Trying to add a deck

So, back to my learning double ender Pirogue.

Having got the hull draw techniques sorted, I want to add a 2' deck at either end (I am working on a half model and then duplicating it ATM). I have just tried extruding the edge of the segment at the end and that would do the job, but for one problem: when I extrude the edge, it straightens that edge.

By that I mean that the Control Net edge is straight, but the resulting line is curved...until I extrude the Control Net Edge. Then the actual plank edge curve flattens to the control curve, so I have a straight section of sheer, rather than a curve.

It also appears to force a compound curve in the plank of the side, because it actually causes the plank to straighten in both planes.

ummm...a picture I guess...hang on guys ...and yes I know the "deck" in the wrong direction .

Sorry about the double screen shot. If I take a screen shot of the secondary screen, it's blank. If I move FS to the primary screen then take a screen shot, I get bot screens. Separate issue, but sorry.

Thanks for any help