cartop aerodynamics

cartopping one canoe seems to decrease my mileage about 2mpg width my van and 3 with my car. Either way about 10%. Have any of you experimented with socks, covers, baffles, etc to fair the underside open side of the boat?

you need a wind tunnel
to really determine where and what is causing the decrease. And an analysis of weight, how you drive with the canoe on top, and a few other things.

Good question. Closing over the
underside of a canoe would eliminate the “resistance” associated with air moving through the canoe interior, but is it better to deal with air rushing up the windshield by blocking it from entering the canoe?

The real problem is measurement of the effect. With all the other sources of variability, the results will be uncertain at best. As another poster said, wind tunnel conditions would be nice.

We need to find a graduate student who needs a dissertation topic, and who is at a school with the necessary facilities.

Some unproven ideas
I’ve never tried to measure mpg changes, but I subscribe to three theories for improving cartopping aerodynamics.

  1. Bow first, which should part the wind better than stern first. Even symmetrical waterline boats may not be symmetrical in sheer line.

  2. All boats – canoes, kayaks, skis, outriggers – should be mounted hull-to-the-sky. This will put a down force on the roof racks, rather than a more dangerous lifting force as when the boats are mounted hull-to-the-roof. Hulls in vertical racks or J racks create incomprehensible wind vortexes.

  3. Mount the boat so there is slightly less bow protruding from the front strap than stern protruding from the rear strap. This should cut down on wind yaw.

    When driving on highways, I also like to drag behind tractor-trailers or buses, but I can’t figure out how far back I should be. There sometimes seems to be a point of maximum turbulence. I don’t know if I should be in that, ahead of it, or behind it.

According to Mythbusters
"When driving on highways, I also like to drag behind tractor-trailers or buses, but I can’t figure out how far back I should be. There sometimes seems to be a point of maximum turbulence. I don’t know if I should be in that, ahead of it, or behind it. "

You should be ahead of it, so close to the wehicle in front of you that you become a serious traffic hazard. Obviously not recommended.

Not sure about bow-first

– Last Updated: Apr-28-12 4:45 PM EST –

Many canoes have a bow that is taller than the stern. I think having the taller end toward the back is probably better. The reason I think that is that when I hang a length of surveyor's ribbon from the tip of the deck surface behind the car (as a warning about potential head banging when a person walks around the back of the car when it's parked), it does not trail backward behind the boat when at highway speed, but instead, it weakly twirls around while hanging straight down, and that tells me that the air around that part of the trailing end of the canoe is a bubble of turbulence following the car, rather than a slipstream that's rapidly passing by. Seems like making the tallest end of the canoe poke downward into that following bubble of turbulence would be better than putting it up front where it would create a larger surface with which to "part the wind", and where it would more effectively "catch" the full brunt of cross winds or bursts of truck turbulence.

I seriously doubt that any other aspect of hull shape matters, in terms of going frontward or backward. However, fish-form shapes are more aerodynamic than swede-form shapes when moving rapidly through AIR (it's even the shape assumed by falling water drops), so the average swede-form canoe MIGHT actually have a bit less air resistance when strapped to the roof backward. However, one of my boats is swede-form, but the widest part of the boat is only about 4 inches behind center. Also, I've never seen any asymmetrical boat where the widest point was so far from center that it was apparent just by looking, so even this has got to be a miniscule detail to worry about.

That said, I put my canoes bow-forward because because that's how they end up from the natural shoulder-carry position when pushed onto the roof from behind the car. It also positions a thwart just forward of the back end of the car to function as a good anchor point for a pair of lines that will keep the canoe from moving forward if the rack or main tie-downs should fail. I guess I'm not too concerned about possibly reducing air resistance by a small amount by carrying the canoe with the smaller end forward.

I bet that putting a cover over the hull opening would help a lot more than picking which end goes forward.

The Answer

– Last Updated: Apr-28-12 6:54 PM EST –

Hi, I did engine nacelle aerodynamic work for 15 years. The answer is - it is complicated, very complicated. The aerodynamics caused by the boat in the near proximity of the car roof is really complex...technical term for that is "junk". The flow in that area can get choked (unless you spend a lot of money designing it correctly), which will really drive drag up.
If there was a better way to do it, jet airliners wouldn't have the engines suspended on a pylon below the wing. I don't think that backwards or forwards will make much difference. Just don't strap it on sideways and you're doing the best that you can do. Adding a fairing might make it better, or worse, or might not make any difference.
You might save some gas by slowing down.

I didn't want to sound cocky, but if I was posed this question in a big fancy meeting I would have answered, "I don't know". It's complicated enough that we figured this sort of stuff out through a couple million dollars of wind tunnel tests.

1. Bow first definitely, that’s just good juju. Plus it can’t hurt to use the cutwater to part the air, and keep the rudder business in the wake zone.

2. I’m not sure about the up-force / down-force you mention. It’s not clear to me one way is better than the other for a kayak - I travel with mine right side up. A canoe I would always carry upside down. I hate the idea of J-cradles and would never carry a boat on its side.

3. Probably a good point about yaw forces.

I echo what’s said below about drafting - the recirculation zone behind a bluff body will be maybe twice as long as it is wide, perhaps 15-20 feet. You’d have to be too close to the truck to benefit.

One could cover the canoe opening while on the rack, but if the cover is able to flap, it will contribute its own drag, as well as being noisy and annoying. As mrmannerz says below, it’s complicated, and is not obvious whether covering the boat with a rigid cover would hurt or help. You could shrink-wrap your canoe and drive around for a while to see if your mpg goes up…

Got a long hill by you ?
Maybe do some coast down tests with various configurations.

Somebody is thinking.
That type of experiment should actually work. I know that when coasting downhill on a bicycle, you can “fine tune” your body position and see a clear result in terms of speed.

Don’t draft tractor-trailers
Have you ever been near one when it blew a tire? This happens frequently, based on the number of tire pieces I see several times on any road trip.

I was well behind one and in the lane to its left when one blew. A big chunk narrowly missed hitting my vehicle, and I also had to dodge the pieces already on the pavement. These pieces may have damaged metal belts protruding, not a good thing to run over or get caught on your underside. Or to have smash your windshield or sheet metal.

One more observance…
I seem to lose about 10% gas milage when roof topping boats on my Tahoe, and a bit more on my Focus, around 12%. Speeds under 65 or so, faster costs much more on the little car.

These boats are narrow, long touring kayaks, with cockpit covers, so I’m inclined to think covering a canoe won’t help much, but there are always two kayaks up there.

Tomorrow, I’ll be car topping only one, so I’ll see what that does to the mix. It’s on the Focus, and the speed should be under 60.


Dumb guy answer
Dear Peter,

Umm, with a canoe on top of the car isn’t it doing more work than when you are simply commuting back and forth?

Why then would it be reasonable to assume it should do the work on the same amount of fuel?

I know people like to tinker and analyze things but I think you’re sort of pissing in the wind. Like the engineer a few responses up said, changing things in a positive direction costs big money.

More than likely it would cost far money than the process would be worth in terms of gas savings.

Load the canoe and go.


Tim Murphy AKA Goobs

I was a little shocked by these response
I have noticed a large (2mpg) increase in my milage with 3 kayaks in J style carriers on my diesel F350 pulling a fifth wheel camper. This has also been the case with several friends that drive similar vehicles. This may be because the empty j cradles create truly terrible milage, coupled with my cinder block aerodynamics of the truck.

I was really hoping for a magical solution.

Thanks, mrmannerz, I think you describe
the state of our knowledge correctly. We act based on hypotheses that are, at best, plausible.

Here are my unsupported hypotheses. It would be best to reduce the amount of air forced up the windshield, where some of it must escape between the canoe and the roof. Perhaps my high rocker 15’ ww tandem pushes more oncoming air up and out to the side than my low rocker tandem. Observation of signs of turbulence indicates that the floatbag in the canoe may reduce turbulence in air rising up along the windshield. Some corner carving on my whitewater minicell pedestal systems might help air flow through the interior of the canoe.

I’ve wondered if it would help to put a hitch on the car, and add one of those towers, so that the entire canoe can be shifted back on the car. This would reduce air entering the boat after rising from the windshield, and would put most of the interior of the canoe back in an area that is turbulent anyway.

But there’s no easy way to find these things out.

Funny this should come up

– Last Updated: Apr-29-12 9:38 AM EST –

just now.

I just spent the day thinking about this while working on setting up racks for a new (to me) truck.
I think the vehicle plays a pretty big part. I've used small engined pick-ups with toppers since the 80's. On average my mileage hasn't differed a great deal whether I was carrying canoes or not. I've notice the rack itself hurts mileage a little when I'm not carrying boats. When I use flags I, like Guideboatgguy, notice the sterns of my boats seem to be in a sort of "bubble". I'd expect hatchbacks, station wagons and SUVs to act similarly. I don't know if this would be the case with a sedan. That could be a whole new ball game.

What seems intuitive to me is that you absolutely don't want the canoes acting at all like parachutes. I therefore try to set my racks up so the bows of my boats are as close to the truck roof as possible and the sterns ride a little higher (relative to the direction the wind will hit it - a slightly negative angle of attack, if you will...) even with a load in the bed. I hope to create a little "bubble" under the canoes by doing this. If I'm carrying two boats, I try to keep them as close together as I can (without any chance of rubbing tumblehomes) and moved side-to- side so as little air a possible can sneak in around the sides of the vehicle.
Problem comes in when I need to carry the only boat I transport with any regularity that has sharply upwardly curving gunwales. What is a perfect rack set-up for all my other canoes puts Prospector gunwales bouncing off the cab of the truck. I put foam pads on the gunwales under the front straps to remedy that, but its not ideal.

Set up like that, especially while driving across prairies like those of Il, I notice that wind direction and speed (and, of course, tire pressure, wheel alignment, and being tuned-up) effect my fuel economy more than how many boats I carry. Staying a bit under the speed limit helps also, though I try not to be a traffic hazard by driving 55 where most others are doing 70+. I'll pay a little extra to be a little safer - besides, I want to get the heck across those flats sooner than later.

I try to stay just a little behind truck turbulence though,of course, wind conditions also play a big role in just how far back that is.

I've always checked gas mileage every time I fill up, so I do trust my long-term averages on my vehicles.
Wind tunnel tests, of course, would be the right way to figure this out - but that ain't a happenin' thing. So we're left making our best guesses based on our own little observations. I'm sticking with very near the roof and a front-low angle of attack for overturned canoes.

the more you know …
…the more you know you don’t know.

The engineer’s response pretty much sums it up. It’s complicated. The key point of complication is the narrow gap between the boat and the roof.

You can have a very aerodynamic boat and a very aerodynamic car. But put the boat 4" above the car, it will destroy the aerodynamics of both. Kind of like driving with a big gap in your windshield and back window. The air turbulance while it squeezing through that narrow gap generates a lot of resistance.

One easy thing you should always do
to reduce turbulence is fit the cover over the cockpit, which otherwise would act as a parachute of sorts.

As for upside down or right side up, I don’t know, but I suspect that if it’s several inches off your roof, it wouldn’t make much difference either way.

I doubt it’s the fault of empty J-hooks
The empty J-hooks will add about as much drag to your rig as as sticking one hand out the window. I don’t believe they play any part in what you’ve observed. However, if putting boats on the roof really does increase mileage rather than being a conclusion based on coincidental conditions (few people have the chance to measure mileage for entire tank-fulls of travel during consistent conditions, but almost everyone can compare distance traveled per tank-full when conditions are changing along the way), it could be that having kayaks on the roof creates some beneficial turbulence in front of the flat front and of your camper trailer. No doubt you’ve seen those little ramps you can mount on your truck’s roof which are supposed to improve fuel economy. They cannot actually start the air upward on its way to meeting the trailer in pure form, but they can induce a circular-running “cushion” of turbulence behind the ramp which has the effect of virtually smoothing-out the point of impact between the front of the trailer and the air. Your kayaks on the roof might help reduce the degree to which air smacks straight into the front of the trailer in similar fashion.

On my new canoe hauler(Ford Focus Wagon) when hauling just one solo canoe,I want to try not using my long yakama bars at all to eleminate their drag and get the boat closer to the top. The problem is that I havn’t found a secure enough way to mount the boat. Foam blocks,dubble bow and stern lines ect just don’t give enough bracing on the road with big trucks. Ideas?