Regarding to carring the kayak by my car

Please secure your boat at the front and rear in addition to the side to side straps

I use Thule Bars with Thule kayak saddles on the front bar and Yakima Rollers on the rear bar OR Malone J racks front and rear. Yakima bars are good too. I know either of these combinations is not cheap but the thought of a kayak flying off at 70 mph is very unpleasant. 

John VB

Some of my friends use the 'V' foam blocks which snap over the bar of a standard roof rack.

Those foam blocks work much much better on 'square' tube racks like the Thule and knockoffs.

Also, you absolutely need both straps to the rack bars (over the kayak) AND end ties to 2 points at each of the front and back of the vehicle.

With 'proper' kayak cradles on rack bars, you can get away without end ties for short, low speed drives to the launch.

On the highway, end ties are necessary.

BTW, a flag on the stern of the kayak is also a very good idea and is probably required by law, depending on the overhang.

I highly suggest a proper j-rack. Especially for speeds above 80 km/h. I used to have foam blocks and almost had an incident on a highway with 110km/h. Sudden wind from the side slipped the foam block and the kayak broke on one of the shoulders on the foam (actually the block broke close to the center where it was thinner). I was able to stop safely but only because of the front tie (front of the kayak was tied to the car and managed to keep the kayak on the rails) as at that speed it tend to rotate - the wind captures the bow of the boat and try to rotate it. Since then I’m using thule j-racks – extremely strong and safe even up to 140 km/h (you have to be very careful about wind at that speeds).

TWO THULE HULLAVATORS

LEFT SIDE - THULE HULLAVATOR

RIGHT SIDE - TWO THULE HULL-A-PORT

Having carried kayaks, singles and double for over 3 decades, tying the kayak to the rack isn't important. They have to be tied to the vehicle and at both ends. Previous writers mention only the front, the back is just about as important. The front end preferably with an inverted V to stop twisting/slewing. From the kayak's bow(s) to two places at the front (each side/corner) and if possible same at the back. Many cars have two tow points at the front. If not then in the gutter that the bonnet (hood?) edge closes into there will be a suitable bolt. Here we can buy straps that the bolt fastens to giving a short webbing loop for the tie-down rope to go to. Easy enough to make if you can't buy them.

The OP didn't tell us the type of vehicle but many now have rails running fore and aft on each side of the roof. A couple of cross bars and foam cradles work well as the kayak(s) can't slide or twist if tied to the vehicle at each end.

As for speeds, 100 kph with strong side winds or at speed on tight windy gravel roads, the kayaks will not move if tied correctly.

Yes, tying to the rack is a good secondary security thing though how sure are you that the rack is fastened to the vehicle properly?

It's pretty easy to test if your kayak is secured properly.

Just grab the end of the kayak and shake the car with it.

Using only one tie at the stern- which has been mentioned and which I see quite often - is just slightly better than nothing.

The Thule Hullivator and most J-bars are not as secure as a set of 4 curved kayak cradles attached to a good rack, and the kayak will definitely wobble in them, so you will need good end ties.

I have carried kayaks with the Hullivator, with the foam 'v' or curved blocks over rack bars, with J-bars, and with the combo of 2 Hully rollers and 2 cradles. All of those are less secure than using 4 cradles per kayak.

Yes, make sure your rack is attached to the car. If this is a problem, you should also check that the wheels are bolted on to the car as well!

You want it tied down so it doesn't move when you do the end shake test, but you also want to be careful about to over tightening it. Particularly with the bow and stern ties, you can over stress your kayak by cranking down too hard.  You'll find that you fairly quickly develop a feel for how tight to go.

Also, try not to leave it parked in a hot sun too long. 

I agree 100% with Snowbound's comments about over-tightening the end ties.

I don't like the ratchet webbing end ties (provided with the Hullivator, for example) which are easy to over-tighten.

I prefer light lines (1/4" diameter at the most), generally in polyester (low-stretch), not nylon.

Thinner lines don't obstruct the driver's forward view as much, and don't vibrate in the wind as much, compared to webbing or thicker line.

Arrange good tie points to the vehicle and learn the trucker's hitch, and you will be good to go.

safety is the first priority, I think we should create a list of things must be needed for the safety gear! 

Just to reinforce the comments here...I've used Thule racks for almost 15 years now. I used the foam blocks on the Thule bar, with great success but after a few years, I got tired of fiddling with them so I have both saddles and J-cradles. J-cradles are great to use on days when the winds are not blowing hard, wind resistance is lower. The saddles, however, seat the kayak with less resistance so I use them on days when winds could be a problem. 

To reinforce the idea, if the car moves when you try to move a strapped in kayak, it's on the racks properly. 

Now you need to purchase something to set the boat on the rack. If you can easily get the boat up onto the rack a simple set of saddles or J racks are ok. But if you cannot lift the boat easily there are options, such as rollers (Yakima Hull Rollers are an example), the Malone lift system, or the Thule system.

   I am 74 and not as strong as I once was. I frequently transport other people's kayaks to repair them.  My favorite system is a set of Malone J racks and the Malone lift system.  Unless of course, I can get another tall person to help me load and unload the boat.  

  Please do NOT use ratchet straps. A lot of the repairs I do are the result of people tightening the ratchet until they hear the gel coat crack.

John VB