When you say old Seadart do you mean a really old seadart? There once was a composite model (fantastic sit on top), then a yellow polyethylene model (very very good SOT) , then in later years the brand got sold and they turned it into a turd brown fishing kayak (don’t bother). The older models had a serious modeled in skeg. If you have one of these, tip the boat over and sight down the hull and make sure the keel is straight . If not then you have a problem. If the keel is straight, then you are the problem. You may be weighting the boat funny, by where your butt sits, and how you push on the peddles or just not using good paddling technique. My old seadart paddles straight as an arrow with no effort. Also was the problem in wind?