Port Credit Marine Survey
& Yacht Delivery
11. Navigation Equipment
Chartplotters -A few years back I was delivering a million dollar boat out of a harbour on Georgian Bay. I stayed up the night before and programmed the three just installed, state of the art chart plotters. I was reasonably familiar with the waters and knew that I would pass rocks awash to starboard and a little later change course. We passed the rocks and I complacently waited for the plotter to tell me I had reached my course change waypoint. When I saw a second shoal I knew something was wrong and stopped the boat. While surrounded by rock shoals and frantically trying to find out where I was, I noticed that the plotter screen had frozen 10 minutes before. I went back to my compass, dividers and parallel rule. 

Over the course of the next 12 hours the other two plotters froze without warning but that was OK because I was no longer navigating by them. Chart plotters are no more than computers. How many times have you tried something on your PC that you have doing for years and it no longer works ? How many times has your laptop frozen? How many times has your PC crashed or simply turned itself off and refused to restart ? .......... So why would you trust your life to a chartplotter that is just another computer ?

To be fair they are an excellent aid to navigation but just that …. an aid. The problem is that like me, too many people get complacent with them and start to believe what they see on the screen. They follow that skinny magenta course line on the fancy colour screen and forget to look out the window.

Complicating matters is the fact that hundreds of marks at river mouths and sounds are moved frequently and many are not charted because they move so often. 

The black triangle is the chartplotter boat, I drew in the yellow boat in the actual position about 250’ from the plotter position. The boat was tied to a wall and the plotter left on for an hour. The black lines around the plotter boat are the tracks of the GPS wandering around. This is a DGPS/WAAS plotter…. Still want to navigate narrow channels in the fog with these things ? This plotter could not be trusted in a narrow channel in the Trent Waterway. I swear I didn't take the boat up there.

Want some more complications ?…. Other than major commercial routes, many charts (not only electronic) are based on surveys done in the 1950’s and in the Caribbean some charts still rely on the work of Lord Nelson.

Now to contradict everything said so far …..

GPS is amazingly accurate !
But ….. you knew there was a but !

GPS triangulations are built upon mathematical models projecting a grid around the globe. These grids are referred to as “datum” with names such as NAD27 or WGS84. You must make sure your GPS/plotter is programmed for the datum of your charts or go through the process of programming offsets (read the manual).

As accurate as GPS is in relationship to the model grid, the flaw lies in the relationship of the grid to the real world. Our real world was charted decades ago without the accuracy allowed by modern methods. News flash … the world is not round, it is a spheroid (sort of round but with big lumps) and while the model tries to match the shape with billions of calculations, it simply can’t with current technology and even if it could the entire planet would have to be re-surveyed to match. This is unlikely to happen in our lifetimes.

The following is reprinted from the newsletter of Peter F. Prowant

Complete reliance on GPS in lieu of other navigation methods is a mistake. The inherent, commonly misunderstood, belief is that GPS receivers capable of accuracy within several meters, place the mariner exactly on the plotted position of the chart or at the exact position of the electronic chart loaded into the chartplotter or navigation computer. The accuracy of a position plotted directly from GPS is, however, limited to the accuracy of the chart. A good example of this is the recent observation by two Bluewater customers within a week, both using electronic chartplotters, that the GPS was not accurate in the vicinity of the island of Bimini, on the western side of the Bahaman Islands. Interestingly, the customers were using chartplotters and digitized cartography from different manufacturers. Yet the complaint and the position errors provided by the chartplotters were exactly the same, placing the vessels approximately 200 yards on the island itself, while navigating into the harbor. How can this be? Simply, the position of Bimini on National Imagery and Mapping Agency Charts and British Admiralty Charts is based on a chart first published in 1844, when surveyed positions were based entirely on celestial observations, hence the apparent error. It is rather amazing to me, that charted positions based on such observations are so accurate! Logically, the question arises, why not just correct the position of Bimini? Unfortunately, correcting charted positions is not that simple, for to move the position of Bimini on the chart would mean that all charted positions of other islands, shoals, coral heads, entrances, lights and so forth on the chart would also have to be corrected relative to each other.

Radar -This topic could very easily be a very long book on its own so I won’t get into the minutiae.

Note that the
Georgian Maritime College radar course is 11 weeks with a very sophisticated simulator, which should give you some idea of what’s involved.

Let’s stick to a few highlights. In clear weather and calm seas radar will help find marks you cannot see in the glare. It will detect thunder storms and see floating logs.
Of course this presumes that it is properly adjusted which is no simple matter. Properly setting up and adjusting radar cannot be learned from the manual. It is a complex piece of equipment and your fair weather or dockside setup will be virtually worthless when the weather turns. When purchasing a new radar there are a few things to insist on.

There is simply no substitute for power, a 5kw radar will pick up things a 2kw unit won’t see except in perfect conditions. Most will pick up seagulls on a calm day but throw in a little rain or wave action and a more powerful unit becomes a very valuable asset especially in places like Manhattan.

There are certain functions you will be adjusting constantly and these should not be buried in a digital menu but be up front controls and include range, FTC, STC and gain.

Even radar can be confused by the sometimes massive RF (radio frequency) interference around US military bases like those in
Norlfolk, Virginia, the mouth of the St.Johns River in Florida or the USCG base in Key West. We have often found our radar screen one solid colour in these areas and even had our depth sounders and GPS display nonsense.

Our radar screen in Hampton Roads, Norfolk, VA Massive RF interference at this and other military installations can render all
of your electronics useless.

Rush hour in the harbour at Manhattan is to be avoided. Try transitting early Saturday or Sunday mornings.

Radar Reflector -
I am constantly amazed by the large number of seasoned cruisers who don’t know that if they are less than 20 meters or made of non­metallic materials are required by Collision Regulations to carry a radar reflector. Those apartment building size ships coming up the Delaware, Chesapeake or running the gulf stream are often running at 20kots plus. I don’t know about you but I want them to see me. If you are one of the few who really know how to use radar you will know that although they are essential there are some things they will not see. We often see large cruisers show up very well abeam or from their stern then disappear from the screen when they turn bow on. FRP absorbs radar rays and sharply angled surfaces can send the beams into space rather than return them to the receiver. I want the big ships and everyone else to see me coming.

Autopilot -
As much as I love my radar I won’t even move without my autopilot. We once waited in Sarasota for over a week for a new pilot pump to arrive. I do not like being tied to the wheel. One of us is always on watch but with the pilot we can watch from almost anywhere. Like every other aid, there are things to watch out for. I once witnessed a Monk 36 make a 90° turn into a moored steel barge. He had passed too close and the large magnetic field of the barge turned his pilot. Be aware of the same potential going under bridges. Currents often make abrupt changes going under bridges due to the constricted waterway and effects of large pilings. Unless you want to take a close look at those concrete pilings, turn your pilot off.

Depth sounders -Why people will pay upwards of $200 for a depth sounder is beyond me when a $90 fish finder will not only show you what the bottom looks like but will also point out dinner. With a little experience and an observant nature you will be able to determine the structure of the bottom from sand waves, silt over clay, sand over rock to weed etc.

Photo at left - Sand waves like these suggest opposing currents and good anchoring.

Photo at right - That square block is heavily aerated water caused by a passing gofast. The screen will be blank within seconds and may stay that way for up to a minute, too long in a narrow channel.

Clearview -A Clearview windshield is so much superior to wiper blades they shouldn’t even be mentioned in the same breath. This glass disk spins at such a high RPM that water simply vapourizes. Where wipers quickly become nothing more than salt smearers, this stays clear at all times. Its even heated so when you leave Toronto on October 15 (that’s another thing I won’t be doing again soon) and get hit by the big freeze in Oswego you will still be able to see .

Compass -This is a no brainer, Why don’t you have a properly swung compass with a corrections card ? (and a backup).

Binoculars - Plural ! You’ll be sorry if you do not have two pair (good ones). It can be frustrating when you need the binoculars and they are not where you left them.

Chapter 12. Maps, Charts & Guides.

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