The active drug part of the plant is the resin found at the flowers of the female hemp plant, created during pollination season. There is neglible amounts of THC in the rest of the plant, as in basically nothing. So you would only get high if you smoked that particular part of the plant, not the rest. So sailors could have saved on tobacco by using the dried leaves of the harvested hemp plants they imported for rope, but it wouldn't have made them stoned in the slightest.