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Old 04-08-2013, 00:00   #31
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Quote:
Originally Posted by goboatingnow View Post

Sheesh , this pinpoint navigation fixation is all due to modern electronics.

Your celestial fix represents your best accurate fix, offshore , the Inaccurracy of that fix can be considerable. Since EP trumps DR , having a DR water distance method of greater accuracy is useless.

Check your trailing log against GPS 24 hour distance traveled ( not SOG ) that's way more accuracy then needed.

Or if you forsake GPS , check it against noon to noon sight runs.
Dave
Who said anything about pinpoint navigation? Plenty of GPS's onboard for that.

It would be nice to know how accurate a log is, in the same way it's nice to know how things work. Because finding things out is interesting.

Knowing how accurate an estimate of how many free miles you're getting from current on a long passage is interesting as well.
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Old 04-08-2013, 02:19   #32
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Re: Taffrail logs - ?

Whilst I agree that knowing how accurate (or inaccurate!) an instrument will be is useful - nonetheless an instrument only needs to be pinpoint accurate if you need to be exactly somewhere at a given time......and are only "flying" on instruments. With boats that is rarely the case, and certainly not when on extended passage. Very informative for sure, but that not the same as need.
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Old 04-08-2013, 03:43   #33
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Originally Posted by David_Old_Jersey View Post
Whilst I agree that knowing how accurate (or inaccurate!) an instrument will be is useful - nonetheless an instrument only needs to be pinpoint accurate if you need to be exactly somewhere at a given time......and are only "flying" on instruments. With boats that is rarely the case, and certainly not when on extended passage. Very informative for sure, but that not the same as need.
Quite agree.

Beyond usefulness, isn't curiosity enough on it's own to want to know more about the accuracy of the info the various bits of kit spit out?

Pretty sure the walker on board is fairly accurate but would be nice to know for sure. And if length of string makes a difference, not heard that one before. One day I might get round to spending an afternoon sailing back and forward somewhere.
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Old 04-08-2013, 05:13   #34
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Re: Taffrail logs - ?

I have used a Walker Log a lot on my old Contessa 26. No worries about the accuracy. It's very good, provided you follow the instructions. I would say between 1% and 2% of traveled distance through the water.
In the end I used it less and less, mainly due to the risk of an awful mess, with the more important fishing line, but also due to that I knew my boat so well that I normally estimated my daily runs to within 3% anyway.

If I streamed it, it would be on an occasion with a landfall coming up night time, or in bad visibility.

Thomas
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Old 04-08-2013, 07:53   #35
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Re: Taffrail Logs - ?

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Originally Posted by conachair View Post
Quite agree.

Beyond usefulness, isn't curiosity enough on it's own to want to know more about the accuracy of the info the various bits of kit spit out?
Yes!
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Old 04-08-2013, 08:56   #36
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Re: Taffrail logs - ?

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Originally Posted by mrohr View Post
Raku:I noticed that no one has answered your question regarding how speed is measured.
Of course there is a counter that is attached to a braided line that is attached to a towed spinner that reads out in nautical miles covered , but there is also a opening in the housing that enables one to count the revolutions of the spinning flywheel and when armed with a stopwatch the navigator could reference a diagram that gave the speed thru the water in knots.
I towed one across the Atlantic and they are very accurate ,totally mechanical devices that use no electricity, fool proof and work well in all weathers but speed is not accurate in drifting conditions or very slow speeds while the distance run still continues to work accurately
As noted, one does not want it to become entangled with a fishing line since the two lines will then be wrapped around each other thousands of turns...If contemplating a voyage RTW I would probably keep it aboard since the modern yacht is only 1 lightening strike away from the 18th century. PM me if interested in purchase of a lightly used one complete with original wooden box and spare spinners( sharks think they're yummy).
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Quote:
Originally Posted by goboatingnow View Post
Sheesh , this pinpoint navigation fixation is all due to modern electronics.

Your celestial fix represents your best accurate fix, offshore , the Inaccurracy of that fix can be considerable. Since EP trumps DR , having a DR water distance method of greater accuracy is useless.

Check your trailing log against GPS 24 hour distance traveled ( not SOG ) that's way more accuracy then needed.

Or if you forsake GPS , check it against noon to noon sight runs.
Dave
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Quote:
Originally Posted by David_Old_Jersey View Post
Whilst I agree that knowing how accurate (or inaccurate!) an instrument will be is useful - nonetheless an instrument only needs to be pinpoint accurate if you need to be exactly somewhere at a given time......and are only "flying" on instruments. With boats that is rarely the case, and certainly not when on extended passage. Very informative for sure, but that not the same as need.
That says it all about the need to accurately know your speed.

Having said that, I trailed a Wasp log (not a Walker - it has distance and speed on the dial) for over 30K miles. I lost one spinner to a shark and needed to clean it up when on the Sargasso sea, the only two "inconveniences", it was an indispensable piece of equipment. I also had a handheld GPS, which I checked the speed of the log against, they were pretty much the same. I still have mine and no, will no sell it.
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Old 04-08-2013, 09:02   #37
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Re: Taffrail Logs - ?

I also had a Walker Log back in 1976 or so. It was a beautiful thing in a fine wooden box. Used it quite a lot and it was very accurate.
Now-a-days I don't even turn on my Ray Marine knot meter. I find that knowing actual speed and distance over ground (from GPS) is much more useful than speed and distance trough water.
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Old 04-08-2013, 09:04   #38
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Re: Taffrail Logs - ?

G'Day all,

This has been an interesting discussion with some informed answers.

The reason that I posed the question in the first place was in response to the many opinions expressed over the years that the Walker log was far more accurate than any through hull paddle wheel type log, and I wondered about that.

By careful calibration vs GPS one can get a modern log to be fairly accurate, and if they are cleaned regularly the accuracy is maintained over time. Having done this, I have often then compared day's runs (GPS) with miles logged. Sometimes they agree pretty well, most times there are significant differences. These differences may stem from currents being present, from poor steering or from instrument inaccuracy... possibly other things that I've not thought of.

I suspect that the primary cause is indeed varying currents encountered at sea. These currents do have a lot of fine structure... little whorls and eddies that, in the days before GPS, were undetectable by mariners. One has only to sit and watch the differential between GPS speed and knotmeter speed as one proceeds across the sea to realize this. Or, have a look at the charts published daily by various government services showing the structure of important coastal currents such as the East Australia current or the Gulf Stream. They are very complex and change daily... those broad arrows shown on the pilot charts are pretty misleading!

If one is using the log reading to establish an EP as an entry point for celestial nav calculations these inaccuracies are unimportant. If using for DR in poor visibility and a hazardous piloting situation, then they become more of a factor. We don't do that sort of thing much nowadays, so no one thinks about the issue!

But I was curious... and I thank all who contributed information.

Cheers,

Jim
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Old 04-08-2013, 09:56   #39
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Re: Taffrail Logs - ?

I've calibrated my paddle wheel knotmeter using GPS on a lake on a day with no wind. When the calibration is done close to hull speed the knotmeter can be 0.2-0.4 knots off at slower speeds. My knotmeter has no adjustment for non-linearity.

On the lake sailing heeled over the knotmeter does not agree with the GPS. One issue with my knotmeter is that the paddlewheel is on one side of the hull aft of where the keel starts. I get different speeds on different tacks for the same speed on the GPS, neither of which agree with the GPS.

One interesting inaccuracy is when I heel between 20-25 degrees on starboard tack the paddlewheel is out just enough to get slapped by waves which spins it up. I got pretty excited the first time this happened thinking I was doing 8.5 knots on a close reach. Didn't take long to think that makes no sense.

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