: I have recently laid my hands on a 4 metre fibreglass whitewater kayak that I
: want to use for an overnighter/play boat. The trouble is that as I get to
: a decent speed on the flat, because it was not desisgned for this, it
: tends to try and change direction right or left! Any suggestions as to
: change this? A guy at a fibreglass company (I needed to make some small
: repairs) suggested I glass a keel onto the back end using EVA foam as a
: core. I am confidant I can do this but any firther advice would be
: appreicated.

Try moving the seat back before modifying the kayak.

Adding a skeg will increase tracking (reduce maneuverability). It will also reduce weathercocking (or increase leecocking) and reduce broaching in following seas. You may want to test paddle your kayak in a crosswind. If it turns into the wind adding a skeg will be a good idea helping to reduce the weathercocking and increasing the tracking. If you want to increase tracking and not effect the turning from a crosswind in addition to installing a skeg you will also have to either move the seat back or add keel to the front.