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Discussion Starter · #1 · (Edited)
Safe clearance from overhear high v. lines?
corrected to:
Safe clearance from overhead high v. lines?


My mast reaches 40' from the waterline. Can I pass under High Tension Lines that are 50' above the water, assuming calm water, no surges or wakes?

Specifically, NSTAR has a cable height of no less than 50 feet at mean high tide, across the Weweantic River from Marion to Wareham near Briarwood Point.
Reference navigational chart 13236.

Is ten foot a safe clearance, in summer, dry air ?
 

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I worked for a company that constructs high power lines between high school and college. Here is the problem. You have a lot of variables.

One, the voltage. Those transmission lines can carry 7,000 110,000 or as as much as 750,000 volts depending on where they are. Obviously, the more voltage, the more chance of an arc.

Two, the humidity in the air makes a huge difference in conductivity.

One of the biggest scares I had working there was picking up a dead cable laying on the ground, under a 110,000 volt transmission line, and getting knocked on my ass, just from the static electricity that passed through the air to that cable on the ground.

The answer is, no one can tell you the answer for certain, but after seeing these lines arc a lot of times, to things near them, I would say let extreme caution be your guide.
 

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Not so difficult I think

Find the reference used in your chart - Norwegian charts use HAT, read more here

My charts give the safe clearance at HAT
This is from a raster chart


This from vector of same area


It won't make sense that each navigator should know how to calculate safe distance.

Around here we need to look out for storm and spring-tide that can go high above HAT.
This is data from 1987 - the black line is the calculated level - the blue i real data during as storm that stuffed data into the fjord.
 

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there's a set of high wires over the Delaware River near the Burlington Bristol Bridge and they SCARE me! I know they are high up there, but I just have this fear and don't touch any metal when going under! ( like that would save me ) LOL
 

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The clearance on charts is typically at the low point of the cable at high tide. If water depth is sufficient pass under the cable closer to the tower where the cable clearance is greater. I have felt the hair on the back of my neck standing up as I pass with plenty of clearance under overhead power lines in high humidity.
 

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gotta wonder why or if wires power companies are not responsible for the problem more or equal to the mistake boaters make when going under.

 

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roper-
You might ask the USCG, or the local electric company. Whoever owns those lines will know how many volts are running through them, and what the safe air gap distance is. I've seen videos of the huge circuit breakers at transformer stations, they need an air gap of several feet to break the connection.
 

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As has been mentioned, there are a lot of variables including line voltage, temperature, conductor loading, etc. But in the US, power lines crossing navigable waters where sailboats are permitted have higher clearance requirements above ground.

The goal of the standards is to avoid the need for every boater to have to attempt to calculate the safe clearance. I would not be overly concerned with a 40 foot mast if there are routinely sailboats passing under the line.
 

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Would the insurance company consider knowingly violating minimum safe clearance and intentional act? They could easily not cover that.
 

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I have worked with high voltage equipment in the past. The rule of thumb was one inch for every 10,000 volts. This was DC voltage, the overhead wires are AC. I don't know if that makes a difference.

Using that rule, a 120kV line needs an air gap of only 12 inches. As Group9 has also seen, the HV can create a static electric charge in objects much further away. I saw a set of metal shelves (on rubber wheels) charge up and arc to one of the building's steel trusses. The shelves were 6-8 feet away from the HV source and a few inches from the truss.
 

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And, one more negative thought, I watched a guy on a power line get his arm blown off because another power line was several feet lower than it was supposed to be, and arced over to the supposedly dead line he was working on.

Lesson I learned: 7000 volts from a distribution line make a mess when it tries to go through you.

And, the amazing thing was, the guy lived (I had to give a deposition in his lawsuit a year later).
 

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That is what would put my mind at ease. If they all made it, I probably will. All the same, I would go as fast as possible, and refrain from licking the mast for a few minutes.
I triple dog dare you!

You are going to feel like the mast is a lot closer to the wire than it really is. I would think 10 feet is plenty, but call the local utilty to put your mind at ease. If you are really nervious you could always swing a bag of water out over the side like this guy:

 

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Hmmm. Should I follow...

* Internet advice.
* "Other people do it."
* Authorized clearance.

I can see the challenge.
 

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Hmmm. Should I follow...

* Internet advice.
* "Other people do it."
* Authorized clearance.

I can see the challenge.
Where are all the experts when you need them? :)

I can't believe that safe navigation i US is that difficult.:eek:
Could not find any description on US chart use of height for power lines except chart datum that is always important. But giving anything else than safe clearance sounds like a bad idea.

A good start would be to learn to read a chart :)
Nautical chart - Wikipedia, the free encyclopedia

I did a quick check on some random charts I found here Atlantic NOAA Nautical Charts

Seems many US charts still use Mean High Water Spring (MHWS) instead of HAT that give more safety margin but still not foolproof.







As for what it looks like from deck vs the chart

The chart give clearance 28 meters at HAT, my mast hight is 17 meters - but still look scary.
Look at the wording difference
Power line (safe clearance)
bridge (overhead clearance)
 

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I would think the local towboat and / or boatus captain would know someone who has a boat similar to yours that has made the trip.

Also local yacht clubs would have people who have been sailing the water for 50 years.

It is not like you are sailing uncharted lands for the glory of the king.
 

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One more thing to keep in mind with power line clearances. Those lines expand and contract as they are heated and cooled by the sun and outside temperature, and on a long span, will actually be hanging as much as several feet lower on a hot sunny day, than on a cool cloudy day (see my above post for a good example of that not being properly allowed for when the wire is tensioned, and what can happen as a result).

In other words, the same sailboat might clear with no problem on a cloudy cool day, but make contact on a hot sunny day.
 
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