 The firefight also alerted the three Union boats on the James — the ironclad USS Galena , and the gunboats USS Jacob Bell and USS Aroostook — which began lobbing missiles twenty inches ( 510 mm ) in length and eight inches ( 200 mm ) in diameter from their position on the James River onto the battlefield . The explosions and impacts of the gunboat fire impressed the Confederate troops , but the guns ' aim was unreliable , and the large shells did considerably less damage than might have been expected . 
