When you are happy with your sketch, go to your ink layer, in this case called DeckLines, go back to the ink tool with your favourite setting and start carefully inking in the lines you want to have at the end. Don't be afraid to use ctrl-Z if you're a bit off. Also, make sure you are working at 100% zoom or above. Gimp can pixelate things if you work at lower zooms and what looks great at 50% doesn't always look good at 100%.

Also, don't worry about areas that are below another object. I drew in the deck without the fallen masts first and went back to create the broken areas later. It's easy to erase a small portion of the line drawing later and redraw it as necessary.

Don't be afraid to add new layers to start working on a separate object.

Here's the finished line-drawing for this ship. Here I have been a little sloppy and deleted the old sketch now that it is no longer needed. Notice that there are 4 line layers that contain the overall ship and deck lines, the textures on the ship's deck - the woodeon planking and the hatch over the hold - and similar breakdowns for the area beneath the water.

Click image for larger version. 

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