Loch Etive power cable clearance

[AngusMcDoon]

I'd like to get to the head of Loch Etive, but there are power cables with 12m safe clearance at Bonawe. I don't know if that refers to HAT or MHWS as that has changed recently on charts & mine doesn't say which, but I'll assume MHWS.

MHWS at Oban, the nearest standard port is 4.0m. Connel, the nearest secondary port MHWS is -0.3m, so 3.7m. If I go under at low water next week tide height is 1.0m, so that gives a safe clearance of 12 + 3.7 - 1.0, 14.7m. According to my boat's manufacturer's manual mast height above water including antennas is 14.6m. That's only 0.1m less than safe clearance, not a lot to account for meteorological tide height differences & the nearest secondary port being 6 miles down the loch.

Would you risk it (assuming I survive the Falls of Lora anyway)?

 

[jimi]

No! You don't want to go there anyway, the midges are the size of hornets and have a voracious appetite! i recall one of the funnier climbing episodes there, we were sitting at the coffin stone debating which route to do when the leader, clad in breeches, on the first pitch of Swastika lost adherence and slid down on his arse. There was a big puff of smoke as he did so. He'd had a box of matches in his back pocket which had ignited ;-)

 

[dunedin]

Absolutely not. But if you are going through, let us know when so we can film it. And not sure putting electronics in the oven as Faraday cage would be enough protection from hitting a power line ?

PS. When I measured our mast and fittings it was nearly 0.8m higher than manufacturer’s figures

 

[JumbleDuck]

Would you risk it (assuming I survive the Falls of Lora anyway)?

No. Absolutely not. What's flashover distance for voltage in the wires? And don't forget that they will stretch and sag with load and hot weather ...

 

[JumbleDuck]

Absolutely not. But if you are going through, let us know when so we can film it. And not sure putting electronics in the oven as Faraday cage would be enough protection from hitting a power line ?

Faraday cages are much misunderstood. An oven isnt one, and even if it was they only protect from external electric fields. A powerline strike will send a massive current down the mast and or rigging, with a resultant magnetic field against which even a real Faraday cage, let alone an oven, will do nothing. It's therefore really a matter of how susceptible electronic is to magnetic pulses.

 

[Jodel]

The Imray digital chart app give the clearance as 13M above HAT and states that is "the safe vertical clearance to avoid risk of electrical discharge which has been obtained by applying a deduction to the physical vertical clearance"

 

[AngusMcDoon]

The Imray digital chart app give the clearance as 13M above HAT and states that is "the safe vertical clearance to avoid risk of electrical discharge which has been obtained by applying a deduction to the physical vertical clearance"

My pilot book, which is old, says 12m, but my electronic charts which are new, also says 13m. HAT rather than MHWS also gives another 0.5m. That's now 1.6m below the safe clearance. The pilot book also says that the safe clearance is given for the lowest point of the span but additional clearance can be obtained by passing under near the edge of the channel, which is 450m wide and deep close to the north shore.

If I shin up the mast & temporarily take the windex & VHF aerial down I can get another 0.4m of safe clearance taking it to 2m.

 

[AngusMcDoon]

No. Absolutely not. What's flashover distance for voltage in the wires? And don't forget that they will stretch and sag with load and hot weather ...

The safe clearance given on the chart will already take the flashover distance into account. My calculation above is not 0.1m from the wire, but 0.1m from the safe clearance. Revised calculation is now 1.6m below the safe clearance.

The safe clearance takes into account heat induced droop.

 

[Daverw]

does not help the op, but recalled this from a couple of years ago and definitely a squeaky bum moment, had to wait for tide to rise and fall before coukd finally move, spent the whole time on deck balancing to stop moving. Luckily the cables were insulated

 

[jimi]

Perhaps the op should consider 2 further options 1) builders bag attached to halyard and filled with water at appropriate time 2)borrow an Andersen 22

 

[AngusMcDoon]

Perhaps the op should consider 2 further options 1) builders bag attached to halyard and filled with water at appropriate time 2)borrow an Andersen 22

Option 1 might have some unforseen technical difficulties on a trimeringue, but option 2 is most attractive. Just think how quickly I could get to the head of the loch and back, especially in a force 10.

 

[cpedw]

Some points.
First, there is a temporary height reduction at Connnel Bridge. For road works.
Secoond, there is a lot more clearance under the wires near the north end where the deep water is. Cruising on Loch Etive - BLUEMOMENT
Third, I asked the local pilot about it. He told me there's more space under the wire than under the bridge without its extra low deck. He once nudged the bridge with a misjudged coaster mast but still got to the quarry without a spark.

 

[dovekie]

Facing similar uncertainty elsewhere, we have used the method of dropping someone half a mile or so away from the height obstruction - with binoculars, sailing up to, but not under it, and the person with the binos can see if there is clearance.

 

[AngusMcDoon]

Some points.
First, there is a temporary height reduction at Connnel Bridge. For road works.
Secoond, there is a lot more clearance under the wires near the north end where the deep water is. Cruising on Loch Etive - BLUEMOMENT
Third, I asked the local pilot about it. He told me there's more space under the wire than under the bridge without its extra low deck. He once nudged the bridge with a misjudged coaster mast but still got to the quarry without a spark.

Thanks. Lots of useful information there. The height of the mast above water in the account is exactly the same as mine. Looking at the OS map the base of the North tower is at 10m above MHWS which gives some scale to the photo. On streetview it's possible to see (just) the mid channel droop, which is considerable.

It's looking like the bridge might be more of a problem at the moment.

 

[JumbleDuck]

The safe clearance given on the chart will already take the flashover distance into account. My calculation above is not 0.1m from the wire, but 0.1m from the safe clearance. Revised calculation is now 1.6m below the safe clearance.

The safe clearance takes into account heat induced droop.

Thanks. I would love to know how accurately they measure the height of the wires. I presume they measure load and air temperature at the same time to calculate droop.

On the bright side, if you are below the safe clearance and still end up with a smoking stump where your mast was, you can probably sue somebody.

 

[AngusMcDoon]

Looking at the pictures I've found there's a big difference in clearance between the lowest point of the catenary, the specified 13m above HAT, and what's available near the north shore. If I'm 1.6m below the safe clearance in the middle of the droop, there will be bags of space at the edge.

Looking at the picture on Blue moment, you can see the hw mark on the rocks below the tower, so the picture will be taken at low water. There will therefore be about 12m from sea level in that picture to the base of the tower. Add another 2.5m and that's my mast height. You can also see to the left of the picture at maximum droop things would get a bit hair-raising!

Every day on a trimeringue is a life endangering risk according to most on here. A 115kV line above my head just adds to the everyday tension I'm used to.

 

[savageseadog]

It is possible to ask National Grid (Scotland?) for information. It's a while since I did it myself for a trip up Manchester Ship Canal.

 

[scottie]

Looking at the pictures I've found there's a big difference in clearance between the lowest point of the catenary, the specified 13m above HAT, and what's available near the north shore. If I'm 1.6m below the safe clearance in the middle of the droop, there will be bags of space at the edge.

Looking at the picture on Blue moment, you can see the hw mark on the rocks below the tower, so the picture will be taken at low water. There will therefore be about 12m from sea level in that picture to the base of the tower. Add another 2.5m and that's my mast height. You can also see to the left of the picture at maximum droop things would get a bit hair-raising!

Every day on a trimeringue is a life endangering risk according to most on here. A 115kV line above my head just adds to the everyday tension I'm used to.

Does one not normally toast ones baked Alaska or am I rang ??

 

[Bilgediver]

Strap two crew members to the boom end and haul it out. A practice often used by our Kissin Cousins across the water to get under low bridges when heading out to sea.

 

[jimi]

It is possible to ask National Grid (Scotland?) for information. It's a while since I did it myself for a trip up Manchester Ship Canal.

Just ask them to switch it off whilst you transit