ORIGINAL: CrateCruncher
....Rock,
I agree drag would occur in the vertical direction and could rob thrust but I'd bet the logging mod was to trim tare weight for more payload. Green wood is extremely heavy and non-essential weight in a commercial transport application can make a huge difference in total profit over the useful life of the equipment.
It's the "downal" area as well as the overall weight. I've seen other references to it but they were well in the past so I can't quote them. My buddy also mentioned that it was due to the exposed surface area as well as the weight.