My Opinion
I asked a similar question on this forum a few years back and got lots of feedback, both pro and con.
My decision was to always use the bow and stern tie-downs. Here was my rationale:
o It only takes a couple more minutes to add the extra lines.
o Although I have an excellent Yakima rack system in which I have full confidence, s*** happens. Several posters to my original question had personal experiences where a top-of-the line rack failed. Didn’t happen often, but it DID happen!
o Using a bow and stern line CERTAINLY adds an added level of protection. It may not be necessary with a properly installed top-of-the line rack system, but it DOES add an EXTRA layer of protection for those rare situations where something unexpected occurs.
o Given our litigious society, the potential financial liability (not to mention the emotional impact) one would face if their yak or canoe killed or injured someone is so great that the extra couple minutes of prep time seems insignificant.
That’s my 2 cents.
When I was in highschool and college
I had a canoe that I want to take places. I installed pipe insulation on the gunwales and put in on the roof of any car or van with two bow lines and two stern lines. I didn’t use any lines over the center. It would shift around from side to side about 6 inches every time a big 18 wheeler past me but it never came off.
It wasn’t until years later that I saw some one used a strap throught the center inside their doors. After I added that all the boats road rock steady.
Later I got cars with roof racks and still added boaw and stern lines on many occasions. The best thing I like about bow and stern lines is that you can see then while you are driving so you know the boat has not moved on the roof.
I’ve never used them either …
The majority of the time I am paddling local, short trips (10-15 minutes) rarely get going more than 40 mph, and have a relatively small kayak (12.5 ft.). I have a Yakima Bow Down rack (I highly recommend this) on top of their standard towers and secure it with the ratchet strap tie-downs at two locations fore and aft of the cockpit - it is rock-solid when tied down correctly in this manner. For longer trips, higher speeds, and a larger kayak, I would agree with the other posters and would add the additional lines.
Tie downs:
There was a good post on this recently, and I passed on a few of my thoughts. Since then I had an unexpected failure, (Murphy?) Using good, new condition tie down straps, a tension spring on the buckle failed. It still held firm, but I don’t know how long it had been hanging on. I use a front tie down similar to the one shown by Fat Elmo. Nothing on the rear. When I was new to the sport, I always used a second tie down over the front cross bar, sort of a belt and suspenders approach. I have gone back to this practice. Carrying deck side down gives me a constant view of my kayaks from the drivers seat. This combination gives me a bit more peace of mind and seems like cheap insurance. I carry on a Dodge Grand Caravan with factory longitudinal racks and Yakima crossbars, set nearly five feet apart.
good tie down procedure
I use two ropes in the front, two ropes over the hull, and one ratchet strap in the rear. I pre-cut ropes for the front, and put carabiners on the ends to clip onto the loops I tied under the hood. Pull the front of the boat down, clip on the carabiners, walk to the back and use the ratchet strap to tighten it up, then throw two ropes over the hull. The rack is the old style rain gutter kind, and padded with 4 inches of high density foam. Using 'biners allows me to load two boats and secure them in about 5 minutes.
ratchet straps
Careful with the ratchet straps. It’s easy to apply far too much pressure to a kayak with those. The spring-loaded cam straps are plenty tight, and they can’t distort or crush a kayak.
keep your loose ends near the boat.
Another important tip for bow/stern lines is to attach them with any extra rope near the boat, rather than near the bumper. If that extra rope comes loose, and gets under a tire, it will rip your boat in half. If the loose end is secured near the bow or stern then it shouldn’t be long enough to reach the ground if it comes uncoiled.
Very good tip
My wife (not me of course) put a ratchet strap on out trailer and the loose end came loose and went under the tire, in the 35-40 mph range. It made a loud pop as the strap broke after twisting the ratchet. I don’t think it would tear the boat in half, but it most likely cause damage to it or the car/rack.
use them
Many years ago, while we were all headed to the Poconos to celebrate graduation, my high school friend died instantly after a big metal canoe came off a vehicle driving locally (just a few miles)
at ~40 mph. She was in the passenger seat of the following vehicle. She was decapitated. The people
who owned the canoe were devastated.
Takes a few minutes to use tiedowns.
P.S. A Yakima or Thule rack are indeed well made,
quality systems. Own a Yakima system myself, love it.
But human error in strapping boats to the rack will always be a factor. Unforeseen wind gusts buffet boats, long ones especially. They can become a big sail on your vehicle.
If you depend on just the gunwhale straps you cannot tell if they are loosening or coming off until it’s too late. Using bow and stern tiedowns you have an early warning system that something is going wrong plus another two points of attachment.
What is a few extra minutes compared to damaged boats or being the cause of a serious or fatal accident?
Not always true…
I have good quality Thule racks, and tiedowns. They are done right, my partner is a sailor, and he is extremely safety aware. We were at 55 mph, and even with bow and stern tie downs, the wind caught one of them, and tried to wrench it sideways. The wind didn’t succeed, and we stopped and did an additional cross-tie, just to be on the safe side. If we hadn’t had the bow-stern tie downs, the outcome could have become a disaster.
“Be safe out there”
Donna
Hooks
I was afraid of that too and modified my Thule tie-downs to use carabiners instead of hooks:
http://good-times.webshots.com/photo/2917662820102304487AmvLCJ
Thank you.
Thank you for all of your feedback. I really appreciate it, although now I’m a little paranoid about using the factory rack. 
I saw the loopy things that Fat Elmo posted, but I’m not sure where I’d find those - any ideas? And I’m still not sure where exactly to attach them on the front and back of the car (Toyota Sienna minivan).
Seen that happen
A bowline not secured to the car wrapped around the front wheel of a friend’s car and imploded a kevlar Looksha 4 on the crossbar.
No matter what you’re doing, you have to do it right.
Those loopy things
are your own fabrication. Open the hood and look for a substantial bolt. Double a piece of nylon webbing, burn a hole through it with a hot nail, and secure it beneath that bolt with a large washer. Use good judgement here, it won’t be effective unless you keep a mind to strength. Get some advice, if needed, from a mechanical friend.
top ties are what they are called
rutabaga has them.
Nylon webbin’ - yer kin find it in any
outdoor store or hardwaar store.
FE
depends on trip
Down to the landing rarely use front or rear tie downs. Highway speeds(over 35mph) use front tie downs primararly because of wind shear from large trucks. Only use rear tie downs if rack is too short. Rear tie downs can be a real diasaster backing up. I always have a tie down place available and rope for rear tie downs if needed due to conditions or local cops…
Probably don’t need them…
However, a good properly installed roof rack such as Yakama or Thule combined with cam-lock buckle straps wouldn't have helped in this case:
http://www.kajak.nu/bilder/bilder2003/div03_1/rails.jpg
Luckily a front tie down prevented the boats from leaving the roof. With a only PhD in structural mechanics I don't think I myself would have been able to foresee that particular accident.
I know of a similar accident at my local paddling club. No tie downs meant that the boats and rack all strapped together into one very solid unit landed in the opposing lane. Luckily the highway was empty and nobody got injured/killed.
Use tiedowns!
It seems that JackL has argued against the use of fore and aft tie-downs for years. This is America and JackL can do what he wants and advocate his own personal beliefs. Those are his rights.
However, the originator of this thread should take note. I and the vast majority of other posters to this thread clearly feel that tiedowns should be used (just re-read the posts). Furthermore, most (if not all) of the boaters I know personally also use fore and aft tiedowns. Have you ever heard, read about, or seen something occur that you could have sworn would never happen in a million years? I certainly have. I don’t believe my rack system will ever fail. However,if it does fail those tiedowns may give me a few extra seconds to avoid a possible catastrophe. Personally, I’d rather have my yak hanging to my vehicle by a rope and tearing up my car than flying through JackL’s front windshield.
Each to his own.