Position of boards on the roof for best MPG

[U2U2U2]

Any data to support a different position of 2boards on the roof, that would gain any better mpg or unruly wind buffeting?

Specifically, side by side, nose or tail forward, OR stacked like land cakes with same fore aft nose positions ?

No other items will be on the roof, just the 2boards. Highway speed .

In the past a magazine posted some ridiculous findings, so not interested in bringing those up.

I intend to try several variations on a 2k trip.

[gvogelsang]

I always go stern (back end) first, and right side up. If you go upside down, you can ruin a rail with your straps.

[techno900]

When I put them on the roof (long ago), I would stack up to three with "bones" in between, tail first and upside down. I used small carpet squares to protect the rails with never any damage. This position seemed to be somewhat neutral with up and down forces from the wind.

Do use a second "safety" strap across the front, especially if you have "old" straps. Mine broke once, but I had just exited the interstate and was only going 25 mph when the front strap broke, causing the boards to turn 45 degrees to the right, but didn't come off the roof. Three boards at the time and a very close call. Two minutes earlier at 70 mph and it would have been bad news.

[coachg]

One of my windsurfing books has a study done by Mercedes Benz in a wind tunnel. Most efficient was nose first, upside down. This was of course with the nose of the board behind the point where the windshield met the roof. If the nose was over hanging the roof, all bets are off.

Coachg

[Number-nine]

Where are the myth busters when you need them

I think if you drive 80 to the beach so you do not miss the session it does not matter which way you load the board.

But it is important to tie it down really well

[isobars]

Search the forums for MANY discussions and lots of opinions. My bottom line is
"keep the front of the boards behind the windshield, close to the roof, and tied down as though they were your favorite dog."

[westender]

Depends on the aerodynamics of your vehicle. I carry upside down nose first in Gorge Rax. I can touch the lower board at highway speeds and it hovers in a neutral airflow moving up and down between the bars. I lose 2 mpg with the Rax and 2 boards. Highway position is to the rear and River position is forward for hatchback lid clearance when I don't drive over 60. YMMV

[rigitrite]

Yay! Something right up my alley! The simple answer is: it's way too complicated to figure out. (see what I did there).
Seriously, you'd have to model various board positions with CFD (computational fluid dynamics) which would eventually give you an idea which configuration is best. The thing about fluid dynamics is, it completely non-intuitive. You think that air flow would work one way and it should be totally obvious, but turns out it's often not obvious what's actually happening. The other thing about CFD is that getting your basic assumptions correct is really really challenging. A small change in your set-up parameters can yield wildly different results, unlike finite element analysis, which tends to be pretty straight forward. It would also be a real PITA to set up the mesh (the surface model of the car, roof rack, and boards).
To give you an idea of how counter-intuitive fluid flow can be, given all the elements involved in our two boards on a roof rack, what single element creates the most drag?

answer: the straps, especially if they're oscillating/vibrating.

Best idea is to go tail first and deck side up, as gvogelsang recommended, so you don't damage your rails. The deck of your board is many times stronger than the bottom and rails.

[U2U2U2]

[swchandler]

As a surfer from way back, tail first and bottom up. You definitely didn't want the wax exposed to the sun.

Of course, windsurfing boards don't involve a waxed deck, but why change a process that worked well for so many years. That said, I've long transitioned to a van consciousness where everything lives inside the vehicle.