For extended surface runs, the small sail intended for greater speed underwater was augmented with the temporary piping-and-canvas structure apparent in the photo. The considerable time required to dismantle that structure made "crash" diving the boat impossible, and that impediment remained until its removal with the inception of the N class, commissioned 1917-1918. Experience in World War I showed that this was inadequate in North Atlantic weather, and earlier submarines serving overseas in that war (E class through L class) had the forward structure of the bridge modified with a "chariot" shield. Starting with the N class, built with lessons learned from overseas experience, US submarines had bridges more suited to surfaced operations in rough weather.