can a Walker log under-read

[phein]

I towed a Walker log over about 500 miles last year and allowing for tides it seemed to underread by about ten per cent compared to the GPS. How come? If the internal mechanism was sluggish, the string would knot up, wouldn't it? So how can a Walker long underread?

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[VicS]

The log records distance through the water while the gps gives distance over the ground. If you sail with favorable tides the log reading will be lower. Also the spinner of a trailed log tends to get fouled by weed etc and under read. It will also under read if it does not stay properly submerged. It should also be trailed in undisturbed water not directly in the boats wake.

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[snowleopard]

there is some internal resistance in a trailing log which may not be enough for the line to 'knot up'. i have a wasp log which does knot up below 2 knots. any of the following could cause it to underread:

damaged rotoator blades

wrong rotator

dirt, corrosion or old oil in the head

tatty string

i would do a timed run over a measured mile and calculate the error then you can apply corrections in future.

p.s. if you filled in your profile someone could advise you where to find your nearest measured distance.

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[snooks]

Another thought

Don't forget there are time when you don't use the walker log, finding an anchoridge, manouvering, going out of harbour, at anchor etc...the last one might sound strange but if your GPS was left on whilst at anchor the boats movment would also go to the total distance travelled...just a thought

We couldn't understand why the boat GPS was different to the hand held GPS which was also on the boat (after a short trip of 10 or so miles there was 2 mlies difference!), until someone confessed they'd left it in their packet when they'd gone for a walk around the town :-D

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[drawp]

Questions re a Walker log.

How long does the cord in a Walker log need to be. If the log is say, 1 metre above the water line and the boat is sailing at 7 knots, how long should it be? Anybody got any idea? If it was too short it might tend to skip across the water. Also what type of braid should you use if you wanted to replace the line?

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[Guest]

The old ones are best aren\'t they !

GPS gives as near as it can .... SPEED / DISTANCE over ground - with no allowance or detection of tidal / current water related influence.

Log gives distance through the water and cannot be compared in any form or way to that over the groundf without correcting for the effect of moving water during the period of measurement that has influenced the movement of the boat ..... PHEW ....

EG : GPS reads 10 miles travelled .... that would plot direct on the chart and be seen as near as damn it 10 miles.
Log reads 15 mils travelled - with a total current against totaling 5 miles .... or LOG reads 6 miles and total current with calculated at 4 miles ......

Get the point ???? The boat moves with the water and the actual movement of the boat relative to the water is imposed on top of that combined movement ....

Oh b*******s ...... the two are not the same !!!!



<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[Neal]

didn\'t the original post....

recognise that difference?

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[Twister_Ken]

Wrong or damaged spinner. Weed, Towed too close to boat, so operating in turbulence. Doesn't work at very slow speeds. Heaved aboard when entering/leaving port. Bashing the bottom in shallow waters. Plaything for an amorous fish.

One or more of the above.

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[Guest]

It mentions tides ....

but there are far more factors than just tides / currents that can do it and to quote 10% is actually not so bad I feel.

There is of course the accumulated error in GPS, so what is the real difference as introduced by the walker log ???? Maybe the GPS is accounting for 2-3 %, plus the factors of water movement that are uncharted / un-tabulated ..... soon starts to mount up.

No-one calculates current drift / set minutely, we all do it to practical intervals and in fact it would be difficult to do it that accurately to really be spot on, as the admiralty only give it a range across a time period ..... and affected by neaps / springs etc. etc.

So calculate as hard as you like, tell me I'm wrong .... but you will never get them THAT close that this question would be voided !!! If you do - I'd claim a fluke !!!! Because go back and read the time period he's talking about over a significant period ... a year if I remember right .....



<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[Guest]

I read somewhere ....

a blue-water guy who kept having his rotator bitten off by hungry fish - painted his log rotator matt black ...... seemed to cure the amorous and hungry fish instinct .....


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[Mirelle]

Some possibilities

Wrong spinner, line too short or wrong type (should be a fairly stiff braided line) no sinker on line ahead of spinner, weed round spinner, bearings in mechanism shot (rare, but has happened to me, only likely to be a problem if the unit has done about 10K miles and has not been well oiled in that time) unit will not read at very low speeds - below about 2 knots.

All very simple and logical mechanical stuff.

In heavy weather, they over-read slightly, as the spinner goes up and down over waves rather than straight through them.

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[Birdseye]

tide data is measured at specific points and extrapolated to fill the intervening gaps.

i zeroed my log by the mile posts in a canal (with embarrassed correction afterwards when i realised it was reading mph not knots) so can compare gps with log to deduce tide speeds with some accuracy.

conclusion is that the tide data is not that accurate where extrapolated.

in any case, a mechanical instrument will never reach the accuracy of a decent electronic one. bit like comparing an old timex mechanical watch with a modern digital one.

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[Mirelle]

Cautiously disagree

I would say that, if anything, the towed Walker is a more accurate guide to distance run than modern through hull logs, which are working in disturbed water flow, and which are much more likely to hang up on bits of weed, etc. At least, such is my experience, having spent some time sailing up and down between mile posts towing a Walker whilst trying to cakibrate the through hull gadget! An excepton is slow speeds in light airs where the little paddle wheel starts to rotate long before the Walker rotator gets going.

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[VicS]

Re: Questions re a Walker log.

From the instruction leaflet for KDO & KDS Mk IIIA (bought 1979):-

"When deciding on the position for the Register, two things must be borne in mind. The rotor should, as far as posible, be streamed in undisturbed water and the dial should be readable without leaving the cockpit."

"For speeds up to 10 knots with the Register not more than 5 feet above W.L. 30 feet of line with one sinker fitted 18in. ahead of the Rotator will be sufficient. For higher speeds, particularly if the Register is more than 5 feet above W.L. it is necessary to use a longer length of line with a maximum of 60 feet at 15 knots. An additional sinker is provided for the higher speeds and should be fitted to the line 3 feet ahead of the original sinker if there is any tendency for the rotator to porpoise. The rotator will normally run only just below the surface with the sinker submerged. At higher speeds, using two sinkers, it is normal for the leading sinker to break surface.

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[drawp]

Re: Questions re a Walker log.

VicS, What an excellent reply! It proves the old expression "When all else fails, read the instructions". Thanks.

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