Years ago, I had a Mirror Dinghy (still have it, but no longer use it). I used to load this by myself onto the roof of my car. The front roof bar had another bar held in place a pivoting bolt on one corner. I used to bring the bar parallel to the axis of the car and lock it in place. I then wheeled the boat up to side of the car, stern first. I would then lift the bow until the boat was vertical, and gently lay it onto the pivoting bar and clamp the gunwhales of the boat to the bar. I then went to the stern of the boat, lifted it up and walked it around the rear of the car and placed the boat onto the rear roof bar - upside down. I then bolted the other side of the pivoting bar to the front roof bar. I could do this easily by myself because at any given time I was only lifting half of the weight of the boat. By having dedicated clamps (U bolts) to clamp the gunwhales onto the bars, I didn't need any ropes to tie the boat down. Now, I have a 4WD instead of a regular car, and this may make it a bit harder to get the boat onto the roof because of the extra height, but the principle still will work.