Is there an addon or a way in blender that would let extrude this edge down over faces below, it’s a curved shape, a bit like a boat hull.
I know I could do it manually with the knife tool, but I’d like it to be more precise and bevel doesn’t really work.
any help would be great
With that kind of topology - no. There isn’t any defined flow there. Your best bet here is to indeed use the Knife and some manual cleanup.
Along with @Stan_Pancakes idea, you could also try adding the included addon, Add Curve: Curve Tools, and there is a Loft between 2 curves … as well as Rail and BRail tools…
Those should help to make the “Hull” shape.
Cheers Stan, I thought that might be the case
In the end I simplified the top edge and below so I could bevel it, thanks for all your help