Guest Posted October 4, 2011 Share Posted October 4, 2011 You have zero visibility. The only instrument you have is GPS. You are approaching a harbour for the first time. You have used the GPS and similar charts before without issue. Would you shoot the gap? Vote for the widest gap you would sail through on GPS. I'm trying to get a feel for how much you you would trust it. Link to post Share on other sites
Grinna 2 Posted October 4, 2011 Share Posted October 4, 2011 Just to clarify here, are we talking a chartplotter or are we talking about plotting GPS positions on a paper chart? Approaching an unknown harbour in zero visibility with only a GPS???? Hmmmm, dunno about that for a start. Can we cheat and use a depth sounder or lead line to confirm depths? Link to post Share on other sites
Guest Posted October 4, 2011 Share Posted October 4, 2011 GPS only, big screen chartplotter if you like Link to post Share on other sites
Guest Posted October 4, 2011 Share Posted October 4, 2011 I have trusted it (big screen plotter) to poke us through between Whale Rock and Okahu in the middle of the night in a CC..... but others havent been so lucky have they...... In conjunction with paper charts and some dead reckoning and common sense too tho, not just point and hope. Link to post Share on other sites
Grinna 2 Posted October 4, 2011 Share Posted October 4, 2011 The problem with the chart plotter is you're not only trusting your GPS, but you're trusting the eletronic charts in the chart plotter. There'd have to be a damn good reason for me to be attempting to shoot any kind of gap like that in zero visibility conditions with no other instrumentation or assistance on GPS alone. Sorry Squid, I'm not being very trusting or very helpful. Link to post Share on other sites
NevP 0 Posted October 4, 2011 Share Posted October 4, 2011 I'd be less concerned about the gap I'm trying to shoot and more concerned about the other wallys out there trying to navigate in zero vis with only a GPS. If I know the chart to be accurate then the bricks ain't gonna move and GPS is pretty damn reliable now (assuming it's installed & configured correctly). It's all the moving objects out there that I'd be worried about to give me grief. Link to post Share on other sites
Guest Posted October 4, 2011 Share Posted October 4, 2011 Don't apologise, just curious to know what people think the limitations of GPS are, there's no right or wrong here. Link to post Share on other sites
PaulR 3 Posted October 4, 2011 Share Posted October 4, 2011 I'd be less concerned about the gap I'm trying to shoot and more concerned about the other wallys out there trying to navigate in zero vis with only a GPS. If I know the chart to be accurate then the bricks ain't gonna move and GPS is pretty damn reliable now (assuming it's installed & configured correctly). It's all the moving objects out there that I'd be worried about to give me grief. I'd suggest a quick check on the number of sattelites the GPS is using would be a good pre-decision step. If 3 or 4, then NO would be better, if 10 or more then a definate maybe. Also look at the HDOP. In this instance, it is useful to have read the manual Ch 2 to see what that all means YESTERDAY if knot before then. One could wait an follow a bigger yacht in, really good to go, if it is a large centerboarder, knot Link to post Share on other sites
Guest Posted October 4, 2011 Share Posted October 4, 2011 Paul and Grinna , which gap are you considering? Link to post Share on other sites
Murky 1 Posted October 5, 2011 Share Posted October 5, 2011 The first time you look down at the screen and see your track [impossibly] passing over land is like the first time someone cheats on you. After that point, the magic is not there. Link to post Share on other sites
Grinna 2 Posted October 5, 2011 Share Posted October 5, 2011 I don't have too many issues with the Little Barrier/Cape Rodney gap. I might consider the Tiri Passage gap. After that it all becomes a bit "Tell me again why do we NEED to do this?". I'm assuming worst case here, i.e.that there are no lit buoys or beacons and there are lot so nasty hard and sharp bits on and around the edges. It does depend a little bit on whether its a well charted harbour or some far flung, middle of nowhere, boondocks type situation (i.e. how trustworthy is the charting?) and what my motivation for shooting the gap is in the first place. Link to post Share on other sites
Atom Ant 0 Posted October 5, 2011 Share Posted October 5, 2011 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. Link to post Share on other sites
Guest Posted October 5, 2011 Share Posted October 5, 2011 I don't know what I'd do with JUST a GPS/Chartplotter. I've never experienced that. But with a GPS/Chartplotter and a deep sounder, I'd go a lot of places that I wouldn't go without the deepsounder working too. By constantly monitoring and comparing the actual depth to the predicted depth on the chart plotter my confidence in me being where the GPS says I am is greatly increased. (That's not to say that other factors, like but not limited to the accuracy of the electronic charts in the chart plotter, don't come into it). But it also depends on the situation... If I have someone on board desperately needing medical attention, then I'd give the entrance to westhaven a go under GPS only (the alternative being sitting and waiting with them in a critical condition). If however we're just cold, wet and/or tired then I'd be more inclined to wait for better visibility. Link to post Share on other sites
Marshy 30 Posted October 5, 2011 Share Posted October 5, 2011 Is the boat Insured? Or is it the Marshall? If its Mercenary then probably not Link to post Share on other sites
Guest Posted October 5, 2011 Share Posted October 5, 2011 I'm using ENC's and they are meant to be made with digital technology, so I think very reliable. I can imagine situations where the benefits of getting in outweigh the danger in doing so. Say when the teenage daughter has had a few cokes and some lollies and I happened to look her way in the wrong way. No seriously though, say a nasty storm is coming. Link to post Share on other sites
Matty 0 Posted October 5, 2011 Share Posted October 5, 2011 I had an interesting experience once with our Navman reading funny things on Sundreamer. It was my first time going to Fitzroy harbour and there was a nice 15-20 knot south west breeze so the trip there from Auckland was great until... we had about 40 knots breeze (reading 55 at channel) and wind against tide waves with low viz and the only option is the narrow gap into Fitzroy. I knew I couldn't trust the GPS because it showed our track going through the cliff. it was a classic case of going as slow as we could without losing steerage and having a guy on each bow. Link to post Share on other sites
John B 106 Posted October 5, 2011 Share Posted October 5, 2011 Man o war passage is definitely out on whatever charts Raymarine use. Navionics? According to my plotter I've been anchored on the beach in that bay to the south of Paget rock too.There's issues around the Noises as well, the northern /NE corner particularly. Always good to know. Looks like we might have hit gottapay dirt in the Wide berth islands too.... Link to post Share on other sites
Murky 1 Posted October 5, 2011 Share Posted October 5, 2011 Looks like we might have hit gottapay dirt in the Wide berth islands too.... ...whereas I completely failed to round the Sugarloaf in a race in February, according to our Lowrance handheld. From memory, it is out of whack with the bays up the eastern side of Whangaruru Harbour too, although the charting may be OK and it may just be the name labels on the bays. Link to post Share on other sites
Guest Posted October 5, 2011 Share Posted October 5, 2011 Is the boat Insured? Or is it the Marshall?If its Mercenary then probably not I have aimed at and got the entrance to Bayswater channel in fog. But as mentioned above it was in a semi-emergency like situation, lots of satellites and I was watching the depth sounder closely. Normally I don't even turn it on in the Gulf, I see no need to. The Sky Tower/ Tiri light combo are far more energy efficient. Link to post Share on other sites
Jon 360 Posted October 5, 2011 Share Posted October 5, 2011 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 Link to post Share on other sites
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