A rudder will add drag, but in some cases the drag will be compensated by more efficient paddling if you don't need to do steering strokes. The rudder needs to be a good design with a good foil shape. Make it as small as possible while still having it do what needs to be done.

> Last week I did some racing with my Nordic Hawk 16km for seakayaks. We
> placed third with the time 1.34 with an average speed of 5.5 knots. The
> boat is indeed fast. But off course a bit tricky to turn So I think I will
> add a rudder for better maneuverability. But the boats that placed second
> and third (me) did not have any rudders. So maybe a rudder may make the
> boat slower? Due to larger wetted surface and a lot of unnecessary
> steering. Maybe I should improve my paddle skills instead? Hummm