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Fuel Economy
October, 2010
John Rich
I know that everyone is concerned about the fuel economy of our motor vehicles, because, for one thing, the more money we save on gas, the more money we'll have available to spend on our favorite recreational past times, like paddling.

People have remarked to me before about how much difference dragging a boat around on top of their car or truck affects their miles-per-gallon (mpg), but I never really know quite what to believe.  There are many variables which affect a mpg calculation.  Sometimes the trips are so short, you can't be sure of the effect.  Other factors can be traffic conditions, hills, changes of drivers and driving habits, and so on.

I participated in a recent trip from Houston to Minnesota to canoe in the boundary water lakes, and that extremely long drive (24 hours of road time) provided an excellent opportunity to gather some statistics about fuel economy with a canoe mounted on top of the vehicle.  The trip was on Interstate-35 all the way, across flat terrain from Texas, through Oklahoma, Kansas, Missouri, Iowa and Minnesota, at the same speed limit, for over a thousand miles.  It seemed like ideal test conditions.

There were two observations made: 1) A boat mounted a little bit crooked longitudinally, and; 2) A boat mounted on one side of a two-boat rack versus a center mount.

1) The first was the effect of a boat mounted on top a little bit crooked.  In other words, the nose did not cut cleanly into the wind, but rather the the nose was slightly off-center, allowing the wind to hit a larger portion of the side of the canoe, creating more drag.

   Ken's Honda & boat
This case arose on Ken Anderson's Honda CRV, with a 4-cylinder engine, hauling a 15-foot solo canoe (see the photo on the right). The boat was tied down in front in a "V" pattern to both sides of the front hood, to keep it stable.  However, after many hours of driving, we noticed that the rope on one side of the "V" was taut, while the other side had grown some slack.  Ken calculates the gas mileage at each tank fill-up, and he was disappointed that he was only getting 20.5 mpg, through several tanks of gas.  We then straightened out the boat and re-tightened the tie-downs.  At our very next fill-up, the mpg calculation jumped up to 23 mpg - an increase of 2.5 mpg, just from straightening out a slightly crooked boat!  And this fuel saving continued on subsequent fill-ups.

2) The second experiment was conducted by Dana Enos, who also carefully tracks his miles per gallon, in his Toyota 4-Runner, with a 6-cylinder, 4.0 liter, 235 horsepower engine.  He drove all the way to Minnesota with his 14-foot Mohawk canoe mounted offset on one-side of a two-boat rack.  In this configuration, he achieved 17.4 mpg. 

   Dana's Toyota & boat
On the way back to Houston from Minnesota, being the ever curious engineer that he is, he decided to experiment, and moved the boat to the center of the rack atop his vehicle, instead of being offset to one side.  And in this mode over the course of his trip home, he averaged 20.5 mpg - an increase of 3.1 mpg!

Those are interesting results.  I would not have imagined that you could increase your fuel economy by 2.5 to 3 mpg, simply by making minor changes in the position of your boat atop your vehicle.  But after this trip, I'm convinced - it really works!

I don't pretend to understand Dana's example, but it obviously has something to do with high speed airflow over the hood and windshield, and how that then transfers to the hull of the boat.  It would be fun to watch some wind tunnel tests on these kinds of experiments.

Let's crunch some numbers and see what this kind of improvement can do for us.  Using Dana's example, traveling 1,500 miles at 17.4 mpg, would use 86 gallons of gas.  With an average cost of $2.65 per gallon, that's  $228 in gas expenses.  Now, if you improve that fuel mileage by 3 mpg, you instead would consume 73.5 gallons of gas, a savings of 12.5 gallons, and would spend $195, a savings of $34.

That's worth doing.  Even if the money isn't that important to you, consuming less gas is an overall worthy goal.

 The author, John Rich