Mere towage service is confined to vessels that have received no injury or damage: and mere towage reward is pagable in those cases only where the vessel receiving the service is in the same conditionshe would ordinarily be in without having encountered any damage or accident which may effect her safety, if left without assistance. A towage service may be described as the employment of one vessel to expedite the voyage of another, when nothing more is required. If something more is required than acceleration of progress, then, although the service may not be attended with any danger of thetowing vessel, the service may be deemed of a salvage character. The danger need not be imminent.It will be sufficient if, at the time the assistance is rendered, the ship has encountered any damage or misfortune which might possible expose her to destruction if the service were not rendered. The extent of the damage is one of the factors usually taken into consideration in fixing the amount of the salvage reward. If by the towing of a vessel towed was taken out of a state of reasonable apprehension of danger, then the towing vessel is entitled to salvage reward.