# Roof rack aerodynamics



## brandob9 (Jun 13, 2010)

It all comes down to frontal area and the coefficient of drag. I just assume horrible for both and throw mine the in bed of the truck now.  

As for the configurations you mentioned, I've seen it all be a wash. Upside down will do better for the air coming over the top, but messes with the high pressure zone coming off the windscreen. Cockpit up runs the opposite trade. 

An object going through the air with a longer trailing side than leading side is more efficient (think of a raindrop), so my nomination would be cockpit up bow forward.


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## caspermike (Mar 9, 2007)

I usually role stern first and cockpit down. Aswell as slide the kayaks to the rear or the car a little more so the air coming off the front of the car can still travel over the kayaks. Gives you more down pressure. Four boats is different though.. 69 them on there sides.. Good luck wind can be unruly


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## peterholcombe (Mar 8, 2006)

Anyone use a cockpit cover to help with drag and MPG. Seems like this would make a difference for boats that aren't 69ed or singles.


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## BCxp (Jun 3, 2012)

Peter, sure does for us. + 3-4 MPG with two 13 ft. tourers hull down on top a Nissan Frontier w/ cap and + 4-6 with two 8 ft. river runners, also hull down. The suggestion to keep bow to the rear if you can, also helps, but the biggest improvement by far for us is the cockpit cover. Above numbers are for speeds between 45 and 70 MPH, burbs & highway, with 2 bods & moderately heavy cargo, and are the overall averages for dozens of trips, cold and hot weather. The boats become huge plenums with no exit without the covers. Beaucoup drag. Be sure to secure the covers well. We use Snapdragons which have an elastic cord with springeye you can warp through a grab handle/safety bar, but we also run the bow tiedown through the ripcord. If it's going to be extra windy/gusty, we'll put also use a short camstrap around the middle of the cockpit. Not sure these numbers would hold up with you,Scott & Pat in the back, though 
A second upside of the covers is less strain on the racks.

HTH


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## ZGjethro (Apr 10, 2008)

BCxp said:


> Peter, sure does for us. + 3-4 MPG with two 13 ft. tourers hull down on top a Nissan Frontier w/ cap and + 4-6 with two 8 ft. river runners, also hull down. The suggestion to keep bow to the rear if you can, also helps, but the biggest improvement by far for us is the cockpit cover. Above numbers are for speeds between 45 and 70 MPH, burbs & highway, with 2 bods & moderately heavy cargo, and are the overall averages for dozens of trips, cold and hot weather. The boats become huge plenums with no exit without the covers. Beaucoup drag. Be sure to secure the covers well. We use Snapdragons which have an elastic cord with springeye you can warp through a grab handle/safety bar, but we also run the bow tiedown through the ripcord. If it's going to be extra windy/gusty, we'll put also use a short camstrap around the middle of the cockpit. Not sure these numbers would hold up with you,Scott & Pat in the back, though
> A second upside of the covers is less strain on the racks.
> 
> HTH


I really find it hard to believe that you get 3-4 mpg differences using a cockpit cover and the 4-6mpg claim is ridiculous. How often do you do the same drive, with and without covers, in identical weather (winds and temperatures) with identical loads?


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## boogercookie (Feb 18, 2011)

I like the cockpit cover idea, but think 4-6 mpg increase seems like you had a different overall load, or a sweet tailwind. 
How about some hard numbers?


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## BCxp (Jun 3, 2012)

Numbers accumulated from December, 2011 through Jan. 10, 2013. For example, Jan 7 - 9, 2013: 3 days RT Salida recorded low of 19.4 MPG to high of 21.6 per fill, two river runners on top with covers, two people, luggage for two for two nights, two paddles, two PFDs, two skirts, medium toolbox, spares kit. Temps from low of 3 to high of 37 F, light wind from W both ways. This was second trip over this route since November with similar results.

Did same route twice last year with same RR boat, once without cover, then next with. Difference was over 4 mpg in moderate wind.

Without any rack or boats, but with full compliment of motor racing tools, tires, spare parts, & fuel (very heavy load) we'll get highway from 19+ to 22+ and have seen as high as 25+. Real numbers. Same route (Hwy 50 to GJ & return) 3x per week for several months.

We do identical kayak route twice a month from April to October with same boats on top so temps are relatively close between any two trips. We were surprised first couple of trips with the dif the covers make, but cockpits are huge drag. Covers will pay for themselves many times over.

Yes winds will obviously affect up or down. 

High numbers *are* doable, and without hypermiling.


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## boogercookie (Feb 18, 2011)

Thanks for the tips and the book keeping. I have kept my boat upside-down since day one...mostly to avoid rain accumulation while driving. Cockpit cover would solve that,too.


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## caspermike (Mar 9, 2007)

Upside down also helps save the life of the hull. advantages to having a boat with a good crown.


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## BCxp (Jun 3, 2012)

caspermike: ++ re saving hull. 

Up or down, don't snug too tight, specially in hot weather. Check hull after trip for oilcanning from being too tight. If a problem, maybe look at wider saddles, extra foam, etc. If using crossbars only, sometimes a board pad that wraps around the bar can help. Even if upside down a cockpit cover will reduce aero drag, though maybe not quite as much as hullup depending on shape of rig, racks, boat, and position of boat on roof.


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## yak1 (Jan 28, 2006)

too tight is better than under the wheels of the semi behind you. Also if the boats are put on facing to the rear some one will go for a swim


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## caspermike (Mar 9, 2007)

I never believed that old wise tale. I practice rolls not irrelevance. Booty beers is about paying respect. Little different


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## carvedog (May 11, 2005)

I hitched a ride on the Lochsa from Three Rivers to....oh well. Riding in the back of this guys car and hear hella noise from the topper - out shoots the guys kayak into the other lane and stoppered into the bumper of a guy going the other way. State patrol has to stop for these guys to extricate the kayak from under the other guy. Stater doesn't even roll down his window or get out. And amazingly little damage to the kayak. Maybe the stater sees this shit all the time - it was a first for me. We did break off an entire roof rack with boats attached going to the Deadwood one time but that was a different story.

Then shortly after got to see a big tree fall perpendicular across the road right behind us. And I wasn't even loaded. 

Hard to believe the covers mileage, but people don't believe me that I get a steady 15% increase running premium over regular gas either - so who knows.


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## freeheeltim (Mar 3, 2009)

Yup, I run premium myself and get significantly better mileage as a result. I'd need to do some research, but I believe that the higher-octane stuff contains less ethanol.


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## gannon_w (Jun 27, 2006)

brandob9 said:


> It all comes down to frontal area and the coefficient of drag. I just assume horrible for both and throw mine the in bed of the truck now.


To be accuate it actually comes down to 4 variables.
Frontal surface area (usually called projected area)
Coefficient of drag
Fluid density
Relative velocity

Directional imperfections (gouges, scratches etc...) in the hull and strap placement can also influence the skin friction/form drag tradeoff also!


All my kiddos have to learn this and a lot about kayaking during my fluid mechanics lessons!


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## BCxp (Jun 3, 2012)

gannon_w said:


> Directional imperfections (gouges, scratches etc...) in the hull and strap placement can also influence the skin friction/form drag tradeoff also!


Not to mention the Rocky Mountain Splatted Mantis.

Hey--maybe one of your kiddos will come up with the first variable geometry kayak when transportation costs go past out of sight!


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## caspermike (Mar 9, 2007)

Gas is under 3 dollars here in montana I'm happy bout that


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## BCxp (Jun 3, 2012)

$2.84 here, it won't stay down.


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## colorado_steve (May 1, 2011)

freeheeltim said:


> Yup, I run premium myself and get significantly better mileage as a result. I'd need to do some research, but I believe that the higher-octane stuff contains less ethanol.


actually to raise the octane level of gasoline, companies add ethanol. ethanol will produce less energy and subsequently lower your mpg..... check it out for yourself. they should put me on mythbusters


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## Avatard (Apr 29, 2011)

You mean "octane rating" which has to do with the apparent point a vehicle begins to knock. Octane rating has nothing to do with the actual amt of octane in the gas

Yes ethanol is added among other things having an effect of raising the octane rating. It of course is used for emissions as well as subsidies etc


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## ZGjethro (Apr 10, 2008)

I can pull a 700 lb trailer, with three dirt bikes on it, with three adult passengers and all gear in the car, and my milage does not drop 6 mpg. Sorry, but there is no way two open cockpit boats will hurt your milage 6mpg.


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