So I love my kayak and my little car, which is the only one I have, but the only racks that fit it are really expensive (with the rear extension) and look like they could easily pull off the door rails. It's a fun car and not meant to car top big things. Take a look at my solution and lemme know what you think:
http://www.phrancis.com/pics/PreludePics/images/DSCF1486.JPG
http://www.phrancis.com/pics/PreludePics/images/DSCF1481.JPG
So it's basically foam blocks with bent conduit attaching the two and keeping them from shifting on the roof. They're spaced as far apart as my roof will allow, but don't quite line up with the bulkheads. The cambuckle straps are wrapped through the doors, around the circumference of the boat (to keep from sliding side to side), in through the deck lines (to keep it from sliding forward and backward), and slightly twisted (to keep them from vibrating in the airstream). There are no bow or stern lines, since the car's lip spoiler gets in the way, but I'm able to cruise at 70 mph on the freeway and almost take corners as fast as normal. On occasion I don't position the boat perfectly or cinch it down as tight and a passing semi-truck send the bow off center. I have to pull over and readjust. So does it look safe?
I noticed this product has a similar design, but the rear bar could only be placed as far as the rear of the door opening, which would offer less lateral support: http://www.paddling.net/buyersguide/accessories/showProduct.html?prodID=1031&manfID=166&refer=gearSpotlightNL Still it looks easier than my complicated strapping sys and I could carry 2 boats!
my thoughts
Three straps around the kayak and through the car - good. Rear strap pulling forward, front strap pulling rearward, good.
Not understanding the conduit/foam connection. I am accustomed to strapping the boat to the foam, and strapping around the boat through the interior. Your way may be fine or even better, I just don’t grasp it from the two photos.
You could find a way to accomodate front and rear tiedowns, I think it’s worth it as an ‘early warning’ system that the boat might be shifting. Lots of threads on p-net about strap loops using factory under hood, or under bumper bolts. I had one occasion when a loose front tie-down alerted me to a loosening of the other straps, long before a problem arose.
bow/stern
I remember reading that some people would fashion loop for the tie down by open the hood, typing a loop around sturdy metal piece and closing the hood so that the loops is sticking out. Then you can tie your bow line to that.
Not sure why you are worried about the spoiler getting in the way. If you don’t care about paint under those pads rubbing away, a little damage to painted plastic shouldn’t really bother you 
It’s clever, but I would not have spaced
the supports quite so far apart.
As for bow and stern lines, there are ways, discussed previously on this board, to attach straps that emerge from the edge of the hood and the trunk. Such straps are attached to existing screw holes under the hood and trunk. I have not done this, but someone who has will almost certainly give you more details on the approach.
I improvised my first rack system, and I didn’t know what I needed to do to perfect it until I had been through some windy thunderstorms.
DITTO
Blocks to the boat, Boat to the car (2 straps)through the interior, use TWO straps for the redundancy; a safety concern…AND I use Bow and Stern tiedowns…
Works great when I take the car, I trailer when I use the truck…
block and conduit connection:
At first I had the foam blocks held down by the boat, but because the boat is such a long lever and the fulcrum point so far away, bumps in the road caused the front to bounce up and air to catch the front block, which sent it back to the middle of the sunroof. The conduit ensured that they stayed exactly spaced apart.
I’ll have to research the bow and stern tie downs to the hood or trunk structure.
spoiler issue:
the front air dam or lip spoiler would be pulled upward and ripped off if I attached bow lines to the pull loops under the front bumper. I will however investigate attaching them to the hood structure.
Yes, the paint on the roof and edges of the door where the straps come out are a little scuffed, but I try to use a car duster to clean up those areas and the foam blocks prior to use. A good polishing and car wax seems to fix the scuffs too.
block spacing:
Why wouldn’t I want to space them as far apart as possible? The boat is so long and the roof so short. I wanted to get the blocks as close to under the bulkheads as possible to prevent oil canning and to provide as much lateral support since I don’t have bow and stern lines.
I will however look at attaching bow lines to the hood structure.
tie down:
Nice, but would engine heat degrade the nylon? Someone once told me there were metal brackets that you could bolt down in the same place, but it sounded like they permanently stuck out of the hood. I suppose small cable loops would work…
Blocks to boat.
Not strapping the blocks to the boat spells trouble. I know a rookie paddler who had block blow-out this summer because he tried to balance the boat on the blocks and whiteknuckle strap it to the roof. Once I mentioned strapping blocks to boat, the lightbulb went off. Much easier. Rear and stern straps would help with air surfing.
I think you missed a point…
a strap through the block(s)and around the kayak and cinched tight to the boat…one in front of the cockpit combing, one behind it…
Think of it this way: the BOAT is the conduit that way…
No one is really answering your question
It doesn’t look safe to me! If I were driving down the highway at 70 mph and saw your set up (and I do check out every boat and tie down system I see along the road), I would stay as far away as possible.
The fact that you get out and readjust the setup after a truck passes you is enough information. It won’t be pretty when your boat shifts and a piece of the foam flies out. I would guess your car will take the worst of it. Heavy cross wind, big truck, and high speed will be a bad combination.
I am guessing that since you are asking the question, you already basically know the answer.
Remember that the primary function of bow and stern ties are protection from catastrophic rack failure. Your “rack” has a high probability of catastrophic failure. YOU need bow and stern lines!
Leverage!
The wind pushes on the front of the boat. The more leverage the wind has against the rack, the easier it will be for the wind to rip the kayak from your car.
Imagine this, you walk up to the kayak, on the car, in the driveway, and push the tip of it to the side. The fulcrum of your lever is the back mounting location of the “rack” and the force is applied to the front “rack” with the appropriate energy for the given leverage. Boy, I can’t manage to say that in a way that makes sense. But…
Now you mount the back “rack” to the back of your car (imagine you have a very long roof line) and then you do the same experiment again. Now the leverage on the front “rack” is much less because your lever has a different ratio between the place you apply force and the thing you are leveraging.
Still, I’m just no good at writing this. The point is, if the mount points of your rack were six inches apart, it would be very easy to take the front of the kayak, apply sideways pressure, and rip the whole thing from your roof. on the other hand, if the kayak were attached to the car about six inches from the bow and about six inches from the stern, you would not really need a bow tie-down, would you?
So it’s all leverage.
Mount the kayak on the same rack but move it back quite a few feet and the wind from driving down the road will try to straighten the kayak on the roof, like the tail of an arrow flying through the air. Move the kayak forward and the bow then starts to act like the tail of the arrow and the kayak wants to invert. there is no need for a bow tie-down in that situation because the bow is placed very close to the roof rack.
So:
- Add bow and stern tie-downs but remember that they are not quite as effective against sideways pull as upward pull.
or
- Move the kayak rearward on the car so that it tends to keep itself straight and so that the bow is close enough to the roof rack that it doesn’t need a tie-down (or tie it down since your rack is crap and no one wants a kayak in their windshield!).
Again, sorry for the accurate but badly worded description of the leverage issue. I’m feeling a bit sleep-deprived today.
Dave
P.S. The fulcrum could also be the front rack mount point with the leverage being applied to the rear rack. Either way, the more the kayak sticks out the front and the closer the rack mountings are to each other, the more leverage the wind has on the front of the kayak. Again, leverage ont he rear tends to push the kayak straight except in a cross-wind situation.
boat to blocks:
No, I get the concept it’s just that implementing that is more difficult than this conduit-block sys. First of all I’d have to place the separate light weight blocks perfectly spaced on the roof. Then I’d have to lift the boat, step up on the door frame and gently place it on the roof, but so many times the wind has blown one of the block off and I’m left straining to hold this boat. Also, running straps through the blocks is very difficult. Strapping the blocks to the boat first before lifting it onto the roof would really grind lots of sand into the roof paint from the dirty block bottoms.
engine heat
I haven’t had any problem with engine heat. The straps will really only be as hot as the body parts their attached to. If your hood or fender get too hot to touch while driving, maybe you’ll have a problem. But I don’t know any cars like that. 
Unsafe leverage:
@ wenoahrider and rectorsquid... I totally understand the leverage thing and the safety concerns. That's why I'm asking on this forum to confirm a few things, get feedback, and possibly learn a few tricks that other experienced people have up their sleeve. The only times I've had to pull over and readjust after high winds or a passing truck was when:
A - the foam blocks weren't attached to each other and shifted in the wind, causing the boat to shift and the straps to become more lose.
B - I didn't attach the front and rear straps as far forward or backward as possible, causing a longer lever.
C - The front and rear straps weren't wrapped around the circumference of the boat, allowing it to slide left or right.
D - The boat was too far forward, not allowing the stern to act as a weather vane and keep it straight. I suppose deploying the skeg could help in that way. One problem I've encountered with the boat further back is that the rear weight causes the bow to point up more in the wind (the chatham's bow has some rocker). I could use a taller rear foam block to keep the nose pointed slightly down I suppose.
The idea of a hidden attachment strap under the hood and or truck structure sounds promising in shortening the lever...
blocks to boat
I completely understand your two concerns. What I have done, is to lay the strap through the block, buckle on the roof, and then load the boat. So far, maybe by luck, but the wind hadn’t moved the foam block enough for me to have an issue with it, prior to getting the boat onto it. Then I complete strapping the block to the boat. This makes the foam and the boat ‘one’ and you just tie down the assembly. 8’ straps worked fine for the blocks-to-boat, and 15’ straps through the interior tied it down.
It was hard to reach over the boat to get the buckle, to strap the foam to the boat after it is loaded, but I’m doing this on a Jeep sitting at least 6" higher than stock. I suspect on your car it’ll be a good bit easier. With the blocks tied to the boat, you’d really only need the front and rear straps through the boat, plus the front and rear tie downs for back up.
it’s ok
I don’t think that it’s a bad set up. Because the boat is strapped to the car and not any sort of roof rack the only way that I can see the boat coming off the car would be for the straps to come completely undone or for your whole roof to rip off. Now if the foam pads to fly off it’ll beat the crap out of you car but will probably stay attached to it. You could always strap your rack to the car first and then put the boat on it.
One other thing that might help is if you run the bow/stern ties through you car like the other straps, if you did that I can’t think of any way that the kayak would come off, and you don’t risk causing damage to your hood/trunk. But I’m no expert on the subject, just my 2cents.
3-point front rope
The fact that it moves sometimes when big trucks pass tells me it’s not safe. However, I think the fix is pretty easy - you need a 3-point front tie down to fix the front point in place. It should be pretty easy to find a bolt or hole underneath. For the side ropes, a last resort would be to drill holes and insert a nut and bolt in the plastic bumper or front of the wheel well on each side.