Roof rack aerodynamics - Mountain Buzz
 



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Old 01-14-2013   #1
 
boogercookie's Avatar
 
Wheeler, Oregon
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Roof rack aerodynamics

I usually load my boat upside-down with the stern forward when I have to put my boat on the rack. Has anyone actually tested to see which configuration results min the best mpg's assuming 2 boats on the rack?
1. Upside-down stern forward
2. Upside-down view forward
3. Cockpit up stern forward
4. Cockpit up bow forward
5. Stacker
6. put boat on someone else's rig

My thoughts are number 1 and 6 would get me threw best mileage...

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Old 01-14-2013   #2
 
Beaverton, Oregon
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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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Old 01-14-2013   #3
 
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Bozeman, Montana
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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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Old 01-14-2013   #4
 
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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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Old 01-14-2013   #5
 
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Staghorn Springs, Colorado
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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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Old 01-14-2013   #6
 
Aspen, Colorado
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Quote:
Originally Posted by BCxp View Post
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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Old 01-14-2013   #7
 
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Wheeler, Oregon
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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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Old 01-14-2013   #8
 
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Staghorn Springs, Colorado
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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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Old 01-14-2013   #9
 
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Wheeler, Oregon
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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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Old 01-15-2013   #10
 
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Upside down also helps save the life of the hull. advantages to having a boat with a good crown.
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