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Twister_Ken

Twister_Ken

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Having installed a new-fangle through-the-hull log, I need to calibrate it. To keep the sums easy, I was looking on my charts for a measured mile, somewhere in the Solent area.

Nary a one could I find. Then I noticed on the western shore of Southampton Water, two contiguous measured distances, marked by transit posts, and each identified on the chart as "measured distance 926.6 metres".

Only when contemplating life, etc. in the reading room did it occur to me that 926.6 metres has to be pretty close to 0.5nm. Turns out is is exactly.

So why doesn't UKHO signpost it on the charts as a measured mile, or more accurately, two contigous measured half miles?
 
They lurk all over the place, for example there are two in Chichester harbour, One below Itchenor and one in the Emsworth channel.
 
Not very seamanlike, I know, but I've given up on the measured distance calibrations and tend to adjust mine against the GPS, at slack water, in the Beaulieu. The b****y thing constantly varies in reading, with hull fouling etc, even though I withdraw the paddle after every trip.
 
I might be missing something here but is it not easier to use your GPS to measure out your own mile?. I would have thought that would be more accurate than trying to run transits as you pass the shore based mile markers.
Just a thought
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“Artificial intelligence is no match for natural stupidity”
Skype id:cliffillupo
 
[ QUOTE ]
Only when contemplating life, etc. in the reading room did it occur to me that 926.6 metres has to be pretty close to 0.5nm. Turns out is is exactly.

So why doesn't UKHO signpost it on the charts as a measured mile, or more accurately, two contigous measured half miles?

[/ QUOTE ]

Probably in an effort to stop all us yotties from cluttering the place up, while we try to calibrate the uncalibratable.
Now you've blown it, there will a queue there on Saturday morning, and colregs questions asked on Monday, vis, "Am I the right-of-way vessel when calibrating my log?"

Alistair
 
I do know how to do things the traditional way but I am not a masochist - If there is an easier, better way then I generally use that unless there is a very good reason to do it the hard way - I think it is called progress.
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hammer.thumb.gif
“Artificial intelligence is no match for natural stupidity”
Skype id:cliffillupo
 
Agreed... but I have also learned the hard way not to rely on electronics... and while preferring to use the easier/newer way, still like to be seamanlike, and know how to do things without the aid of electronics..... and that often means (for me anyway) just having a go.....
 
"Wouldn't a calibration against the GPS need zero tide to be accurate ?"

Not on my gizmo, apparently you do one run uptide, and one run downtide and it's clever enough to average them. The problem (relatively minor) with doing it on the jeeps is that you need to start and finish at the same place, so you need to put it a coupla waypoints. Whereas, if you start and finish on transits at least you don't need to button-push the jeeps.

Naturally, it'll be foggy on the day I try it!
 
Has anyone noticed a non linear calibration with their log ?

In my last two boats, if gps(measured mile or whatever) and log show the same reading at say 5kt, the log will under-read or over-read at any other speed, by not much but there is a consistent 0.2/0.4kt difference ?

is this a common occurence ? [log is AH st6000]
 
UKHO has gorn metric, or at least that's what that little symbol with an "m" in it means - I think.
We got a good 'un twixt Looe and Polperro you could try! It's even got lights on it (sometimes).
 
It happens on my Simrad IS15. I only use it as a guide or when it's outrageously out on the high side - everyone feels good at 9 knots!
 
I haven't got a log, well, I have. It's a lovely big EMI Electrolog sitting in a big hole in my bulkhead. The through hull is beautifully designed so the little propellor can be retracted and then sealed off from the 'oggin when not in use. Trouble is, I haven't got a little propellor thingy and can't get one.

But, I digress. Correct me if I'm wrong, but for navigation and most other purposes, isn't it SOG that matters, rather than speed through the water?

If so, the GPS is a far more important bit of kit, and the log is really only an aid to sail trim, when all that really matters is the change in speed. To oversimplify, if it goes up you're doing something right, if it goes down you're doing it wrong. Likewise, if you used to do an indicated 6 knots under power and now only do 5, it's probably time to visit the scrubbing bay.

The absolute reading is only really useful for bragging in the bar, so an inaccurate log is an advantage - as long as it's optimistic /forums/images/graemlins/smile.gif
 
I callibrate my log with that measured mile in Soton water beginning of every season. I was also foxed by the 926.6m when I first did it and it took me a few minutes to realise that was half a mile too. So, I agree, why don't they just mark it as a measured (half) mile on the chart in the first place?
 
If the log (speed) and gps are in general agreement, in slack water, then the calibration ain't going to be too far out. I then tend to compare the distance reading after a gps mile as a double checkand make any compensations that may be necessary. Keep meaning to get meself a nice trailing jobbie, that can be properly calibrated, and trusted, without worrying about hull fouling error etc.
 
[ QUOTE ]
Agreed... but I have also learned the hard way not to rely on electronics... and while preferring to use the easier/newer way, still like to be seamanlike, and know how to do things without the aid of electronics..... and that often means (for me anyway) just having a go.....

[/ QUOTE ]Ah, not relying on electronics, knowing the proper way -
oh, sh!t, the gps has just blown up... where the hell did I put the hand-bearing compass, OK used that 5 mins ago... where is that dratted chart... OK, few teastains, but we'll be allright... now where's the Breton plotter.. or the parallel rules... or that gadget my wife gave me for Xmas.... hell, what was that big bump..............
 
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