Log v GPS

And here is one I prepared earlier:-

2007-07-29%20Rhein%20Maas%20029.jpg
On the mighty River Rhine through Germany this 'summer'.

On the left, depth= 7.9m. To the right the LOG pointer shows just under 10km/hr ‘through the water’ however the lower LCD displays (set to GPS ‘Speed Over Ground’) shows 15.4km/hr.

This is not inaccuracy on my part in the log calibration; simply that we were going downstream at the time.

You can imagine the discrepancy between the log and GPS after a day or two on this river!
 
Re: Why?

It's not the GPS that I'm sceptical of - just the problem of mis-match with charts that can lead to GPS positions that plot as well inland (and the much more dangerous converse). The missing factor in this discussion is Mk 1 eyeball and the need to use all possible inputs to establish a position.
 
Thread drift?

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The missing factor in this discussion is Mk 1 eyeball and the need to use all possible inputs to establish a position.

[/ QUOTE ]Really can't see how relevant that is to a discussion on logs.
 
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Has the GPS been left switched on whilst at anchor etc?

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I am intrigued by this. I can understand that the log if left switched on could clock up a lot of mileage due to tidal flow whilst at anchor, but the GPS ???

Does the GPS position really wander around enough to show significant mileage
 
Re: Why?

Tony,

I agree absolutely, it is no good saying that “The GPS said it was OK”, when you hit a car in Boots car park, to the left of Dover high street! (Isn’t there a rule about this somewhere, perhaps it should be called Collision Avoidance)?
Even on the inland waters of Europe we find GPS accuracy is almost always better than the length of our boat, and according to Bill Gates we are usually well within the constraints of the channel we are supposed to be in.

Methinks ‘RTFM’ and ensure that the geophysical datum is setup correctly for GPS, the charts and that the navigator understands the constraints of such digital info.

For instance:- Try using a cars ‘GPS Navigator’ just 100 meters from the border between Poland and (what was Eastern) Germany the results can/could be terrifying, due to the differing ‘chart datum’s’ however we use our eyes and navigate according to what we see:- It’s simply common sense; which I fear, is no longer taught in English schools?

CRB
 
Re: Thread drift?

Does anyone who doesn't have GPS actually do that these days, outside the confines of a Yachtmaster course? When was the last time you worked out an EP based on time and distance run and plotted that on the chart? This is a legitimate question - maybe I'm out of touch and people are doing it all the time.

Having said that, I agree that it would be prudent to ensure the log was properly [edit] calibrated [edit] in case you needed to use it for this purpose.
 
Re: Thread drift?

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Having said that, I agree that it would be prudent to ensure the log was properly calculated in case you needed to use it for this purpose.

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Except that in my experience the (through hull) log can NEVER be relied upon unless you have just withdrawn and cleaned it.

Vic
 
Re: Thread drift?

[ QUOTE ]
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Having said that, I agree that it would be prudent to ensure the log was properly calculated in case you needed to use it for this purpose.

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Except that in my experience the (through hull) log can NEVER be relied upon unless you have just withdrawn and cleaned it.

Vic

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Which is why you should remove the paddlewheel when leaving the boat.
 
A REAL sailor (I don't necessarily include myself), with accurately calibrated log would always have GPS reading more than the water log!

Simply because a REAL sailor (I still don't include myself) would on average sail far more with a favourable tide or current than against one.
 
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