Drifter, a knee tube is any piece of pipe (often PVC) of any appropriate diameter, that is affixed inside the boat to the underside of the deck just before the cockpit. The forward end is closed (capped). It utilizes the unused space there for storage of whatever--water bottle, lunch, anything that will fit into the tube. Some people glass theirs into position; you could also use straps bolted into place--whatever works. I think the Brits invented them. The tube shouldn't be so big as to interfere with getting in or out of the boat, but big enough to hold enough to make it worthwhile. Any length that will work. Good luck.