Yes tie downs are very necessary. However, I don't like tying down someplace low creating a good chance of the ropes rubbing the paint in the highway winds. Instead, I created my own tie down points that work marvelously. I took an extra nylon strap and cut out two 18" and two 12" peices. (I'm guessing that's how long they are, but you could change to whatever you like.) I folded each peice over and used grommets to hold each one together creating a loop on one end and a hole to put a bolt through on the other. The front needs the longer ones because the bolts I put them under are back a few inches from the front of the hood. I used the bolts that are under the hood and back a few inches from the inside of the headlights. Those are strong since they are right into the frame. However you can choose whatever bolts make you feel comforable. The back is a little different since there aren't any bolts to use. I ended up deciding to actually drill a hole on either side of the trunk lip. There is a wall on the left and right side of the trunk towards the back that works great. (This is not IN the trunk, but the flat vertical wall just outside the weather seal.) I bought all stainless hardware and also used rubber washers on the inside and out to make sure it's good and water tight. So now there are 4 straps available to tie down to. The ones in the front easily fold down into the area behind the headlights and can be pulled out whenever needed. Close the hood and there are two neat and clean loops to tie too. The trunk is the same way. You don't see the straps other than when you have the trunk open. Fold them over and close the lid and they are gone. It's just what I did and I'm sure others will have better ideas, but it was cheap and does the trick. I hope this helps a little. If you think it's a good idea and wants some pics, I could take a few if you'd like. Good luck
gwarstadt@yahoo.com. Have fun Kayaking!
P.S. You mentioned that yours are longer, so I'm assuming that they are touring or sea kayaks. I weigh 260lbs so those don't suit me all that well. I have a 14' Wilderness Systems Pungo 140. It's also a hybrid. It tracks like a touring kayak, but it has a large cockpit like a recreational as well as the very comfortable WS seat. It's a great fit for a guy my size who wants to kayak! My wife is just learning, so she's started off with a 10' WS Palmico 100. It's short, but wide and overly-stable. So it's a decent kayak to use to decide if you like kayaking or not and not sink a lot of money into.