The Y Mounts also work very well with Stand Up Paddle Boards (SUP) and Surfboards. For these lighter items, often fiberglass, it is usually easier and better to leave the foam on the mounts and simply place the boards on top. The slight bend in the Y Mount, combined with the contouring foam will allow just about any board to sit nicely with the bottom down.
You can click the Mounts into any holes available; however, if you use the outer most hole and carry the kayaks or boards past the edge of the crossbars this allows the center of the bars to be free for other items or for a J Kayak Mount, Paddle holders, Gear, etc.
To strap down Kayaks and Boards on the Y Kayak Mounts you want to use the EXR 9 foot or 12 foot strap - one for each Y Mount. The strap should start UNDER the crossbar itself at the inner side of the kayak/board (side closer to center of vehicle). Then both ends of the strap come straight up, then over the kayak/board toward you (outside edge). Then the end of the strap without the buckle should wrap around the lip on the edge of the Y Kayak Mount and feed up through the buckle. When all the slack as been removed, pull down on the strap and push up on the kayak at the same time. This will remove any slack and tighten the kayak/board down.
It is the installers decision as to whether the front and rear of the kayaks and boards need to be secured down. In these instances they prevent the items from shifting forward or backward. In some instances, for example on kayaks that have a hole in the middle, a professional installer may run the strap through the kayak/board itself and mitigate any chances of the kayak/board shifting forward or backward. If you are not a professional kayak roof top installer we recommend you use front and rear straps as well. See video of Y Kayak Mounts being used here.