Brick-built sails for a pirate ship
Great question. It sounds like you are building a fairly large ship. If the scale is large enough, brick-built sails can look quite nice. You can add shape, contour, and texture using various building techniques. 

If you want to keep things simple, you could just use a single layer of bricks as was done in this basic sailboat model that was an in-store build back in 2009:



That example obviously isn't the most detailed. It's challenging to develop sail-like contours with studs on top.

Here's one example just using plates that is similar to what you mentioned in your question:



Notice how it uses wedge plates to round off the square corners of the sails. Wedge plates come in a variety of shapes and sizes, so that gives you some flexibility in the shape of your sail. Here's another shot showing that technique more clearly:



Here's a second example of that technique in a physical model to try to show how the sail sections can be connected using hinges:



Here's another where layered plates are used to provide more depth and contour:



And one more example of the same technique:



I personally like the aesthetic of studs showing in those examples, but you could also smooth your sails out using tiles and curved slopes if you'd prefer.