Radar units that are typically found on tugs usually just meet minimum requirements. 

Primarily because of costs and additional personnel training and certification requirements, automatic radar plotting aids (ARPA) are rare, despite ARPA’s proven navigational safety track record. When navigating in dense traffic areas, particularly in reduced visibility, an ARPA unit can quickly sort targets based on collision risk. This permits watchstanders to focus on targets that pose the most immediate danger.

If you don’t have ARPA, a technique can be used with some conventional radar units to quickly separate out greater risk targets. It’s called parallel indexing, or PI. After switching on the PI feature, turn the cursor to either 090º or 270º relative on your screen so that the grid lines are parallel to your heading flasher on the side with the most targets. Some radar units only have one PI grid, but if your unit has two it works even better because you can set up the grids on both sides simultaneously. Then just adjust the grid lines to whatever spacing you prefer. I like to keep them tightly spaced to make target movements appear as obvious as possible as quickly as possible.

So what good does this do? It allows you to quickly pick out the targets that aren’t moving straight up or down scope. This means that the targets are either on the same course as you, the reciprocal course, or are stationary. Then you can concentrate on targets that have a relative motion that’s at an angle to you and are much more of a collision risk. It’s all about identifying dangerous targets ASAP so that you can take any required action early to avoid close quarters situations.

This technique is useful anywhere, regardless of traffic density. However, the denser the traffic, the greater the demands are on watch officers and the more help it will offer. 

In the Gulf’s offshore oil fields where the radar screen can be covered with dozens of stationary and moving targets, you’ll see just how useful this trick can be.