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Old 19-08-2018, 06:36   #1
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Log vs GPS Speed - Why the Difference?

Iíve been observing log (paddle wheel type) vs GPS (SOG) speed. In general they show about the same while reaching or running, but the log shows as much as 1 kt higher when close-hauled or under power.

This happens consistently enough in all directions that I discount currents as the cause, so can anyone explain other causes?

Itís not a big issue, but Iím curious why itís happening.
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Old 19-08-2018, 06:55   #2
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Re: Log vs GPS Speed - Why the Difference?

Pure speculation, but

1 one is more accurate than the other
2 the current

#2 can be tested by reversing / changing direction, see how the delta changes
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Old 19-08-2018, 07:00   #3
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Re: Log vs GPS Speed - Why the Difference?

It might be related to the position of the paddle wheel in relation to the keel or .... something????
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Old 19-08-2018, 07:00   #4
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Re: Log vs GPS Speed - Why the Difference?

Not current - Iíve tested that and it doesnít make a difference if I turn 180 degrees.

I assume GPS is accurate, which suggests that the log is accurate under some conditions but not all. But I donít understand why.
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Old 19-08-2018, 07:02   #5
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Re: Log vs GPS Speed - Why the Difference?

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It might be related to the position of the paddle wheel in relation to the keel or .... something????
Hadnít thought of that but it might make sense. Close-hauled and motoring would be similar in those conditions, while a reach would be different. Will have to ponder this one.
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Old 19-08-2018, 07:03   #6
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Log vs GPS Speed - Why the Difference?

Mine reads higher on port tack vs starboard tack. Makes sense when I look under the boat.

Faster reading under engine makes sense if the log is getting some intake effect from the prop.

No ideas about why close hauled would cause that effect though.
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Old 19-08-2018, 07:11   #7
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Re: Log vs GPS Speed - Why the Difference?

Could be the way the water speeds up locally when flowing around the keel or any edge or chine on the hull.
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Old 19-08-2018, 07:22   #8
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Re: Log vs GPS Speed - Why the Difference?

Interferance on the GPS when close hauled?
Paddle wheel moves more freely on different points of sail?
Water speed across the hull changes when heeled over? Like an airplane wing the air is faster over the top of the wing?
Maybe youíre going so fast the GPS canít keep up
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Old 19-08-2018, 07:23   #9
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Re: Log vs GPS Speed - Why the Difference?

Whatís a ďchineĒ?
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Old 19-08-2018, 07:31   #10
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Re: Log vs GPS Speed - Why the Difference?

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Whatís a ďchineĒ?
GIYF

https://en.m.wikipedia.org/wiki/Chine_(boating)
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Old 19-08-2018, 07:33   #11
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Re: Log vs GPS Speed - Why the Difference?

Btw. some of the cheaper GPS receivers have considerable lag when it comes to displaying speed.
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Old 19-08-2018, 07:33   #12
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Re: Log vs GPS Speed - Why the Difference?

A keel is of course an airfoil, if the paddle wheel is in the lifting side, it will likely read high
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Old 19-08-2018, 08:36   #13
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Re: Log vs GPS Speed - Why the Difference?

As A64 sez.

Start by considering the GPS speed. GPS speed is SOG averaged over some time interval.

Then consider the waterflow around the hull which is what drives the paddlewheel. IF the paddle is positioned on the weather side of the keel where it can be affected by the waterflow over the "upside" of the foil that is either built into the keel by virtue of its section (like an aerofoil) or by the "pseudo-foil" that is generated by differential pressures in the waterflow around it, then as the keel "lifts" the boat to weather, as fin keels are intended to do, the waterflow - fore to aft - over the keel will be faster on the weather side than it will be on the lee side.

The paddle will give the instantaneous STW subject to inaccuracies arising from fluctuations in waterflow around the hull and therefore over the paddle wheel.

But at 5 knots, why worry about it - except for curiosity's sake :-)

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Old 19-08-2018, 08:39   #14
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Re: Log vs GPS Speed - Why the Difference?

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A keel is of course an airfoil, if the paddle wheel is in the lifting side, it will likely read high
This. Local differences in water flow. Coupled with a long averaging time on the GPS. Go into the gps settings and set the averaging time to as short as possible. Do the same with the knot meter if you can.
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Old 19-08-2018, 08:42   #15
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Re: Log vs GPS Speed - Why the Difference?

Might be due to water flow over paddles when boat is heeled. Flow may be less as paddles become more at an angle to the flow of the water. Why this happens at downwind sail is still a question.
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