The soldered edge along the keel is originally the outer edge of the elipse,after rolling out the dents whilst its still a bowl,I place it upside down on a flat surface and mark all the high spots and gradually trim down untill it sits perfectly flat. Then carefully cut the thing in half,this flat outer edge becomes the keel and the cut edge becomes the gunwale.The keel edge is simply bent using flat pliers to give two flat mating surfaces,after soldering any overlaps are filed away. it sounds complicated but its quite easy to do hope this gives you an idea.Cheers Neil.

How do you keep the edges straight to . Butt or lap joint mate and solder.

No mate,just an old brass caster off an old sofa. its fitted to a file handle, then you just roll the tin on a rubber mat on the bench it takes a bit of pressure initially but once you're away its easy after practice. No special tools involved and its disgustingly cheap to do!
Cheers Neil

Do you use an English wheel to roll out the planishing marks.
Cheers Marky