How far do you trust your GPS

[Megwyn 2]

Well, we only have a small hand held etrex - no charts. It seems to be reasonably accurate.

 

We did have a crew member on board for the gulf classic last year who had a chart plotter. Not sure how accurate it was, but we did get rather close to the rocks on the north side of Rangitoto.

 

So , my lack of experience would make me hesitant.

 

Dead reckoning from what I can see, and using the gps to give me a feel for where we may be, then compare that to the chart - yes.

 

Just my 2C

[Battgirl 23]

I don't trust gps alone, use only handheld garmins on board. First time use was a 2handed night sail to Fitzroy, no probs getting in with charts, good visibility and spotlight as well as gps.

 

On a later 2handed night sail we got total blackout just out from Pigeons about 3am so much to my crew's disgust we turned around and had a nice sail back to Kawau.

I wasn't prepared to go in just on gps and my inbuilt proximity alarm ie churning stomach was working. I have learnt that I can trust that.

[grant 40]

if it was dark & foggy and your draft was 9.2 metres would you trust a GPS to get you past astrolabe reef?

[wheels 543]

I know my GPS/chart is very accurate. However, I would never rely on it in the dark or no visibility. It even says not to when you power it up. When we came across Bay of Plenty and arrived at the Mercs, it was the middle of the night and no moon. I could make out some of the land around us, but Rocks in the water would have been impossible. I used the paper chart to confirm my Plotters chart and B lined straight to a bay and anchored for the night. We carried on in Daylight. In hindsight, we would have been fine if I kept going. But I know I made the correct and proper choice and will do the same again if faced with a similar situation.

[Chewing Gum 17]

Lots of boats go inside the reef off red head in the coastal classic (including us !) but I always split the distance exactly and watch the depth sounder. Still probably bloody stupid !

[sailfish 0]

what about radar backing up your position on the gps? would that add some assurance?

 

...........because thats what we may have accidentally done going into tory channel our very first time in pitch black I found out afterwards not to trust GPS which is weird because I've always trusted it in 4wding

[Murky 1]

Lots of boats go inside the reef off red head in the coastal classic (including us !) but I always split the distance exactly and watch the depth sounder. Still probably bloody stupid !

Interesting one that. It is a decent-enough gap, there is now a buoy "on" the rock and in most conditions you can get a visual on Red Head as you are coming up to it. My guess would be that a lot of the boats that have found that rock in the CC knew they were cutting it fine but were just plain reluctant to give away the ground that it would take to put a tack in.

 

Danaide, thanks for the tip re settings - off to the manual I go...

[sailfish 0]

Danaide, thanks for the tip re settings - off to the manual I go...

 

I did exactly the same. I wonder how many people even know about this?

[Bogan 8]

Where's red head?

 

We scared ourselves with Whale rock a couple of years ago. Navigator forgot to point out that the lit buoy was substantially north of the rock. I watched the depth go from 100 foot to 30 foot in space of about 20 seconds. Didn't know which way to go though so just eased off and went more slowly until the depth started rising again.

 

I've checked the GPS trace against the eyeballed course a few times. Looks pretty accurate to me. I'd rely on it for the harbour bridge, and maybe smaller if: a) I had run the same course and checked the plot previously or I absolutely had to get in. I wouldn't trust it for any place I didn't trust the accuracy of the maps.

[Guest]

How many have been shouting about the innaccuracy of gps without having it setup correctly?

[PaulR 3]

Paul and Grinna , which gap are you considering?

 

I was thinking Tutukaka in a strong easterly, a narrow gap between lots of rocks, a dog legged channel and cliffs closeby so that in fog / bad visibility, a strong breeze large lumpy seas etc would make it very interesting indeed.

 

After a coastal 5 years ago, I really enjoyed using the GPS plotter going in there in nice calm twilight conditions, with the paper chart on deck as well. I was comparing the distances off the rocks the whole time, pondering how it would be in bad conditions, where many times the summer easterly has keep many yachts "weather bound" in the harbour.

 

I remember one trip in, doing hand bearings, to stay in the channel many many years before, when I was just a lad. Strange but true.

 

Further comming back from Noumea, many years ago, after sailing past Norfolk Is during the day, one crew member was tossed across the cockpit, hitting the winch and making a bit of mess in the face, damaged teeth, broke ribs and punctured his lung. So we made Tutukaka our port of entry, with customs and an ambulance waiting oour arrival. Good old ZLD radio was excellent putting the skipper and those below immediately in contact with medical and doctor assistance, whilst I steered onwards to NZ.

 

So a hard decision if Tutukaka was your destination in a howling cyclone and you had to simply get there. However in a howing easterly, going to Whangarei Heads would be a much safer decision IMHO, but if it had been an emergency in rough weather, more and different pressures to trust the GPS can occur. Lucky then I was knot the one who had been coughing up blood for 4 days.

 

I think I've been recently dropped from Obama's list to be personally informed in ADVANCE if and when the GPS system will be switched off or strategically offset when he decides to invade the next country on the master enemy terrorist list.

 

Don't you hate being dropped like that when it happens

[PaulR 3]

For me the operative word is "trust". I'd never trust a GPS. It's great as an adjunct to other proven navigational methods and yes they're alot better than they used to be and most of the time far more reliable than DR, or sun fixes, or coastal nav methods - BUT - trust? Nope, but then again if it was all I had and there was no other option but that, and I wasn't confident of where I was, then I might choose to wait until I can see where I am going.

 

Anchoring has been a traditional safe decision, and I agree.

[John B 106]

Johnb & Murky

my Garman chartplotter used to do the same thing until one day anchored in a bay I thought that I must be able to calibrate it manually ?

That's when I found a setting for NZ. It wasn't the wgs 84 thing but something like, once I changed that setting it moved us across the bay to were we actually were.

Since then it's been spot on

But I still like a second or third reference

 

Back to the original question, let's put it this way I wouldn't on arriving at Suva at 2200 hrs hove to until daylight in a NZ to Fiji race

 

Thanks Danaide, I will look into that although to date , those examples I noted are the only areas I've noticed are out . The usuals, Whangaroa Cavallis BOI and most of Barrier / Gulf etc seems to be on.

I regard those examples as little reminders to remember that the plotter/ gps is a tool but not infallible... which is the message behind the thread I imagine.

[PaulR 3]

OR

 

RTFM carefully and again every six months as with more experience and practice you will find previously glanced over features are now useful, and those deeper menu options mean something now.

[Guest]

I think the casualties from turning off the gps system would be more than a war! As far as I know all aircraft use the system also all commercial shipping. I think it may be political suicide to turn gps off.

[Guest Rocket]

I have gone into Tuts many times in big seas both in yachts and game boats. I would never even look at the GPS going in there - if you don't know that entrance and know where to be - particularly when you have breaking waves across the entrance then you would be very wise to head north or south with a big arc out to sea....

[Steve Pope 243]

Have been in and out of Toot in quite strong easterly conditions both day and night, luckily it has leading lights for night navigation excellent except in foggy conditions. Only problem experienced was pushing against short steep seas going out and haveing the prop loseing grip, just took a little longer. Have been into Fitzroy at night through the MoW passage useing GPS and eyeball, we would still be stuck on the cliff on the STBD side of the entrance if we had relied on the GPS alone.

Our basic Nav principle, if the eyeball can't see, the Boat don't go.

[Battgirl 23]

Steve. You are right. Minimum is mk1 eyeball and gps, couldn't have put it better.

[smithy09 50]

Is the boat Insured? Or is it the Marshall?

If its Mercenary then probably not

Nice.. I saw that.

[Jon 360]

Thanks Danaide, I will look into that although to date , those examples I noted are the only areas I've noticed are out . Theusuals, Whangaroa Cavallis BOI and most of Barrier / Gulf etc seems to be on.

I regard those examples as little reminders to remember that the plotter/ gps is a tool but not infallible... which is the message behind the thread I imagine.

 

IIRC we were in the bay beside the mussel farm in manowar passage when I did it and prior to changing it, it was always correct in areas of high traffic but the anomaly only showed up in spots like that.

We were anchored in that little bay west of the farm dead centre but the plotter had us off to one side.

On correction it moved us about 2 or 3boat lengths but only in either lat or long not both