bars placed below the main bar for each tug. The number of lighters and car-floats towed simultaneously is clearly shown in the chart, as well as the time at which each was picked up, and the time at which each was delivered by the tug. In the case of car-floats, a frequent break will be noticed at the end of the towing, one-half the width of the horizontal bar being marked with black. This convention was adopted to show that the idle time was occasioned by the necessity of waiting to obtain an unoccupied float bridge into which the car-float could be shifted. Though the tug-boat was standing idle it was not feasible for the dispatcher to take the tug away from the particular car-float to which it was attached, for the tug would be necessary to place the car-float in the float bridge as soon as the float bridge became free.
Charting of information frequently brings out points which would be entirely overlooked if charts were not made. The tug-boat chart from which Fig. 60 was taken at once calls attention to the time required for the tugs to obtain water. As the service of tugs was valued at about $9.00 per hour, the time spent in taking water was a serious loss. As soon as the chart showed the extent of this loss, it was comparatively simple to remedy the situation by providing different methods of getting boiler-feed water.
A time chart like Fig. 60 can often be used advantageously in conjunction with other time charts covering the same period of time. Thus, in studying tug-boats, the information on a chart for lighter operation such as is shown in Fig. 59, and on charts for the operation of car-floats or of float bridges, may be valuable. All the various kinds of equipment with which the tug-boats may be employed could be considered in the study, if it is to be determined whether or not the most effective use is being made of the tug-boats. If all the charts used in the study are drawn to the same horizontal scale, the charts can be placed immediately above each other so that the operations of all related equipment for every hour of the day and night can be instantly seen during the whole of a test period, of, say, a week or a month.
The charts thus far considered in this chapter have shown time in the horizontal direction only. In Fig. 61 we have time shown by days in the horizontal direction and by hours in the vertical direction. This type of chart is extremely valuable in determining whether or not schedules are maintained uniformly over any period of time.
