Carrying Capacity - Kayaks on Car Roof

Looking for roof top rack carrying capacity on my 2007 Prius, I found some overall capacity recommendation <850lb for people + luggage, but nothing about weight capacity on the roof.  Rack Attack installed a fixed track Yakima Q Control-tower system with crossbars, so a quality roof rack.

I've successfully carried one kayak (~65lbs) and buzz along the interstate at 70mph... but now I'm considering hauling a 2nd kayak (probably another ~65lb). 

Any opinions if this is safe to haul 2 kayaks rooftop? 
Any experience/knowledge/advice?
Don't worry, I'm OK with the resulting MPG.  Just want some help in Smug Town.  Thanks.

Yakima's specs say that the max load capacity is 125 lbs on a 2007 Prius with the Q towers.

We routinely carry 4-6 whitewater kayaks on Thule and Yakima racks on the roofs of various vehicles without any problems.  4 whitewater kayaks at 45 pounds each is 180 pounds, and six whitewater kayaks is 270 pounds.  I have noticed that when the handbook for a vehicle or the specifications for a roof rack system lists a weight capacity for the roof rack, the weight seems suspiciously low.  I figure that if the roof gets dented or a rack bar gets bent, the car or roof rack manufacturer wants to be able to say "we warned you not to put so much weight on the roof rack."

However, each vehicle is different.  A load which the roof of most vehicles would handle with ease might crumple the roof of your Prius like an aluminum beer can.

Here is a typically loaded roof rack at the Jarbidge River launch site in southern Idaho.

Don't forget that cars are designed to survive roll over crashes.  Two tasks should be fine.  I put four on my 2008 civic a few times with no problem.

Oopsie, I have the Control Tower, not the Q tower setup -- it seems sturdy. 

I understand the integrity of the body has some rigidity for crashes, but the Control Tower Track is only affixed to sheet metal on the roof with the flared plus nuts along the track base.  Seems like the wind load on 2 kayaks could possibly damage the roof, and worst case lift those nuts right through roof.  Visions of a sardine can come to mind.

Langcod - where did you find the Yakima info?  Since I actually have Control Towers, I'd like to look that up.  Control Towers are not the typical rack install on a Prius (the Qtowers with Qclips are typical).

Oopsie, I have the Control Tower, not the Q tower setup -- it seems sturdy. 

I understand the integrity of the body has some rigidity for crashes, but the Control Tower Track is only affixed to sheet metal on the roof with the flared plus nuts along the track base.  Seems like the wind load on 2 kayaks could possibly damage the roof, and worst case lift those nuts right through roof.  Visions of a sardine can come to mind.

Langcod - where did you find the Yakima info?  Since I actually have Control Towers, I'd like to look that up.  Control Towers are not the typical rack install on a Prius (the Qtowers with Qclips are typical).

The wind load is what puts kayaks in the road at speed.  Racks handle downward forces well but fail when you have upward forces.   Make sure you tie down the bow and stern to your cars tow hooks.  It makes all the difference.

-Spot-

Bow and stern lines are critical.

I pulled that from the Yakima Fit List book. Unfortunately they don't put any specs on the Control Towers and Landing Pads for a Prius.

Bow and stern lines are critical.

I agree that bow and stern lines are important.  If you have several kayaks stacked on their sides, it also helps to tie the bows and/or the sterns of the kayaks together so a kayak can't squirt out of the middle of the pile.  I've seen a lot of kayaks fall off vehicles because they weren't tied on adequately.  I've never seen a kayak rack fall off a vehicle.

A couple of "kayak off car" stories:

1.  I was driving down the interstate in a friend's vehicle with our kayaks on his roof rack.  My friend had only attached his kayak with a pair of rubber bungie cords, a technique which I don't recommend.  All of a sudden my friend's kayak shifted, we heard a big thump on the roof, and we looked back to see his kayak cartwheeling down the highway.  He screeched to a stop, and as he jumped out of the vehicle he muttered "Damn, I wish that had been your kayak."  His kayak was fine except for some scrapes and road rash.

2.  I was driving with the same friend on a bumpy dirt road along a river with four or five kayaks on top of his brand new vehicle.  One or more kayaks started sliding out of the row of kayaks.  He braked to a stop, and one by one each of the kayaks slid off the roof rack and bounced off the hood, scratching the shiny new paint with the sand on the kayaks en route.  He jumped out of the vehicle and was stomping around in the road, fuming.  I walked up to him and asked what happened, hoping I guess to receive some reasoned explanation for the failure of his kayak tie-down system.  Bad question!  He roared at me "The @#$%& kayaks fell off the car!" and stormed off.  It took him about fifteen minutes to calm down.

3.  After a day of whitewater kayaking, I absent-mindedly forgot to tie my kayak to my roof rack before heading home.  When I reached home, I discovered that the kayak was missing.  I drove 45 miles back to the take-out, watching for my kayak the entire distance.  I found it, completely unharmed, in some bushes about half a mile from the takeout.

4.  A friend was driving down the interstate with a hot new kayak on the roof.  The kayak came loose, and got pulverised by a semi which was following him.  He picked up the pieces, and they were on display for several weeks at a local kayak shop for the amusement of other kayakers.  It was a pretty sad sight.  But it could have been much worse - the kayak could have gone through the windshield of a vehicle behind him and injured somebody.
 

PmmPete -- Thanks for the good tidbits!   

I've always used my bow-stern tie downs and even tie the kayak body to the rack when i transport on the roof top, so that does help to secure things.   I just hope I'm not posting a new thread in the DAMHIK forum next week.

Thanks guys for your opinions.

+1 on the bow/stern lines. Just don't ratchet them down to where you're bending your kayak (seen that!) - snug is fine.

I have no experience with your exact setup, but just in general I've found that the weight recommendations for car-top stuff errs pretty far on the side of caution. I'm not recommending you exceed any of the stated limits, but I will suggest that if you properly secure the kayaks you'll probably be fine even if you're outside the limits by a significant degree. Most of my experience is with accessories installed onto factory racks/bars on SUVs - misc. kayak cradles and a Thule 2100 cargo box.

I've always used my bow-stern tie downs and even tie the kayak body to the rack when i transport on the roof top, so that does help to secure things.   I just hope I'm not posting a new thread in the DAMHIK forum next week.

What do you mean, "EVEN tie the kayak body to the rack when I transport on the roof top"?!?!?  ALWAYS tie or strap the body of your kayak to your roof rack!  The bow and stern tie-downs are to back up the straps around the kayak by helping to keep the kayak centered on the rack, both from side to side and fore and aft.  If the bars from your roof rack are close together, the bow and stern tie-downs also keep the bow and stern from bouncing up and down.  But I wouldn't try to keep a kayak on the roof of a vehicle solely with bow and stern straps.

http://yakfish.isaac-online.com/wp-content/uploads/2012/02/photo-1024x634.jpg

FWIW, with two kayaks on the roof I got better gas milage than no kayaks. Go figure.

Anyways, strap it all down nice and tight. Towers can fail, as shown above. The manufacturer would not cover the loss of the kayaks because I exceeded the maximum weight limit: 100lbs, minus 15lbs for the cradles...

Poor wording on my part.   I ALWAYS use the bow and stern lines "in addition" to the body straps for transporting a kayak.  So I have four tiedowns - 2 around the body to each of the rack crossbars, a bow and a stern to the chassis of the car.

I'm still reluctant to put that much dynamic load on the beer can roof involving two 65lb 13' kayaks.  If we were talking about a suburban like you showed in your photo, I wouldn't be as concerned.  It's probably OK, but doesn't seem worth the risk.

http://yakfish.isaac-online.com/wp-content/uploads/2012/02/photo-1024x634.jpg
Towers can fail, as shown above. The manufacturer would not cover the loss of the kayaks because I exceeded the maximum weight limit: 100lbs, minus 15lbs for the cradles...

That is a mighty sad picture.  What kind of roof rack were you using?