Aerodynamic question

Anybody have anything other than an opinion on this?

Small car, Thule Roof rack, two pairs of J racks, two 12’ kayaks.

For traveling at highway speeds, is it better to have the two kayaks:

(a) close together and as close to the center-line of the car as possible

(b) as far to the outsides of the car as possible

© whatever looks good

(I know it makes a difference getting them on and off the racks, I’m talking about a four day drive)

This is a serious question, at least it is for me - thanks


I liked them…
Close to the outside edge for easiest loading…

"easier loading"
It will be four days of highway driving - so, easier loading is not a big deal. I’m just wondering if air flow would be different with different positioning?

Don’t know about air flow…
…but the closer to the towers, the better. Especially with not quite square bars.

You might get…
less rolling moment with them closer to the center.

Yes, the air flow will be different

– Last Updated: Jul-06-16 11:49 PM EST –

But I have no idea which method will provide less drag (hey, at least it's not an opinion).

When packed close together, there will be a slight amount of "funneling" of air into the narrowing gap between the forward ends. On the other hand, a wider spacing will expose more surface area.

With canoes, I think widely spaced is better on account of that "funneling" aspect between a pair of much larger surfaces than will be the case in your situation, but there's less opportunity for wide spacing in that case too (wider profile of the boats).

Best case is flat
If you can fit them width wise without J racks, that is best. But if not, then it doesn’t matter a whole lot. I would think the biggest gain you could make is by making a draw string skirt that would cover the opening. This would promote as laminar flow as possible. And try to reduce places where you are forcing the air to compress into a channel.

I agree that flat is better, which can be done with V-bars (I love posting this picture):

If you stick with the J-cradles, for a long trip I’m guessing that having them close together will be marginally better. In the confused air flow over the top of a vehicle with bars and cradles and boats, two boats close together may tend to act as a single obstacle. So in that respect, there may be less effective surface area, per what GBG said above. Also, if air is funneled between the boats, the aerodynamic effect will be to suck the boats together, which is preferable to having them waggle back and forth independently.

BTW, there will not be any laminar flow anywhere over the top of any vehicle traveling at highway speed, but fewer obstructions and smooth lines are better in turbulent flow.

I would agree

– Last Updated: Jul-07-16 12:50 PM EST –

having space for air to pass below and between the boats gives it the path of least resistance. A kayak or canoe on its own is a pretty aerodynamically efficient object, save for the cockpit (no surprise since they're designed to be hydrodynamically efficient).

Car makers are cutting all sorts of holes in front ends and fenders to allow air to pass through rather than around.

try different setups

– Last Updated: Jul-07-16 8:01 PM EST –

n watch the tach/speedo.

travel same direction road wind direction n speed..

as my rig with an upside downSolo or Rendezvous both bow to thwart sealed off with ply, a small foil against the first upright to gunwale....

and a Solstice Titan on foam blocks are slipperier noses apart allowing a wind channel down the center than noses together with sterns apart.

? guess is wind eddies inward onto the roof between the hull Vee then causing extra suck against the rear van doors....noses closed together formed an extra strength vortex back there...

that is blown off the doors with a center air channel.

akin to the Nissan JeepVagen requiring a larger motor when the design crew didn't wind tunnel test with results.

my usual aero canoe kayak spiel falls on deaf ears.

Not so when speaking with a Nissan Drogue owner plus rack n canoe ........

he said, I'll tack an airfoil on this week.....


IF airflow under the rack is eliminated AND airflow is maintained continuously off the windshield THEN\

Wide at front with narrowing at rear, narrowing as an experimental device ie your vehicle is not my nor the hull shapes……may work better than other alternatives.

Narrowing may force a plumenous eddy up over the vehicle's rear. My vehicle is a Ford Econoline with a barn door rear. Such a plume upward would disturb the suck the usual rear eddy has on those doors.Like this:

or at a higher speeds without ground effects, a single airplane vertical tail.

But once NOT forcing air up over hulls …then the eqauuuution changes.

But try that n see what.

A roof rack’s leading edge is airfoil covered from roof to above bar….best is to bar then as a U shape holding the hull and slightly above in height with 6” extra either side…

And canoes have a front plate cover interior hull from bow plate to first thwart….my thwarts are directly above the vertical mounting 2x6”

The front airfoil has another airfoil plate above it to continue into the canoe bow cover plate

Additionally as the rack is a solid plywood platform not an open space tween bars, canoe side are covered with cardboard held in with plaster mesh screws…in good weather.

on airfoils is impressive…figure that…

serious mistakes are made.