– Last Updated: Oct-07-16 1:35 PM EST –

for explanation.

This fairing/airfoil rack is in Padnet at 2 other searches

Rack is on Quick n Easy mounts crossed with 2x3" 1/8th steel well painted and drilliumed rectangular tubes, 2x3 joist on ? 18" centers over the steel. Then 3/8th's plywood painted with Rusto. Joist are rustoed, primer n topcoat white, steel has prob 3-4-5 coats, 2 primer then tops.

After sun curing, seams are done with GE silicone. There are intermittent rumblings on roof sealants, exotic 3Mroof sealants (me), RV midlands sealant...GE apparently lays there sealing, no problem with surface prep

airfoil is cheap HD paneling, painted well and replaceable.

The canoe rack, kayak goes on foam blocks atop the solid platform, is done with drywall electric drill screws thru standard wood, and as seen, uses a center angle brace from deck to front canoe support.

The eyes on front brace are a new mod with cam strap running thru eyes n around the tranverse 1x ends then over hull and tightened. Effective now with one bow line n 2 rear lines, a second cam strap now without eyes at rear support. Am adding a second bow line.

On dry days, 2 precut cardboard sides cut from Wal box stock are lathe screwed onto the 1x for/aft side runners with tops inserted into canoe n bottom laid on deck with a 90 degree bend n flange...from transverse 1x to 1x

If doing this again I would buy an aluminum welder n jig the rack framing with aluminum. Time spent maintaining a permanent wooden structure is consuming. If accounted for the Al draws more than even.

What a drag it is without the foils. Parachute attached vs no parachute. Slippery .....

I am criticked for not placing a wind cutting inverted V foil on the beneath hull plate but ran out of time for this trip west. We acknowledge.

So PYFOOYAH n place a small foil before the front support then cut a bow plate from paneling n scrw down with #8 SS machine screws...we have SS self tappers from McMaster Carr

rack is electric with....burglar alarm sensors n Hella auto horn alarm, yellow side panel LED running lights ( on other side panels) 2 front runners on foil....Interstate trucker approved ..

n 2 2" LED turn signals at rear corners for interstate ON RAMPS. With an extra under rack deck red stop LED in center.

Top boasts a 6 LED fog unit on strut as BACKUP lamp going with a chirp chirper n 'warning vehicle is backing' unit is Spanish. Freezes the action at Wal or McD. All lighting hooked to a separate flasher unit. Both turns at same time flash for road hazards n tie ups. Fog from Amazon 2 for $24

Lighting from Levine Truck Lights NY/Conn

Rear is left open between deck n van roof allowing airflow from between front foil n deck n around front foil sides an exit at slower speeds than the blowbye.

Things aren’t always what they seem

– Last Updated: Oct-07-16 6:07 PM EST –

This reminds me of all those people who thought they were saving tons of fuel by replacing the tailgate of their pickup truck with a plastic net. On the surface, it seems to make sense, but actual air-flow patterns were not as simple as those folks expected. Grad students at MIT did wind-tunnel tests and founds that the swirling air within the bed of the truck when the tailgate was closed provided a "false surface" which the airstream could follow more smoothly toward the rear of the vehicle than would happen if the tailgate were open or missing. It turns out a pickup truck has less wind drag with the tailgate up than with it removed. I think you may be looking at only a partial aspect of air flow too, just like those pickup-truck drivers.

First of all, what's your evidence for reduced drag? There's no better measurement of engine effort available to most of us than miles per gallon. I happen to know that my average boat-less MPG ranges from 22 to 23, and with one or two canoes on the roof, and sometimes two canoes and a kayak, it usually ranges from 20 to 22. To me, that's not an indication of "parachute" levels of drag, and since nothing you do will get the drag down to zero, you will only recoup some portion of your loss in MPG, not all of it, making the potential benefit even smaller than it looks at first glance.

As one example, regarding upward air flow, if you observe canoes carried on foam blocks which are just barely tied down, they are not tending to pull upward with tremendous force, and to me, that suggests that the air blowing up along the windshield is pretty free to just travel through the canoe as it then passes over the roof. If what you describe were such a problem, many of the huge number of car-topped canoes I saw on my last trip to the BWCA (as described in a recent post about roof racks) would not have been as secure as they were. Also on a similar note, when I was a kid, my dad and I forgot to tie a Jonboat down on the roof of a van. All we attached was a single front rope. Using your reasoning, the part of the boat behind that bow line should have lifted due to the wind rushing up the windshield, but it did not. It stayed tight against the rack even at highway speed, though it did slip sideways a bit and that's when we noticed our mistake. No parachute effect happened at all (though I'm sure that would have happened if the boat had been allowed to rise up from the front).

Also on a similar note, during a recent trip of a few hundred miles, we noticed that my girlfriend's canoe had bugs smashed against the rear float tank, but nowhere else. To me, that indicates that air was rushing down the length of the canoe and only encountered a major obstacle when it abruptly hit the rear inside edge, but at that location, a length of surveyor's ribbon tied to the boat (to warn of head-banging risk) shows a substantial downward flow anyway, so I suspect most of the air already has angled downward by the time it reaches that spot. There were no bugs smashed against the floor, where they would have done so if the wind really needed to continue for a greater distance in that upward direction after passing by the windshield. Think about it. With nothing on the roof, where does the deflected air from the windshield actually go? Not up. It immediately levels off and follows the contour of the roof. By my way of thinking, giving that air a tunnel to flow through once it reaches roof-top level could easily provide less resistance than preventing it from having a direct path to roof-top height in the first place.

Again, my fuel-economy comparisons bear this out, that whatever improvements you can make to the aerodynamics of a roof-top boat (remembering again that you can only get your fuel economy part of the way back to that of a no-boat situation, no matter how clever you are), it's clear that opting to do nothing won't break the bank in terms of fuel cost. Heck, simply driving upwind instead of downwind makes just as much of a difference on a lot of days, and most people don't fret much about how much that adds to their fuel costs.

Feel free to cobble away with add-ons. It's your boat, and your car. I won't bother, and can't see that I'm suffering for it, and now I've provided my brand of logic for thinking that way. And remember, it's my choice. Scolding people for not doing as you do, as you've done several times in the past regarding this topic, is pretty pointless. I can see that you enjoy playing around with this stuff, and that's fine.

yeah well
there’s an odometer, a GPS, a fairly precise fll method so we take mileage n divide by gallons consumed.

I doahn wanna stress out MIT with this method but there it is …

As for the pickup, ??? driven both…gateless goes faster at 2100 rpm.

reading that unique instrument, the speedometer in calm air


I never heard the story about the mesh vs solid pickup gate, interesting and sort of obvious in hindsight.

GBG makes an important point about fluid paths and flow blockage. If the hull of a canoe is able to act as a somewhat low-resistance flow duct, that will be much preferable to blocking off the entire area with plywood and sheet metal. The blockage will almost certainly increase pressure drag, and the increase could be significant. The increase in surface area exposed to flow will definitely increase friction drag (probably a small effect).

There is also the added weight to the vehicle (not very significant) and a bunch of parasite drag from bolts, 2x3 ends, sharp corners, etc. (which may or may not be significant).

I have to agree that the reduction in drag (if any) provided by the large rack additions is likely not worth the time, effort and complication they entail.

I’ve seen enough

– Last Updated: Oct-08-16 11:07 AM EST –

As soon as you said that a pickup goes faster at 2100 RPMs with no gate, I'd seen enough. You are either mistaken or you are making it up.

Obviously this can't be true of a manual transmission, so I won't even go there, but the same is true of an automatic. Modern torque converters lock under cruising conditions, and at those times, nothing about the load on the engine can alter the distance the vehicle travels per engine revolution, and in case I have to explain that to you, that means that the RPM at any given speed will not be changed by changes in load on the engine either.

Under conditions of higher load, when the converter unlocks and slips, there's far too little control of the situation and too many variables for a person to conclude anything about drag induced by the tailgate. The driver won't even be able to create a steady-state condition at all when the converter is slipping, much less be able to stop, change the tailgate position, and accurately duplicate everything in the same way for additional replications.

Oh, more could be said about that, now that you (should be able to) see that the tachometer is of no use whatsoever in making these comparisons. The idea that comparisons can be made with the GPS or speedometer is wrong too. Comparisons would need to be made using a precise, mechanical adjustment of the throttle, not a simple foot pedal. In that case, you could set the throttle, note your speed, then do it all again at some later time after making wind-drag adjustments to your vehicle. You could never do this based on foot-pedal pressure and speed observations alone. Ultimately though, this still comes back to one basic thing, which is how much fuel it takes to keep the vehicle moving at a certain speed, and how that changes with changes in air resistance. Can you see that this just comes back to what I already said about MPG? You are imagining ways to measure something that can already be done in much simpler fashion.

I'll add one more thing about tailgate thoughts. Back when the subject was popular, my first thought about "why doesn't somebody just measure this?" was to tie a rope to the tailgate, attach it to a spring scale, then raise and lower the tailgate while driving and measure the drag directly. Not long after, I realized the problem with that, that if overall drag on the vehicle changes as a result of changes in tailgate position, the force of drag on the tailgate itself would be meaningless (and as the engineers at MIT demonstrated, this is exactly what happens). The idea I came up with next was a torque meter to be built into the drive shaft, and that meter would be read while raising and lowering the tailgate with a rope. That actually would work, but the engineering of the torque meter would be terribly difficult, and the fact remains that any difference in torque would be reflected in changes in MPG, so ultimately it would make more sense to make long-term comparisons based on fuel economy alone. Then along came some guys with fancy instruments and a wind tunnel, who put the question to rest once and for all.


– Last Updated: Oct-08-16 11:44 AM EST –

you are writing not about this roof rack or applications of aero roof racks but associated aerodynamics theory.

apples n oranges.

if you want an argument over an open canoe hull at 65 mph into the wind and a closed canoe hull are comparable in drag ....

not from me

I practice drafting. Cross country, we do this all day. Commonly seen crossing West Texas.

yo may check your observations on road drafting a flat bed vs tanker vs moving van from upper midwest headed to La Jolla.

Several months ago we had an opportunity following a long flat bed with a tailgate ! Huge draft ball oscillating the van while flatbed driver pulled a wagging trailer north.

a basic reality in airflow aids is adding surface area designed for reducing drag by promoting air flow direction ( as a tailgate or rear wing above motor, trunk lid, roof ) is adding surface area thus adding drag. 2+2 does not equal 4 ....prob equals 5... or 6. Good luck with 3.8.

Shops develop months of aero body work only to find the car is a drag down the straight. Then aero aids are added correcting out of shop design flaws n the stone goes slower.

a baseline:


– Last Updated: Oct-08-16 12:25 PM EST –

You make a correct statement that streamlining is accomplished by adding surface area to redirect flow. Added surface area does increase drag - so the alteration of the flow must change the flow in a way to reduce drag somewhere else, and enough to offset the increase. It's not always obvious that this is the case, as some aspects of fluid friction are counter-intuitive, or just plain odd.

I didn't really say so before, but in your case the fairings may be helping to reduce excessive drag in your system caused by the huge and bulky wood rack system on the van. That rack on its own no doubt creates a huge amount of drag.

If you truly want to reduce drag overall, an open canoe on modern aero bars would probably be a big improvement over the setup shown.

And GBG is correct - measurements from available on-board auto instrumentation will never give a reliable estimate of drag. MPG is the most relevant proxy for drag, as it is a direct measure of mechanical effort involved, integrated over a range of testing conditions. Even detailed lab testing is restricted to the testing conditions, although insight into specific drag mechanisms in a given geometry can be gained (e.g. in the MIT testing).

As an aside, I think part of why people disagree with you is your inability or unwillingness to take the time to write coherent posts - it's often quite difficult to understand what you're talking about. You sometimes write in glib and opaque statements that make you seem like a con man who can't be trusted. I don't think you are, and I know you are interested and often have valuable insight, but sometimes it doesn't sound that way.

open canoe ?

– Last Updated: Oct-08-16 8:34 PM EST –

does not compute. try the front plate. easy

buy paneling, paint primer (roller) 2 top coats...

buy #8 screws with drill bits

buy a plastic roofing/stair triangle...daze orange

measure from desired bow point over vehicle hood...back as far as possible...poss 2 positions, local and longer for interstate.

place ply somewhere longer than that on canoe

tape down so it doahn move n mark the position

felt pen the perimeter adding a 1/2" ply spacer to the pen so the piece is wider than hull gunwales

jig saw panel

place panel on marks

nowbthe tricky part.gunwales are extrusions of various inside shapes that is not all gunwales are simple rectangles.

adapt to this fact when drilling.

use the triangle measuring where all the holes go relative to the outside hull. Missing a few does not screw the works so proceed.

drill 2 holes at rear aft cut corners carefully observing the panel sits on hull with 1/2" around

then proceed drilling forward poss with a temp drill/screw in at bow.

saturate edges with paint. primer topcoat. Rustoleum is AAA


the van rack photoed is a mobile shade tree on quick n easy rain gutter mounts. NRS. ask.

all air flowing under rack is extra drag plus with eddy n clearance for flow problems.

all air flowing into open hull is a drogue.

simple airfoil panel before front rack mounting eliminates this extra surface area. Nissan has an airfoil rack option here for their double roof vehicle ?

Guessing, with the flow problem, 2-3x actual surface area. barn door.

Van airflow then continues off windshield up onto the airfoils then twisting off as a drag eddy down the roof. By that time, the drag eddy has mostly left the van. hahahah ....

I can write legal documents or science research grants. I chose to not do so in favor of beebop.

how's the surge ?

NASCAR has all the data you want. Trucks are faster and burn less fuel with a tailgate.

You’re ignoring relevant real world data.

We’re talking roof canoes not Duluth TP


– Last Updated: Oct-09-16 12:15 PM EST –

...and nonsense. The carpentry-gibberish is impenetrable and unenlightening.

The statements made about flow, however, are incorrect, at least the ones I could decipher. I want to make it clear that these two in particular are baloney:

"all air flowing under rack is extra drag plus with eddy n clearance for flow problems."

"all air flowing into open hull is a drogue."

Remember, all blanket statements are wrong...

Reminds me of the old motorcycle GP days
When early GP bikes started to sport “streamlined” fairings, they looked faster - but often weren’t. Modern fairings resulting from more correct understanding of fluid dynamics and more real-world testing look much different. Either way - we are still pushing a barn door in the wind, relatively speaking.

Bug splatter air-flow method
I remember reading about this in a motorcycle mag years ago. The author used the degree of bug splatter evidence as a gauge for determining how efficient his fairing project was and where it needed improvement. It was interesting and unscientific - but seemed to give some usable results.

Given that method, maybe it would be more productive to fair the inside of the rear end of the racked canoe.

Nah. Not worth the bother.

Regardless of the validity…
…of your aerodynamic experiment - you better not count on those drywall screw holding it all together. They aren’t intended for structural work of any kind. Much less for exterior use. They won’t last much more than a year or so in the weather (if that), and will tend to let go without warning. You should change to a better fastener, for safety’s sake.


more humor……

screws go in with pilot hole first member, using an electric drill.

No hammering.

Is not an experiment.

naming these nails as dry wall screws is archaic.

screw nails come in deck, framing, and lathe, special uses …at HD

stop in…bring money.

a guess…screws nails have 3x holding power as a common nail.

use the coated deck screws.
Cheaper than stainless, and still last a long time.

An example

– Last Updated: Oct-12-16 12:54 PM EST –

is SS.

straight grain construction grade wood n HD standard #8 screw nails.

Paint is Rustoleum. Tried Wal Glidden houseplant finding G breathing water. Used on vertical surfaces.

The foils are HD cheap paneling Rustoleumed

And the 3/8th ply rack deck. No local choice on 3/8ths. Elsewhere try for a quality ply then let dry before assembly.

Voids in paneling filled with Wal epoxy.

Good observation, and …
… very logical conclusion!