Marine navigation
Duncan Heenan
----- Original Message -----
From: Dennis Pogson
Newsgroups: uk.rec.sailing
Sent: Friday, February 27, 2004 7:48 PM
Subject: Re: Marine Navigation
Sailor wrote:
> SNIP<.
> The Kerrera sound isn't difficult as long as you know about the dog
> leg in the bouyage half way up - may don't and get confused and the
> level of detail isn't great - I have seen too many go straight on and
> stop rather suddenly!!!!!!!!
I had a peculiar experience in Kerran Sound last summer. I was sailing
happily about half a mile off shore in about 100m depth, when my shallow
alarm went off. the depth gauge showed under 2 metres (the boat draws
1.85m). Naturally I nearly shat myself, double checked the chart, GPS etc.
in double quick time. About 100 metres further on the depth gauge went back
to about 100m. On looking at the chart carefully, notes in the margin said
'Local magnetic anomalies' without stating exactly where. So in the spirit
of experimentation, I sailed back over the same patch and the same thing
happened. I concluded it was a local magnetic anomaly upsetting the
instruments in some way I don't understand. It happened again several times
in the area later - every bit as frightening each time!
Alisdair Gurney
"Duncan Heenan" <duncanheenanR...@tiscali.co.uk> wrote in message
news:403fa...@mk-nntp-2.news.uk.tiscali.com...
Probably just jellyfish, plastic bag or turbulence. Happens a lot
in when there are strong tides or rivers around. If it happens
regularly then turn the sensitivity down.
Alisdair
Ian Sandell
<ne...@agurney.com> wrote:
>
>"Duncan Heenan" <duncanheenanR...@tiscali.co.uk> wrote in message
>news:403fa...@mk-nntp-2.news.uk.tiscali.com...
>>
SNIP
>> to about 100m. On looking at the chart carefully, notes in the margin said
>> 'Local magnetic anomalies' without stating exactly where. So in the spirit
>> of experimentation, I sailed back over the same patch and the same thing
>> happened. I concluded it was a local magnetic anomaly upsetting the
>> instruments in some way I don't understand. It happened again several
>times
>> in the area later - every bit as frightening each time!
>>
>
>Probably just jellyfish, plastic bag or turbulence. Happens a lot
>in when there are strong tides or rivers around. If it happens
>regularly then turn the sensitivity down.
On many (most?) modern echosounders the sensitivy adjustment is
automatic.
Ian
Mike James
I think I have seen electronic sounders go round the clock : pulses sent
out during one measurement interval actually arrive early in the next
interval. A depth will jump from 100 metres to 2 metres (actually more
like 102 metres)
On the old whirling dial Seafarers this was also obvious but scary as
the bottom would appear to be only a few feet down on the 'shallow'
range in moderately deep water, switching the range to 'deep' would show
that it was not the case.
This is why I prefer to use a fishfinder as the shape of the echoes and
the sea bed is plotted against time, as well as the computed depth.
Looking at the two together shows whether the sounder is confused.
fraggy
you think that is scarey, once I was sailing up the east coast and took the
quick inshore route past Orfordness lighthouse. about 50yds off the beach in
about 30feet of water.
I had gotten past and was nearing the Gov Listening post and my depth
sounder was lit up all the way round like a xmas tree. Added to that, at the
same time I had church bells coming over the VHF radio ??????
I wasnt far from Dunwich (sunken Village ) and they say that some times you
can hear the church bell ring.. SPOOKY
fragged
"Duncan Heenan" <duncanheenanR...@tiscali.co.uk> wrote in message
news:403fa...@mk-nntp-2.news.uk.tiscali.com...
>
Gerald Simons
Am happy with the software and backup support they have provided, also
has a free user group. Reads many chart formats as well as scan-own
capabilities. http://www.globenav.com/
Regards
Gerald Simons
"fraggy" <frag...@SPAMhotmail.com> wrote in message news:<c1omja$3fl$1...@hercules.btinternet.com>...
Alisdair Gurney
"Ian Sandell" <i...@sandell.co.uk> wrote in message
news:2vhv30pmta1j7mpij...@4ax.com...
> On Fri, 27 Feb 2004 22:18:17 -0000, "Alisdair Gurney"
> <ne...@agurney.com> wrote:
>
> >
> >"Duncan Heenan" <duncanheenanR...@tiscali.co.uk> wrote in
message
> >news:403fa...@mk-nntp-2.news.uk.tiscali.com...
> >>
> SNIP
> >> to about 100m. On looking at the chart carefully, notes in the margin
said
> >> 'Local magnetic anomalies' without stating exactly where. So in the
spirit
> >> of experimentation, I sailed back over the same patch and the same
thing
> >> happened. I concluded it was a local magnetic anomaly upsetting the
> >> instruments in some way I don't understand. It happened again several
> >times
> >> in the area later - every bit as frightening each time!
> >>
> >
> >Probably just jellyfish, plastic bag or turbulence. Happens a lot
> >regularly then turn the sensitivity down.
>
> On many (most?) modern echosounders the sensitivy adjustment is
> automatic.
..which you can override if there are too many false readings, rtfm.
Keelworm
> message news:403fa...@mk-nntp-2.news.uk.tiscali.com...
>>
>> ----- Original Message -----
>> From: Dennis Pogson
>> Newsgroups: uk.rec.sailing
>> Sent: Friday, February 27, 2004 7:48 PM
>> Subject: Re: Marine Navigation
>>
>>
>> Sailor wrote:
>> > SNIP<.
>> > The Kerrera sound isn't difficult as long as you know about the dog
>> > leg in the bouyage half way up - may don't and get confused and the
>> > level of detail isn't great - I have seen too many go straight on and
>> > stop rather suddenly!!!!!!!!
>>
>> I had a peculiar experience in Kerran Sound last summer. I was sailing
>> happily about half a mile off shore in about 100m depth, when my shallow
>> alarm went off. the depth gauge showed under 2 metres (the boat draws
>> 1.85m). Naturally I nearly shat myself, double checked the chart, GPS
>> etc. in double quick time. About 100 metres further on the depth gauge
>> went
> back
>> to about 100m.
W had similar experiences off Ithica (Ionians). Apparently it's quite
common to get false readings at large depths. Something about the echo
returing late for it's own reading, but early for the next...
ie.
send ----------------> receive
beep 1 ..................
beep 2 ................ Beep 1.
beep 3 ................ Beep 2
beep 4 ................ Beep 3
etc. etc. etc.
(Or this was how it was explained to me by some retired bearded mariner who
had total distain for modern nautical gadgetry...)
Cheers.
Phil.
--
ALTERNATIVE SAILING TERMS:
Compass:
Navigational instrument that ... indicates the presence of machinery and
magnets on board ship by spinning wildly.
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Hugo 'NOx' Tyson
Keelworm <tr...@REMOVETHECAPSgoldingweb.co.uk> writes:
> > "Duncan Heenan" <duncanheenanR...@tiscali.co.uk> wrote in
> > message news:403fa...@mk-nntp-2.news.uk.tiscali.com...
> >> happily about half a mile off shore in about 100m depth, when my shallow
> >> alarm went off. the depth gauge showed under 2 metres (the boat draws
> >> 1.85m). Naturally I nearly shat myself, double checked the chart, GPS
> >> etc. in double quick time. About 100 metres further on the depth gauge
> >> went back to about 100m.
>
> returing late for it's own reading, but early for the next...
>
> ie.
>
> send ----------------> receive
> beep 1 ..................
> beep 2 ................ Beep 1.
> beep 3 ................ Beep 2
> beep 4 ................ Beep 3
>
> etc. etc. etc.
>
> (Or this was how it was explained to me by some retired bearded mariner who
> had total distain for modern nautical gadgetry...)
;-) I'd believe that. In general the word is "aliasing" - as in why wagon
wheels spin backwards in the Saturday afternoon cowboy film, effects like
that.
- Huge
PyroJames
Mike James <mi...@hamble.demon.co.uk> wrote in message
news:c1olqg$ghk$1$8302...@news.demon.co.uk...
> On the old whirling dial Seafarers this was also obvious but scary as
> the bottom would appear to be only a few feet down on the 'shallow'
> range in moderately deep water, switching the range to 'deep' would show
> that it was not the case.
This why I like the old spinning dial. It's generally very obvious when you
have false signals. Even if you do get jelly fish etc, giving a shallow
echo, you still have the original bottom echo, so you don't panic. You can
also read them at great distance without need to see the numbers as the
depth reads as a foot (or fathom) for each minute past the hour. Mine is
mounted over the chart table, and visble from the cockpit. Well protected
from the elements and easy to read in each place.
--
PyroJames
"Every existant is born without reason, prolongs itself out of weakness,
and dies by chance." JPSartre