Walker log accuracy

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awyatybw

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On our recent trip we trailed the Walker log. I generally do this on long trips as I consider it more accurate than the Raymarine paddle-wheel thingy (at least with the state my transducer is in).

But this time it was clearly overreading - best estimate was around 8%. I had always thought that these things were pretty bloody accuracate. I have not noticed a significant discrepency from reality in the past.

Sorry, don't have the model number etc. to hand. Any ideas about the possible causes or other experiences with these logs?

Alan.
 
Nope, flat calm. And I checked that there was no sign of the spinner surfacing.

I guess that there was a reasonable stern-wave cause by our motoring at around 6.5 kts. Surely that could not have a significant influence. I think that the line to the spinner is around 15m long.
 
Only way to check accuracy of any log is to do the measured distance both ways ....

Any other is just an indicator and insufficient to rely on.

How did you arrive at your 8% error ? Give us some relevant data to work from and we may be able to deduce where errors may have got in .... What data / source did you use to deduce such error ?
 
Can't find the 'structions BUT . . . at what speed should one double the string length?
AND . . . is it mounted on the correct corner of the transom?

Anyway, you'll have to run a measured distance to check it - so you might as well calibrate your paddlewheel on the other bit of kit!
 
30 feet of line is OK up to 10knots provided the register is not more than 5 feet above the WL. For higher speeds or if the register is mounted higher, double the length of line and maybe add another sinker too.
 
[ QUOTE ]

How did you arrive at your 8% error ? Give us some relevant data to work from and we may be able to deduce where errors may have got in .... What data / source did you use to deduce such error ?

[/ QUOTE ]

I'm afraid that I have left the logbook with all these details on the boat and will not be back to it for a couple of weeks.

I agree that my estimates may not be accuracate, not having been done against a measured-mile etc. Two of us agreed with the results from a number of comparisons with GPS-derived data over several days and I felt sufficiently confident to post the question on that basis.

But I'm not asking for judgement as to whether the log was overreading - I am quite certain that it was, and 8% was at the conservative end of the estimates.

The effect of squirt from the engine is the most plausible one so far, as I am confident that the sinker was keeping the spinner well below the surface.

I have not recently oiled it - in fact that is a maintenance item on the "one of those to do while sitting in the cockpit of an evening" list. My simple mind fails to see how lack of oil might cause it to overread but I am happy to be corrected.

Any more ideas or other experience of these things overreading?

Alan.
 
Re: Walker log accuracy .... comparison ?

"Two of us agreed with the results from a number of comparisons with GPS-derived data over several days "

Sorry but you cannot compare GPS with thru water ... unless you wish to derive current prevailing at the time ...

It's actually why I asked about how you derived the error - as it sounded like that ... my suspicion was confirmed.
 
Re: Walker log accuracy .... comparison ?

If you are using speed readout from GPS and comparing to log read out I would agree.
To use a GPS to calibrate log, set up two way points a given distance apart running with/against direction of tide and do a few runs to and fro with stop watch and then apply same calcs as you would do for measured mile this will give you a pretty fair approximation of speed thru water.
 
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