by Captain Sean Blaise
While I was recently in the Gulf of Mexico, I was exposed to extremely dense radar target groupings while on night watch.
In the Gulf of Mexico the circumstances are unique, with over 10,000 structures for the oil and gas industry offshore as well as numerous support vessels working in the area. Some are marked on charts, most however are not. Some are lit, and some derricks and simple markers are not lit. Determining which targets are vessels and which are structures can be quite a overwhelming and terrifying experience without a little insight from someone who has done it before. Now with all this going on its very important to have radar information as I like to call it, Fast Essential and Obvious.
FAST: You need information FAST. You can’t spend 15 minutes on the radar stopping to acquire each and every target one after the other only to find that most of the targets you acquired were stationary objects.
ESSENTIAL: You need to have ESSENTIAL information only. Not target data from 12 miles away and moving away from you. You need to have only information that matters to you.
OBVIOUS: Everything needs to be obvious. Whether vessels are moving from right to left on your screen or left to right. Dumb it down. Keep your radar in heads up mode always, this makes it easiest to determine where to actually look for a vessel (off your port bow, starboard bow?, behind you?). You need everything to be obvious, because if you are a one person bridge (as most yachts are) you need to be able to quickly look at your radar and determine developing points of concern with other vessels.
ARPA –Automatic Radar Plotting Aid is the gold standard for determining target position, orientation, CPA (closest point of approach) and so on and so forth. It is a vital tool in determining what and where a risk of collision exists and if so what rules you and the other vessel are required to observe.
Now ARPA is great. It is a wonderful tool that perhaps eliminates any excuse for developing close quarters situations with vessels in restricted visibility. There is no excuse to have a .2 mile CPA when you know well ahead of time what the CPA will be. ARPA also makes it fast and easy to extend your CPA. When you alter course, you can easily see your CPA getting larger or smaller accordingly. Always remember the fastest way to increase CPA is by altering course, NOT SPEED.
However, in a target dense environment, you cannot sit at the radar and acquire every single target in order to determine its threat. If you do that you will be standing at the radar the entire time, acquiring and un-aquiring targets that are neither relevant nor important to your vessel.
Now some mariners might say,” well then just use Auto Acquire on the ARPA and problem solved”. I can tell you from first hand experience that it does not work. Auto Acquire does a fine job of acquiring targets in your guard zone, but then whenever a target leaves that zone or enters it you will get an alarm. It will drive you absolutely nuts if there is a target rich environment around you as a target will be entering or leaving the zone constantly. Not to mention having the target vectors of tons of targets interweaving and crossing each other will confuse you completely.
ARPA should be only be used on targets which you already have established are in fact vessels and vessels that you want accurate CPA, TCPA and other information from that will help you make course decisions. So what should you do in target rich environments? To identify those targets?
Use a little talked about feature called TRAILS.
What is Trails? It’s a smudge of radar green that grows behind each target on the plotter like some weird green sea algae. It tells you which direction a target is coming from and therefore in which direction it is going.
So why is this feature important? Well it makes it easy to determine which targets are vessels and which are stationary objects. If you are in TRAILS (true) mode and you have a bunch of stationary targets, they will have no trail. All vessels on the other hand, will have very long and obvious trails behind them and will stand out on the radar.
Should you just use Trails to determine how to approach another vessel? ABSOLUTELY NOT! Trails will not give you the valuable CPA, TCPA, Range and Bearing information that ARPA will or AIS will. The point of Trails is to help you determine who is what and once you know that a vessel is crossing your bow or your stern, you then should acquire the target with your ARPA and proceed with that information to determine your required course of action. What Trails allowed you to do, was determine the important targets from the non-important stationary ones.
Trails losses its validity when you make rapid course changes because the trails will drag in a big circle all around every target which will cover the screen in green. Changing the range (which you should be doing often anyway) will wipe out previous trails which makes it a good way to clear the screen of long trails if you need to.