THE LOGARlTHMIC SCALE
COMPUTING TIME, SPEED OR DISTANCE
When plotting radar contacts on a plotting sheet I like to use the 5089 Radar Transfer Plotting Sheet these were designed to help off the scope plotting of radar contacts so that rapid and accurate estimates could be made between two or more vessels. It is not used as much as it used to be due to ARPA radars and the operations that your ship might be doing because of the time involved and more than likely turning your back to whats in front of you. With all the electronics on board its easy to forget the big picture and thats keeping a watch of what is ahead of you.
Using The Logarithmic Scale:
The logarithmic scale found at the bottom of the plotting sheets can be used for computing TIME, SPEED or DISTANCE problems. Given any two of these three quantities, the third may be obtained with the use of dividers and the SCALE. The right leg of the dividers always indicates time, the left leg always indicates distance or speed.
To find time:
First, place the point of the right leg of the dividers on 60 of the scale (minutes per hour). Then move the left leg of the dividers until it points to the known speed on the scale. Pick up the instrument and, without disturbing the span between the legs of the dividers, prace the point of the left leg on the distance to run, the right leg on the scale will indicate the time required to run that distance.
To find speed:
Place the point of the right leg of the dividers on the time required for the ship to travel a given distance. Move the left leg of the dividers to the known distance the ship traveled in that same time period. Without disturbing the span between the legs of the dividers, pick the instrument off the paper and place its right leg on 60 of the scale (minutes per hour). The left leg will then point to the correct speed.
To find distance:
Place the point of the right leg of the dividers on 60. Move the left leg until it points, on the scale, to the known speed. Pick up the instrument and without disturbing the span between the legs of the dividers, place the point of the right leg on the time run, the left leg wiII point to the correct distance the ship will travel in that amount of time.
