Most wool fabrics are Fulled, this tightens the weave and raises the Nap. The nap on fabrics like Tweed and Loden is left quite long and forms a 'slicking' layer that water tends to run off rather then soak through (double-coated animal fur does the same thing). Its very hard to get a well fulled woolen fabric totally soaking wet, even a quick dunk is unlikely to do it.
During the Peninsular war (1807-1814) soldiers used blankets as tents, and had to wring them out in order to pack them into their knapsacks (backpacks in modern)...... They were a lot lighter 'once all the water fell free'
Nerd-fact for you. In the British army during the post WWII National Service days, one of the punishments available to NCO's was to make the recruit hold a soaking wet issue blanket at arms length for a few minutes (thats five to seven pounds of blanket plus over a gallon of water....... So the thick end of twenty pounds)
