It's the flutter thing.
Posted by: bartc on Sep-20-12 5:16 PM (EST)
That's the purpose of the ridge and it works well to stop any flutter, which can be a problem if you don't cant a GP.
You should trust the Aleuts and the Inuit on anything with a kayak. They were sophisticated in their technologies for their intended uses, and this wasn't entertainment to them, it was literally life-saving!