If you get the keel where it is supposed to be and use paper or cardboard to draw a shape similar to what is on the F1 drawing, you won't have anything to worry about.

Re: bow stem: If you straighten it out (i.e. make a straight line from the keel to the front of the gunwales, no curve) it will change the shape of the hull slightly, which could alter performance. How much will it alter performance? Likely, very very little, and if you don't have one of Brian's F1's to paddle side-by-side, you'll never know!

The stern stem is already a straight line, so you shouldn't have to worry about the shape here.

It doesn't need to be exactly the same as what is shown in the drawing, although you could try to print the outline on the F1 drawing to actual size and use that as a template. The photo below from Brian's "step by step" F1 page shows two stems that vary a bit. This level of variation is extremely minor, and likely doesn't change performance in the least.

: Just want to get a discussion going on stem design. I'm working on a boat
: based off of the F1 (in link below) and am wondering about making the
: stems. On yostwerks.com, Tom recommends putting a piece of paper in
: between the stringers and tracing a shape that 'looks right'. For those of
: us who may lack the magic eye, what is there to know about stem design?
: How do length and angle impact performance?