I'm not exactly sure where the 14% comes in. You say 17 days of rain + 3 days for the rain to dry up = 20 days and 20 days / 14% = about 143 days. So this is suggesting that you are seasoning the wood a little under 5 months?
Anyway, covering or not covering can (and will
) be debated ad nauseum. Not that it is necessarily a bad thing, but it is kind of like religion...no internet debate will ever change anyone's mind. I can only offer my reason for going tarpless:
For the wood to dry or season, the air must be able to pick up moisture from the wood. When does this happen? Well, the technical answer would be "Anytime the air is below 100% relative humidity" Practically - the dryer the air, the more drying of the wood. So in a rainstorm, the air approaches 100% RH, so there is very little drying of the wood, even though it may be covered and dry itself, the air is full of moisture and has very little "room" to pick up any from the wood.
The second issue is wood getting wet in the rain. The answer is "yes it does" but since the wood is essentially dead at this point, the penetration of water is limited to a little osmosis at the surface and perhaps a little capillary action on the end grain. But unless your wood pile is setting in a lake, this is essentially surface water that will dry up at about the same rate as the ground, standing trees, etc around the wood pile.
The third issue is that the atmosphere offers a huge reserve for the moisture in the wood. Picture one 4' x 8' cord of wood sitting out in the open in a 5 MPH breeze. In one hour, the cord of wood is exposed to a horizontal column of air 4' x 8' x 5 miles long. Or about 845,000 cubic feet of air or about 14,000 CFM! That is one big natural blower!
My last issue is that as long as I keep my one year rotation schedule...cutting one season ahead...for the local climate and species of wood...I know that the wood will be dry in one year without a tarp. So no need to go to the extra expense and hassle of working with them.
So, to sum it up, (to my way of thinking) when it is raining, the wood isn't really drying regardless of being covered or uncovered. When the rain stops, the wood can start to dry, but will only really get "cranking" after the surrounding grass, dirt, trees, etc dry out...which would be about as long as it takes the woodpile to dry out anyway. So the tarp is really only helping for that tiny amount of time after the rain, but before the ground is dry - and that is the slowest portion of the drying process anyway.
If the conditions were different and I needed to shave a few percent off the drying time, I might start looking into the tarp option. Same could go for you...if you think your wood is dry months ahead of time under the tarp, you may try eliminating it on the next small pile of wood. If that wood is still seasoned in time for use, then the tarps are just an extra expense and hassle that you could probably go without
Corey