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28-07-2018, 23:51
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#1
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Moderator
Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Posts: 21,156
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How high is that wire... REALLY?
Well, we had planned to move on up the Clarence from Ulmarra to Grafton today. We knew that there is a HT cable crossing a few miles downstream from the bridge where we wanted to anchor. We had two cruising guides for the Clarence: Alan Lucas (a somewhat elderly edition) and one produced by the locals, utilizing official NSW Maritime charts. Lucas says that the wire clearance at HAT is 19.8 m. The local guide and its official chart shows 21 m. Our air draft is 21 m, so we thought that by easing over near the shore, somewhat up on the catenary of the cables we would be OK, especially if we were at less than HAT.
So today we moved on up the river, and as we passed the end of Elizabeth Island and the wires came into sight there was a big ass sign saying maximum height of vessel was 17 meters. Yikes! Anchored and had a look by dinghy, and the wires sure looked lower than the 20.3 m wires we pass under regularly in the Broadwater in Qld. My batteries were not in need of a really fast charge, so we abandoned the idea of moving further up and returned to Ulmarra, somewhat discouraged.
So, my question is does anyone have personal experience of going under those wires with a similar height mast, or access to more exact data? I'll try calling the local Maritime officer tomorrow, but since he drives around in a small rescue boat, I dunno if he's up to speed on such matters.
Jim
__________________
Jim and Ann s/v Insatiable II, lying Port Cygnet Tasmania once again.
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29-07-2018, 00:21
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#2
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Moderator
Join Date: Aug 2009
Location: Solent, England
Boat: Moody 31
Posts: 18,429
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Re: How high is that wire... REALLY?
Do you have a sextant on board? would you trust the maths enough?
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29-07-2018, 00:32
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#3
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Registered User
Join Date: Aug 2011
Location: Petersburg, AK
Boat: Outremer 50S
Posts: 4,229
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Re: How high is that wire... REALLY?
Jim,
no personal experience at that location, but I've been in similar 'energizing' situations with our own 20m airdraft.
Have you seen this from RMS?
Upper-Clarence-River-insets-Grafton-Brushgrove-Ulmarra.pdf
It shows 17m.
My electronically updated chart NSW2C6 also shows 17m.
Lucas' 6th Ed. Update issued late 2017 says:
Quote:
Page 305. The 19.8-metre high overhead cables across the main channel at Elizabeth Island have been lowered – or at least re-signed – to 17-metres.
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Sorry, but it looks like all the 'official' sources now say 17m.
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29-07-2018, 00:47
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#4
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Registered User
Join Date: Nov 2006
Location: San Francisco Bay
Boat: Fantasia 35
Posts: 1,251
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Re: How high is that wire... REALLY?
Jim,
I have seen YouTube videos of boats using water ballast on the end of the boom to heel the boat to clear under bridges.
Just a thought!
Best regards,
Paul
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29-07-2018, 00:49
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#5
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Moderator and Certifiable Refitter
Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: C.L.O.D.
Posts: 20,305
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Re: How high is that wire... REALLY?
Sorry can't help with local knowledge.
Perhaps the 17m is a CYA approach by the "authorities"; perhaps it used to allow for flood heights; perhaps it is global warming but whatever is the real clearance, I hope you know for sure before proceeding...
There was some flood migration works carried out around that area in the last decade or so but surely that didn't raised the water levels.
Large weight on the end of a guyed out boom, lots of induced heel???
Don't want to see new thread "Simulated lighting strike at Grafton"
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
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29-07-2018, 01:25
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#6
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Moderator
Join Date: Nov 2011
Location: aboard, cruising in Australia
Boat: Sayer 46' Solent rig sloop
Posts: 28,438
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Re: How high is that wire... REALLY?
Quote:
Originally Posted by Wotname
Sorry can't help with local knowledge.
Perhaps the 17m is a CYA approach by the "authorities"; perhaps it used to allow for flood heights; perhaps it is global warming but whatever is the real clearance, I hope you know for sure before proceeding...
There was some flood migration works carried out around that area in the last decade or so but surely that didn't raised the water levels.
Large weight on the end of a guyed out boom, lots of induced heel???
Don't want to see new thread "Simulated lighting strike at Grafton"
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No, way, Wottie,
We went up there, took a look, difficulties of accurate enough measurement were discussed, and to our disgust, we moved off. Had a nice dinghy trip further up the river, and returned to Ulmarra. No electrifying moments for us.
Ann
__________________
Who scorns the calm has forgotten the storm.
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29-07-2018, 01:40
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#7
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Moderator
Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Posts: 21,156
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Re: How high is that wire... REALLY?
Well, DANG! Looks like we are barred from further up river. I thought about the sextant angle measurement but don't trust my skills to determine height with sub-meter accuracy. Determining the distance off accurately enough is problematic, and besides, I'd have to unpack the shed to get down to my sextant. Don't get it out very often these days!
They appear to be building a new bridge there in Grafton as well as at Harwood. Lots of tax dollars wafting away!
Jim
__________________
Jim and Ann s/v Insatiable II, lying Port Cygnet Tasmania once again.
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29-07-2018, 01:50
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#8
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Registered User
Join Date: Aug 2013
Location: Back in Montt.
Boat: Westerly Sealord
Posts: 8,180
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Re: How high is that wire... REALLY?
Method #1
Measure height of pylons ( with vertical sextant angle )
Measure distance between pylons ( horizontal sextant angle )
Calculate catenary.... must be a formula for that.
Allow for tide, apply suitable fudge factor, etc.
Method #2
Catch a bus.....
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29-07-2018, 02:29
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#9
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Registered User
Join Date: Sep 2006
Posts: 9,398
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Re: How high is that wire... REALLY?
Can't give you a definitive answer, but we went under with our air draught of around 18.5m, and from what friends on another boat told us, we had plenty of clearance. I took a photo of their boat, similar air draught, and yes, it looked like the best part of 10m clearance. Close to the pylon, obviously.
I'll have to search for the photo.
__________________
"You CANNOT be serious!"
John McEnroe
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29-07-2018, 02:51
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#10
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Moderator
Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Posts: 21,156
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Re: How high is that wire... REALLY?
Quote:
Originally Posted by 44'cruisingcat
Can't give you a definitive answer, but we went under with our air draught of around 18.5m, and from what friends on another boat told us, we had plenty of clearance. I took a photo of their boat, similar air draught, and yes, it looked like the best part of 10m clearance. Close to the pylon, obviously.
I'll have to search for the photo.
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thing is, you can't get close to the tower, for it is quite a long way inland from the shore. As the thing is structured now, there isn't a huge catenary. I was pretty disappointed, but not inclined to experiment!
I've seen some pretty long arcs back in the day job, never wanted to be on the receiving end!
BTW, when was that? apparently they have lowered the wires by some several meters (dirty buggers).
Jim
__________________
Jim and Ann s/v Insatiable II, lying Port Cygnet Tasmania once again.
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29-07-2018, 02:52
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#11
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Moderator
Join Date: Nov 2011
Location: aboard, cruising in Australia
Boat: Sayer 46' Solent rig sloop
Posts: 28,438
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Re: How high is that wire... REALLY?
Youse guys are great! Thanks for all the encouragement. !
Ann
__________________
Who scorns the calm has forgotten the storm.
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29-07-2018, 03:02
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#12
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Registered User
Join Date: Sep 2006
Posts: 9,398
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Re: How high is that wire... REALLY?
Quote:
Originally Posted by Jim Cate
thing is, you can't get close to the tower, for it is quite a long way inland from the shore. As the thing is structured now, there isn't a huge catenary. I was pretty disappointed, but not inclined to experiment!
I've seen some pretty long arcs back in the day job, never wanted to be on the receiving end!
BTW, when was that? apparently they have lowered the wires by some several meters (dirty buggers).
Jim
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We were able to get close to the tower, but on reflection, we passed to the East of Elizabeth island, theres a tower on the island you can get quite close to, but the water on that side of the island might be a bit shallow for you.
This was last year, around easter.
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29-07-2018, 03:22
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#13
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Registered User
Join Date: Sep 2006
Posts: 9,398
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Re: How high is that wire... REALLY?
With your fast dinghy, you could anchor downstream of the power lines and visit Grafton anyway. Even if you could pass under the lines, there's a bridge you definitely can't get under less than 2 miles upstream.
So it's really just a few extra minutes in the dinghy into town.
There's a good floating dock maybe a mile past the bridge, right at the end of the CBD.
__________________
"You CANNOT be serious!"
John McEnroe
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29-07-2018, 03:24
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#14
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Moderator
Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Posts: 21,156
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Re: How high is that wire... REALLY?
Quote:
Originally Posted by 44'cruisingcat
We were able to get close to the tower, but on reflection, we passed to the East of Elizabeth island, theres a tower on the island you can get quite close to, but the water on that side of the island might be a bit shallow for you.
This was last year, around easter.
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Umm, yep, that branch is verbotten to us lead miners. There are some undeniable advantages to shallow draft!
thanks for the thought, though.
Jim
__________________
Jim and Ann s/v Insatiable II, lying Port Cygnet Tasmania once again.
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29-07-2018, 03:31
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#15
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Moderator
Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Posts: 21,156
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Re: How high is that wire... REALLY?
Quote:
Originally Posted by 44'cruisingcat
With your fast dinghy, you could anchor downstream of the power lines and visit Grafton anyway. Even if you could pass under the lines, there's a bridge you definitely can't get under less than 2 miles upstream.
So it's really just a few extra minutes in the dinghy into town.
There's a good floating dock maybe a mile past the bridge, right at the end of the CBD.
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Yep, we did anchor and dinghy into town and reached that pontoon (Memorial Park). Aboout 3 1/2 miles. Looked nice, but there were thunderstorms threatening and we were not too happy about the anchorage there at Elizabeth Is, so returned to Ulmarra for the night. Just heard the first thunder clap, too (now 2020 local time).. No wind so far, and we've got lots of swinging room here, and no sparky things to drag into.
We will see what the morrow brings...
Thanks to all for the thoughts, guys. I guess it just wasn't meant to be!
Jim
__________________
Jim and Ann s/v Insatiable II, lying Port Cygnet Tasmania once again.
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