One of my biggest fears, is going down a two lane highway, with a cross wind from my Left, and meeting a Semi coming the other way, at or above highway speed !!
It will rock my little S-10 something fierce !
Had one instance, that required repair, to my canoe.
There is little one can do, but since, it has nearly become a phobia for me !
Jim, try it going across Kansas with boats atop a ’67 VW bus! Experienced VW bus drivers turn slightly towards the semi as it passes, and then slightly away from the sucking vortex as soon as the semi passes. It was like piloting a sailboat in confused winds.
Wind can be an issue anywhere and anytime. Crossing long, tall bridges for example. The worst windage experiences have been travelling across the Great Plains. We tie our boats down seriously tight, always with two belly lines, bow & stern lines and gunwale stops. Even so:
The gunwale stops on the Quick and Easy crossbars are strategically located pieces of 1” tall wood, positioned to trap both the inwale and outwale betwixt. That gunwale trap works very well, until it blows so freaking hard that one hull jumps the too-short gunwale stop.
It blew so fiercely on that trip across the plains that we had a hard time getting one of the truck doors open at a rest stop, and when we did a mini-tornado swept through the maps and paperwork inside the cab (and my partner regretted leaving some loose cash in a cup holder, although watching him chase a fiver across the parking lot was schadenfreude funny. He did not catch it, but he chased it for a long dang ways).
On a similar trip across the plains the force of the wind simply broke one of our gunwale stops. Which was not fun to DIY fix on the side of the road with the wind still howling. IIRC YellowCanoe had a crossbar tower destroyed in the Great Plains wind, which necessitated an awkward adventure.
To the original spacing question, with two canoes on the truck I will space them at least a few inches apart, not so much for aerodynamics as for assurance that they can’t possibly shift and rub against each other. We carry four boats on the van racks, all gunwales-down and offset nestled, two positioned fore and two aft, with each hull resting on two of the four Quick and Easy crossbars (11 feet of level rain gutter on the van). In order for all of the boats to be gunwales down they need to be positioned on the racks without too much cantilever overhand they are barely an inch away from each other.
One boat shifted in the wind, still in the gunwale stops, unnoticeably in bowline view. In the worst possible shift; the bolt head from a rudder pedal slider was pressed against the gel coat of the hull next door. A couple hundred miles of jiggling around left a deep, jagged scar in the gel coat.
About the actual aerodynamics, boundary layer and etc, I have little idea. I think about car advertisements that show a streamlined vehicle in a wind tunnel with smoke blowing tight across the hood and roof. We do not have a home wind tunnel, and our usual boat toting vehicles, old CR-V, older Ford van and the Tacoma, are not especially aerodynamic.
I do kinda wonder where the airstream, deflected by a non-areodynamic hood and windshield, ends up; trapped inside an open hull or streaming along the roofline underneath?
In empirical evidence carrying a couple of decked canoes sees a little less reduction in MPG vs a couple of open boats. No boats on the roof rack vs two open canoes on the racks sees a 3 – 4 MPG difference on the CR-V or Tacoma (in ideal conditions, no-traffic flat eastern coastal plain inter-State cruise control). The big 8-cylinder van only looses a couple MPG with four canoes on the racks, even when the van is burdened with a four people and a half ton of cross country gear.
One final
Please Don’t note. A friend bought a little used Bell Magic. The seller offered to meet him half way. The seller elected to put the Magic on J-cradles, and cinch it down tightly with cam straps.
A bit too tightly, or maybe it was windy, or both. The little used, once pristine Magic arrived with the Plexused foot brace broken out and cracks through the foam ribs. I had harbored no doubts that gunwales down and flat on the crossbars was best, but that convinced me.
