valued rewards though. Treasuresense skill has alot to do with this as well.