I recently purchased a new chartplotter sonar combo.

When comparing data against my existing units I notice some variations eg old shows 12.6 volts new 12.1 both at same power supply connections (I know 12.6 volts is minimum I have ), gps coordinates are also very different as is depth even after adjustment eg the deeper the water the error gap increases.

Sadly I hate to say my 10 yr old smaller screen/price fish finder is clearer and shows more fish plus is more capable in dirty water despite lower specs.

Anyone know what is going on here?

My older Garmin chartplotter (276C) is accurate to a metre or less against the GPS fixing points, and everything else I go near. I have never compared its GPS coordinates against another GPS. I have a hand held, an old Garmin Gecko, so I'll do a check next opportunity.

Fish finders don't find fish they detect air bubbles in the water that reflect sound and the animations on the screen turn them into pictures of fish. The assumption being that most fish have air bladders but it could be gas bubbles on sea grass or any other gases in the water. Unless you spend serious money you can't determine fish size at all or depth of fish very accurately. Same with bottom depth you will often get a false bottom on the hydrograph that will affect the depth readout.

Shaggybaxter said...
Hi Warwickl,
Sorry, nothing springs to mind as both above and below water seems to be an issue which rules out transducer/antenna issues, pretty unlikely both are install related.
Gps issues without a error on depth soundings of course can be atmospheric related , nothing proves that better than the tragic loss of Shockwave, an event that I still remember every time I look at my Chartplotter still.
For those who need reminding never to rely on electronics, scroll down to the paragraph titled "bad timing/bad luck".
www.soundingsonline.com/features/knowing-when-to-doubt-what-you-see
Still gives me shivers.
Could it just be a faulty unit??
SB

A good report produced by an excellent navigator in Chris Oxenbould. A maximum 100 metre GPS inaccuracy was only a contributor to this accident, not the cause. Fatigue and poor crew work as raised in the CYCA report and poor seamanship in closing to within 100 metres of the rocks at 0230 are the primary causes. The conditions that cause a good GPS to be 100 metres out are very rare. As with other recent threads, there are other far more likely potentially catastrophic risks we need to worry about.
A chart plotter combined with good seamanship will see you safe. If the conditions happen to occur when you are crossing Wide Bay bar and your chart plotter is 100 metres out, good seamanship is the key to your safety. A good lookout, use of other aids (leads) and spatial awareness will reveal the GPS error which you are aware of.
The Yachtmaster Offshore theory course covers these navigation risks in great depth.

I fly my jet with 170 pax to within a couple of hundred feet of the ground using GPS as the primary approach aid. We have numerous systems with complicated error detection capability and also predicted GPS position degradation before we even take off. I have never in around 15+ yrs of flying GPS approaches, had a position error that caused a missed approach or limitation on a GPS approach.

Having said that, I always employ experience, other navigation means (DME, VOR etc) and rules of thumb to agree my position Is where I think It should be.

I was in that race and passed around Flinders Is. within 1hr after Shockwave hit the bricks. The weather was pretty average but not that bad, my personal view is that the accident was entirely human error. They overshot, tried to correct visually to get back around the island and stuffed it up.

I could write a thesis on contributing factors to such events with latent and active failures along the way, but in the end, it comes down to what the skipper did in the end, and that was a tragic error!

Ive been involved in manufacturing and testing electronic measuring devices a long time.

The 12.1V versus 12.6V would at face value be a very bad error in the instrument. Especially since +/-0.1V on a 12V battery means a lot. Some of those instruments draw a bit of power and there can be a voltage drop along their own leads so the length of the leads can cause a small error. But 0.5V is terrible.

I test a lot of GPS parts for many different purposes. I found that quantity of a given same GPS part, if they have a clear sky view all produce the same latitude and longitude within around 2m. For example Maestro A2035H. Thats good.

But If I test a different manufacturers GPS in exactly the same set up they will indicate a slightly different lat and long to the above units but amongst themselves they all show the same position. ie. The exact GPS position is repeatable for a given brand part but different for a different brand.

In some cases for some projects the actual exact position was important and its really hard to check that. We tried going to various trig. stations to check various GPS parts but even then there was always a position error of a number of meters. More than the marketed and quoted accuracy. Its possible we were actually seeing movement of the Australian continent since the installation of the trig station and the GPS measurements were right.

Hi Warwickl,
I guess an accurate multi meter would help in the voltage situation, also is you g.p.s. set to WGS84, and is the time accurate,
I dont think you set a gps to daylight saving time.

He was steering from the port side (port tack) and the plotter was on the starboard side. The report said he "occasionally" ducked over for a better view of the plotter and never expressed concern about position. As you said, you can navigate around it by the silouette against shore lights, but having overshot, the concentration of shore lights is less as you go further North along the coast.

I think it quite plausible that he was eyeballing the island but that it became increasingly difficult the more they veered away due to less concentrated shore lights. You might then increase checking the plotter or keep trying to visually acquire and navigate. Since the report doesn't mention increased checking of the plotter, I'd assume it likely meant primarily visually attempting to navigate around it. Nobody will ever know for sure.

I did that race on three different boats and crews, and every time approaching and departing the island, the navigator was at their station monitoring position and confirming with the helm's visual cues.

Thanks alot for this thread.. makes me realise that i need to reinstate chart plotting on paper charts again.

Freeradical

it seems the other lesson is about crew resourace management.
we can learn a lot from studying these incidents,but having a crm framework can be very useful
emegencies on the ocean have much in common with airline industry approach
as you said,you could write more,and we would be most interested,as you have experience of both fields

Sectorsteve said..
Thanks alot for this thread.. makes me realise that i need to reinstate chart plotting on paper charts again.

I keep paper charts always.

Lets say someone believed in conspiracy theories. How would you gain control of the world for your benefit? Step 1. Make everyone dependent on a navigation and time system you can turn off anytime it suits you. Step 2. Promote cloud computing. Whereby you can read every ones data (but deny it) and prevent access to it anytime you want. (A journalist acquaintance found that out).

Building a dependence on unknown 3rd parties is dubious to me. I dont want to find out one day Im lost because the GPS system is faulty or turned off. Or find I cant get my maps because some computer company in an unknown place has done something to my files.

Paper charts in my hands, a compass and a street map in the car for me. The electronic systems are the luxurious backup!

Thanks for all the information and I now feel I have a greater understanding of the navigation aspects.

I am now trying to understand the depth, fish identification and structure identification.
I understand that the traditional sonar is best at fish identification and down view for structure.

What I am finding is a very limited benefit of down view as its performance seems to drop off in less than clear water.

Anyone else have experience with down view?

Also in the traditional mode my old finder has a 600 watt transducer where as the new one is 500 watt - does this make much difference?

the poles reverse frequently (in geo terms) and we are maybe overdue for one. The north pole is currently moving much faster than expected....so GPS will be quite useful tool when it does move.

nods said..
the poles reverse frequently (in geo terms) and we are maybe overdue for one. The north pole is currently moving much faster than expected....so GPS will be quite useful tool when it does move.

Indeed the poles locations are moving fast. In my real life I designed recently a "Magnetic Anomaly Detector" for measuring that exact thing. It measures the Earths X Y Z flux changes and reports it on the Net. Theoretically there will be dozens of them.

The consequences of the pole reversal will be very interesting. At least since its happened before we can assume the Earth will stay intact and we can keep on sailing.

Apparently no one knows the real cause - except that whatever it is it may be even worse a problem than the poles shifting. (That book sound interesting Cisco but maybe too scary for me!).