In these cases the fundamental questions are the distance of the Alvena and tugs from the piers, and the rate of speed of the ferryboat. Upon the evidence contained in the record, much of which was put in in the pres-enee of the district judge, we concur in his conclusion that the collision was occasioned solely by the fault of the tugs in towing the Alvena so near to the slip of the ferryboat that the latter, when about to leave the slip, on discovering the tug was unable to avoid collision by the exercise of reasonable care.»