GPS speed V Through the water speed

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BarnacleBill
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GPS speed V Through the water speed

Postby BarnacleBill » Tue Jun 21, 2016 6:23 pm

Hi all
I can't quite get my head round this :D While sailing from Millport last weekend, my GPS displayed the boat speed as 4.3 knots, while the NASA Target Log with a clean paddle wheel displayed 2.2 knots sailing into the oncoming tide. I'm assuming the tide was running 2 knots (didn't bother to check) therefore knocking 2 knots from the GPS speed. What I don't get is why the difference between GPS speed over ground and speed through the water and what speed was the boat actually doing?
Cheers
Billy

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claymore
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Re: GPS speed V Through the water speed

Postby claymore » Tue Jun 21, 2016 8:25 pm

Your log is telling you how quickly you are sailing
Your GPS is telling you how quickly you are covering the ground
Are you sure you had your tide right? It would suggest the tide was with you, not against.
Hope that makes sense!
Regards
Claymore
:goatd

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BarnacleBill
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Re: GPS speed V Through the water speed

Postby BarnacleBill » Tue Jun 21, 2016 11:02 pm

Hi Claymore, I was sailing...well, motoring into an ebb tide

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Booby Trapper
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Re: GPS speed V Through the water speed

Postby Booby Trapper » Wed Jun 22, 2016 7:41 am

GPS is your correct speed. 2.2 knts on the log sounds wrong. If you were sailing into the tide I would expect to see more than the gps speed on the log. Imagine you are doing 5 knots over the ground (gps speed) but you are sailing into a tide of 1 knot the log should read 6 knots.
Your Log is under reading. Around Millport I would not expect to see a tide of 2 knots having said that the tide through the Tan can in some places not be going the way you would expect it Also the bit in from of Keppel pier can have quite a bit of flow sometimes.

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Nick
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Re: GPS speed V Through the water speed

Postby Nick » Wed Jun 22, 2016 9:23 am

.
If one of the two magnets is missing from the log paddle wheel it will read half the actual speed.
- Nick 8)

Image

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mm5aho
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Re: GPS speed V Through the water speed

Postby mm5aho » Wed Jun 22, 2016 10:25 am

I found the need to calibrate my log recently, so did it against the GPS in the Crinan Canal at a couple of speeds. Needs to be still water to do that, and most tidal places have moving water.
Previously had a discrepancy that I could not explain.
Self calibration is easy, according to the book.
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aquaplane
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Re: GPS speed V Through the water speed

Postby aquaplane » Wed Jun 22, 2016 5:14 pm

The stretch of water off NE Arran, where the measured mile thing is, has low tidal streams, I wonder why they put the measure there?

You will still need to measure going both ways and take an average but doing it at slack water neaps will help.

Sorry if Gran has just been given an egg sucking lesson.
Seminole.
Cheers Bob.

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Booby Trapper
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Re: GPS speed V Through the water speed

Postby Booby Trapper » Wed Jun 22, 2016 7:52 pm

aquaplane wrote:The stretch of water off NE Arran, where the measured mile thing is, has low tidal streams, I wonder why they put the measure there?

You will still need to measure going both ways and take an average but doing it at slack water neaps will help.

Sorry if Gran has just been given an egg sucking lesson.


I think those posts were used by the QE2 when she was commissioned . We've got GPS now :P . Easier ways to measure a mile and you wouldn't have to go all the way to Arran to do it.

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Re: GPS speed V Through the water speed

Postby Aja » Thu Jun 23, 2016 3:17 pm

Booby Trapper wrote:
aquaplane wrote:The stretch of water off NE Arran, where the measured mile thing is, has low tidal streams, I wonder why they put the measure there?

You will still need to measure going both ways and take an average but doing it at slack water neaps will help.

Sorry if Gran has just been given an egg sucking lesson.


I think those posts were used by the QE2 when she was commissioned . We've got GPS now :P . Easier ways to measure a mile and you wouldn't have to go all the way to Arran to do it.


You just have to go as far as the measured mile at Skelmorlie

Donaldq

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BarnacleBill
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Re: GPS speed V Through the water speed

Postby BarnacleBill » Thu Jun 23, 2016 9:40 pm

Cheers guy's
I think I'll read the instructions on how to re-calibrate the Target Log. I take it that the log speed should match the GPS speed?

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Re: GPS speed V Through the water speed

Postby marisca » Thu Jun 23, 2016 9:49 pm

Aja wrote:
You just have to go as far as the measured mile at Skelmorlie

Donaldq


Except that the north back post has disappeared.

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Re: GPS speed V Through the water speed

Postby Booby Trapper » Thu Jun 23, 2016 11:26 pm

BarnacleBill wrote:Cheers guy's
I think I'll read the instructions on how to re-calibrate the Target Log. I take it that the log speed should match the GPS speed?

Only if you have no tide

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BarnacleBill
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Re: GPS speed V Through the water speed

Postby BarnacleBill » Sat Jun 25, 2016 12:59 pm

Getting my head round this now. One of those Doh! moments :lol:


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