I would like to travel to Europe from the U.S. East Coast with a whitewater kayak (approx. 8' long, 40+ lbs or 240 cm 17+ kg) but no airline seem to be willing to take an item that large. Does anybody have experience with airlines that have more liberal luggage policies, understanding that it may cost some more?
Icelandair takes large sport items, such as Vaulting Poles, Windsurfing boards, Kayaks, between Europe and North America, for a fee of 16.400 ISK/117 EUR/168 USD each way.
Their hub is (unsurprisingly) on Iceland, and they fly to a number of destinations in North America and Europe. For the U.S. East Coast, they currently fly to/from Boston, New York, Washington D.C., and Orlando. Their European connections are mainly in western Europe. See their destinations page and their routemap.
(I took a bicycle with Icelandair, and it was only 39 EUR extra from Stockholm to Toronto. Not bad at all, in my opinion)
(I do not work for Icelandair, but I can still tell you that as a bonus, you can take a long layover on Iceland, I'm sure it has excellent whitewater rafting! This doesn't cost any extra for the ticket. I don't know if the no extra cost applies to baggage fees.)
"willing to take" is a bit different from "will it fit in the hold". A kayak is pretty big, and it may simply not make the bend between the cargo door and the hold itself. Bicycle boxes, for example, just fit in medium-haul planes like the 737. They don't fit at all in smaller planes. And a very large plane isn't a solution either if the entire hold is set up for containerized cargo.
It sounds like Iceland Air has enough demand that they set up a process. So just call a few airlines. There's also the option of air cargo.
A very fast internet search shows things are not so expensive - it may be cheaper to buy one in Europe and resell it (or just leave it behind) than to fly yours over.