While doing the quest "Keeping the Enemy at Bay", you don't have to "set the sails on fire".
While in the air, the ship name should appear in white in the middle of your screen (like whenever you zone). At that time you just need to click on your Flaming Oil and toss a charge into the ocean or wherever you want.
You will be checked off for the completion of the Sin'oren, Bloodoath, or whatever ship you're at sails being burned---even tho you didn't actually throw the charge at the sails.
This is useless and I bet a lot of people already know this but there are no posts about it so maybe some of you don't.