A couple other thoughts come to mind.
Radar, originally developed for naval gunnery, is entirely too simple on the face of it. You turn it on and, viola! there's the whole harbor right there on the TV. You watch that tug-boat track across it, look out the porthole and there he is doing just what your TV showed. Hey!, I'm a
radar navigator!
Were it so simple. As I and Nolatom intimated, there are such things as
radar assisted collisions. The ageless rules of the road had to be modified to confront their occurences. They all stem from the
radar operator "thinking" he knows what he is seeing. It took about twenty five years after
radar became common to ensure that the average mate on watch knew how to properly operate it. Many were prone, as the average
radar equipped boater surely is, to take for granted what they were seeing. The results of this practise can best be seen by googling up an account of the Stockholm and Andrea Doria collision and resultant loss of life. It is the prototypical
radar assisted collision.
For collision avoidance, it is essential to plot the other vessels. On board ship we used to have (and still do) reflection plotters which were part of the
radar screen. You dialed up the illumination on the reflection plotter and used a grease pencil to make a mark on it. A few minutes later you made another mark on the contact. (Just to illustrate how confusion can result: if that second mark is on top of the frist mark, the target isn't moving right? Wrong! The target is moving, and at approximately the same course and speed that you are.) At the six minute mark you make another mark on the plotter; you now have the basis for course and speed resolution of the target. Six minutes is a tenth of an hour and, if you connect the dots of your plot you have the resultant vector of your's and the target's combined speed and courses. If by vector analysis ( doing a little trig exercise on a triangle) you apply your course and speed vector to the resultant course and speed plot, the third leg of the triangle is the target's course and speed.
You can do this in a three minute plot with substantial degradation in accuracy. Six minutes, with continued monitoring of the target down the resultant vector is required.
For those
radars without reflection plotters one must transfer bearing and distance to a plotting sheet. We know you carry those, right? (g) They really are quite simple to use, for a multitude of purposes.
All of the above is predicated on your, or the target, not changing course. If either does, you must re-plot the resultant vector. And this is where the true value of the reflection plotter comes in. If the target continues to move along the original grease pencil track you've plotted (assuming you yourself have not altered course) the target is on the same course AND speed that you originally plotted. And his closest point of approach will be at the distance off that the grease pencil plotted vector comes closest to the center of your
radar screen.
To plot accurately you must make your mark while your vessel is directly on her base course; a few dgrees of yaw will throw you off substantially. Hence the desirability of gyro stabilized
radar units. Merchant ships send a gyro signal to the
radar and thus the screen becomes stabilized with the
radar picture stabilized to the gyro and the ship's heading
line free to swing without "smearing" the targets into 5-10 degree blobs. If your display is not stabilized you will have to operate like you're on an old submarine, calling out for a "mark" when the helmsman is directly on course, so that you can take your bearing and range accurately and consistently. This part ain't optional.
Boat
radars may have input from such as fluxgate
compasses and
GPS units. These are really not good substitutes for gyroscope input but it's all we've really got (in terms of space and energy consumption). The fluxgate
compass is actually, in my opinion, a better source of stabilization as all you are really interested in is relative bearing stabilization with a reference to true North being of secondary or tertiary importance. The
GPS input, to
radar, must be accomplished in a manner that leaves the picture stabilized and the speed input averaged, or degraded, to a point at which viable resultants can be obtained. Rapid
Radar Plotting, as it's known, only needs your SMG and this can be determined even by previously determined engine rpm cards. The effects of current and wind will be resolved automatically within the plot, both on your vessel and the target vessel.
ARPA devices make all of the above relatively automatic. It should be remembered though that what form of ARPA presentation you select will effect your perception of what is actually going on out there. If you select to display other target's true course and speed you will display only vague information as to closest point of approach, and small changes in course by you or the target will not be readily apparent. Another reason that you wish to start with course alterations in the area of 45 degrees or greater, being well prepared to continue on to 90 degrees or more. Ten to twenty degree course alterations are not readily apparent on
radar, at least not in the way you intend them to be. It is common practise to make a large course alteration and then, when it is apparent that the alteration has been observed, to walk her back somewhat towards original course. I cannot condone the practise is good conscience, but it happens. By doing thus you are making an assumption about what the other vessel has observed, be careful about such assumptions. It is assumptions about what the other vessel's watch keeper must see and understand that leads to wet sailors or worse. Often a much better display format to select is the relative motion
line that would have been your grease pencil plot on the old reflection plotter. This display will result in electronic
lines of relative motion being displayed on the screen. If none of them intersect with you or come close to you, you're not going to hit anything. common practise is to switch back and forth between displays while leaving the display in the cpa mode in between. This allows a quick glance at the screen to determine if anything has changed substantially or if your recent course change has resulted in an imminent close quarters situation.
The other thought I had on the subject concerns the rules of the road and True Blue's post. One will note the Rule's requirement to take all way off and navigate with caution until danger has passed. It is my opinion that
radar has given us often a false sense of security. Prior to
radar it was imperative to reduce speed, stopping completely if necessary, to assess the situation or to avoid collision. It is still good policy. It is much more difficult to hit a stationary target than a moving one. Keep in mind that I am operating on the belief that a miss is as good as a mile. The tendency is to adjust course slightly to miss the oncoming target. Failing to take the time and action to make a radical change in course one is often best to take all way off and await developments. If you are plotting a dangerous contact, or you know which way shallow water lays (safety for the boater from the big boys), or you know the course of the dangerous contact-you can lay in such a manner as to be ready to motor in the direction of safety. Knowing that another vessel in a panic situation is most likely to alter course to starboard will help determine which way you may decide to lay-to. The largest of ocean-going ships have a breadth of 106 feet. (Called Panamax, supertankers will exceed this but will generally only be found in limited areas) So, if you are dead in the water, you have only to cover something in excess of 100 feet to live. Two hundred feet would be nice, yes? I'm not talking about not having the beejeezus scared out of you or even a more imtimate acquaintance with the plimsoll mark of a large freighter, I'm discussing survival. If you continue underway you are more likely to become part of a slow motion collision, removing all possibility that the larger vessel will be able to take action alone passing you safely a mile off. So my opinion is that you either make large readily apparent changes in course, early on, or you sit tight until action, if any, is required.
Fog signals. Most of us are incapable of carrying truly effective fog signals. When I rolled under the Verazzano Narrows Bridge on the "American Hawaii" and let loose with a blast, everyone in Battery Park knew I was there. The apparatus most of us carry may be heard on the relative quiet of a ship's bridge deck or bow, it is much less likely to be heard on the bridge deck of a tug which commonly runs with her engine room doors open. While the Rules require a signal every minute or two minutes depending on vessel or circumstances it may be well to exceed that frequency in congested waters. Pointing the signalling device in different directions, not only in the direction that danger is perceived as coming from, may increase effectiveness as well. Sound over water does funny things. Many's the time when I've heard the echo of a signal without hearing the actual signal.
A final note on traffic seperation schemes and vessel traffic control systems. You may monitor the
VHF channel they utilise but you must monitor Ch 16/13 as well. Most ships will do this and use traffic control frequncies only for updating their position as required. The pilot on board may make greater use of the system, but your best bet is always Ch 16.
Before you depart, in that hour or so of prep work, it is a good idea to be monitoring the
VHF. If the "President Madison" is announcing her unberthing at Middle Harbor Terminal, Oakland, outbound for sea it would behoove you to make note of it by jotting it down and glancing at your chart to see where you might meet up with her. You might make the decision to brew a pot of coffee, enjoy a cup or two, and follow her down the bay rather than come up from below, after putting the kettle on for a cuppa, and see her bow looming over your transom.
For most of us it is a good idea to stay well good and away from traffic seperation schemes. The vessels in them may be able to alter course out of the scheme but that is really not the intent or the practical use of such schemes. The outside of the scheme is likely to be populated with fishing boats, nets, and floats as well as other small craft. It's bad enough with those fishing in the seperation zone. A ship is very reluctant to alter course out of the scheme as she will, once out of it, encounter one collision threat after another. Within the scheme the ship may be able to reduce speed but it is usually of marginal use in collision avoidance-you don't just scrib off ten knots of headway on 50,000 tons of ship in less than a mile, if that. The requirement in the Rules to cross the scheme at as close as ninety degrees is important. If that takes you off of your course, you will have to realize that the longer you are in the scheme the greater time there is for things to change and become hazardous to you and, in any event, you should always have a good sense of where you are at and be prepared to scoot out of the scheme asap. If you think that your particular area ia a hassle, pull up a chart of the English Channel where there are multiple schemes for inshore and offshore traffic. Transitting the area in fog is one of the most migraine inducing experiences available to the mariner.
True Blue's post brings up one other point, specific to his situation. Prior to
radar it was assumed by the Rules that you would be able to take all way off your vessel or alter course to miss another, at the point of visual sighting. If you think about that and consider that you may have 200 yards of visibility, a tenth of a mile, and the ship you are meeting may be a almost a quarter of a mile long (or her tow) a ten knot speed of advance may put you in harm's way much more quickly than you realize.
Good luck and safe sailing to all.
Linear Mode
