Will my mast clear it? - invention needed
- Thread starter jdc
- Start date
PembrokeshirePromise
Well-Known Member
This shows you how to do it ...
http://www.youtube.com/watch?v=epz6BBZm__0
(You did say you knew it was insane ..)
http://www.youtube.com/watch?v=epz6BBZm__0
(You did say you knew it was insane ..)
MASH
N/A
Use 2 dinghies, a known length of light floating line tight between them - say 20-30m, and a sextant or homebuilt protractor sight.
Place one dinghy directly beneath the cable (build a simple gravity sight) and measure angle to overhead cable from other dinghy with sextant ensuring the sextant is on or close to the hypotenuse of that triangle - or the baseline distance won't be accurate. (Draw it - you'll see what I mean) Add a Cosine table and SOHCAHTOA, bingo! Should be accurate to a couple of feet with care. And don't forget to account for tide!
ps. some of the figures posted here for arcing distances are staggeringly inaccurate and misleading, as is some of the arithmetic used to "calculate" them. Do people do tidal calcs with the same diligence? If so you'd easily be able to sail over the pylons on a high tide by their reckoning!
An air gap will arc at about 30kv/cm, so a 375kv line would arc at c. 13cm.
A metre gap gives a massive eightfold safety factor. But are you really that brave?
Me, I'd just ask the elecrtickery company.
Place one dinghy directly beneath the cable (build a simple gravity sight) and measure angle to overhead cable from other dinghy with sextant ensuring the sextant is on or close to the hypotenuse of that triangle - or the baseline distance won't be accurate. (Draw it - you'll see what I mean) Add a Cosine table and SOHCAHTOA, bingo! Should be accurate to a couple of feet with care. And don't forget to account for tide!
ps. some of the figures posted here for arcing distances are staggeringly inaccurate and misleading, as is some of the arithmetic used to "calculate" them. Do people do tidal calcs with the same diligence? If so you'd easily be able to sail over the pylons on a high tide by their reckoning!
An air gap will arc at about 30kv/cm, so a 375kv line would arc at c. 13cm.
A metre gap gives a massive eightfold safety factor. But are you really that brave?
Me, I'd just ask the elecrtickery company.
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AngusMcDoon
Well-Known Member
I shall go and find the electricity company's phone number and completely confuse the customer service helpline bod in Mumbai.
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jdc
Well-Known Member
Thanks all for many helpful replies and ideas. I loved the video, but not sure I've the balls required for that trick!
That was what I remember, but also I agree with lw395; I never can judge the clearance from the deck and am always convinced I'm going to hit a bridge (even when it's huge like the Skye bridge or in the Swellies). btw my mast is 18m approx - clearly I will have to measure that v. carefully. Boat is a 42' monohull.
The rope of known length as one leg of the triangle is a jolly good idea - wish I'd thought of it. I'd rather ignored the need to calculate the tide as I hoped for a measurement of delta rather than having to measure absolute height of mast and absolute height of cable - the propensity for quantities which are the difference between two large numbers to give error multiplication is notorious. I think a recce in the dinghy, and some ball-park messurements with sextant, is a fun winter project.
PS: in my long-ago youth I was part of the team working on a particle accelerator: I designed a tetrode based 100kV cc source to feed the belt of the 14MV Van der Graaf, so I'm reasonably comfortable calculating air gaps (and had enough scary flash-overs to know that a good margin is needed; and that the shape of the conductor really does count). I had 100kV jump 40cm given the right (or wrong in that case) geometry. In this case I really want a 2m margin, and will probably remove the windex and VHF aerial.
That was what I remember, but also I agree with lw395; I never can judge the clearance from the deck and am always convinced I'm going to hit a bridge (even when it's huge like the Skye bridge or in the Swellies). btw my mast is 18m approx - clearly I will have to measure that v. carefully. Boat is a 42' monohull.
The rope of known length as one leg of the triangle is a jolly good idea - wish I'd thought of it. I'd rather ignored the need to calculate the tide as I hoped for a measurement of delta rather than having to measure absolute height of mast and absolute height of cable - the propensity for quantities which are the difference between two large numbers to give error multiplication is notorious. I think a recce in the dinghy, and some ball-park messurements with sextant, is a fun winter project.
PS: in my long-ago youth I was part of the team working on a particle accelerator: I designed a tetrode based 100kV cc source to feed the belt of the 14MV Van der Graaf, so I'm reasonably comfortable calculating air gaps (and had enough scary flash-overs to know that a good margin is needed; and that the shape of the conductor really does count). I had 100kV jump 40cm given the right (or wrong in that case) geometry. In this case I really want a 2m margin, and will probably remove the windex and VHF aerial.
prv
Well-Known Member
Fantastic!
If you watch to the end you see how he winches the bags back to the centreline and the boat smoothly comes upright again. A really slick technique. I suppose a bit of trial and error is needed to know how much water a given boat needs.
My mast is only 25 feet tall, so I doubt I'll ever need this, but it's now filed away in the back of my head just in case.
Pete
MASH
N/A
Angus, you're right, I meant the National Grid. Surely they have a legal obligation to publish such info?
A Sextant can be used without a horizon by using a horizontal reflective surface - ie a tray/bowl of used engine oil.
Find the reflection of the body (usually a star, but here the wire) in the oil, sight this with the telescope - it will be at a strange downward angle - as if it were the horizon. Bring the body down to it using the mirror in the usual way. The angle obtained is double the elevation of the body. You'd need to arrange the elevation of the body to be less than 30' or the sextant won't cope with the large angle, (unless you have access to a quadrant, in which below 45' is the max useable). This procedure would be relatively easy to achieve in a dinghy in flat calm. (account for eye-height of the sextant too). You still have the difficulty of measuring a baseline accurately though.
For £60 or so you can buy laser rangefinders with a 50m range accurate to 1.5mm (!) . Even if the wire is not very reflective you'd surely get a reading off it from a dinghy directly beneath at twilight or evening. Compared to fooling about with two dinghys, ropes, sextants and bowls of oil I'd be inclined to shell out the £60 first.
A Sextant can be used without a horizon by using a horizontal reflective surface - ie a tray/bowl of used engine oil.
Find the reflection of the body (usually a star, but here the wire) in the oil, sight this with the telescope - it will be at a strange downward angle - as if it were the horizon. Bring the body down to it using the mirror in the usual way. The angle obtained is double the elevation of the body. You'd need to arrange the elevation of the body to be less than 30' or the sextant won't cope with the large angle, (unless you have access to a quadrant, in which below 45' is the max useable). This procedure would be relatively easy to achieve in a dinghy in flat calm. (account for eye-height of the sextant too). You still have the difficulty of measuring a baseline accurately though.
For £60 or so you can buy laser rangefinders with a 50m range accurate to 1.5mm (!) . Even if the wire is not very reflective you'd surely get a reading off it from a dinghy directly beneath at twilight or evening. Compared to fooling about with two dinghys, ropes, sextants and bowls of oil I'd be inclined to shell out the £60 first.
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DaveS
Well-Known Member
The originally quoted value of 375 kV appears to be accepted unquestioningly. This, however, is not a standard UK distribution or transmission voltage. 33kV, 132kV, 275 kV and 400 kV are standard voltages, so perhaps one of these is meant? The quoted voltage is in each case the line voltage, i.e. the nominal RMS value of the AC voltage between the power carrying conductors.
To get the voltage wrt earth of the lowest conductor (which is what we are interested in) we need the phase voltage which, assuming perfect load balance, is line voltage / sqrt(3) giving either 19kV, 76kV, 159kV or 231kV, again RMS. As said in an earlier post, for discharge calculations it is the peak value which matters, which we get (assuming a perfect sine wave) by multiplying the RMS value by 2 / sqrt(2) giving 27kV, 107kV, 225kV or 327kV.
Having determined which of the possible voltages it actually is, you could then add a %age to cover the possibility of the actual voltage being a bit high, calculate the likely discharge distance based on assumed humidity etc. then add a safety margin of your choice.
So much for the theory. However...
Note in the above I said "you". For myself, I would be most unwilling to "stretch" the charted safe clearance (and yes, I did like the "balls" video!).
Get this one wrong and it's the last mistake you'll ever make.
To get the voltage wrt earth of the lowest conductor (which is what we are interested in) we need the phase voltage which, assuming perfect load balance, is line voltage / sqrt(3) giving either 19kV, 76kV, 159kV or 231kV, again RMS. As said in an earlier post, for discharge calculations it is the peak value which matters, which we get (assuming a perfect sine wave) by multiplying the RMS value by 2 / sqrt(2) giving 27kV, 107kV, 225kV or 327kV.
Having determined which of the possible voltages it actually is, you could then add a %age to cover the possibility of the actual voltage being a bit high, calculate the likely discharge distance based on assumed humidity etc. then add a safety margin of your choice.
So much for the theory. However...
Note in the above I said "you". For myself, I would be most unwilling to "stretch" the charted safe clearance (and yes, I did like the "balls" video!).
Get this one wrong and it's the last mistake you'll ever make.
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PeteE
Active Member
Don't try this at home!
In similar vein, many, MANY years ago I was working on the construction of a causeway in the Gulf as part of which we had some guys drilling from a small towed barge. At one point the rig had to get to the upstream side of the power line supplying a processing plant and tank farm. Tides in the area were 8kts. Sights were taken (theodolite) and calculations done (neither by me) and the barge set off with the driller in his usual position directing operations from atop the "A" frame. Against an 8 kt current the inevitable happened and an uncontrollable rig bore down on the catenary. Later, back in the Mess, a blackened Albert told us how as he approached the cable (and before he jumped clear) he had considered lifting it over the top of the "A" frame..........
The tank farm were not too amused either.
In similar vein, many, MANY years ago I was working on the construction of a causeway in the Gulf as part of which we had some guys drilling from a small towed barge. At one point the rig had to get to the upstream side of the power line supplying a processing plant and tank farm. Tides in the area were 8kts. Sights were taken (theodolite) and calculations done (neither by me) and the barge set off with the driller in his usual position directing operations from atop the "A" frame. Against an 8 kt current the inevitable happened and an uncontrollable rig bore down on the catenary. Later, back in the Mess, a blackened Albert told us how as he approached the cable (and before he jumped clear) he had considered lifting it over the top of the "A" frame..........
The tank farm were not too amused either.
starfire
Well-Known Member
Might Give you some clues :-
http://www.nationalgrid.com/uk/LandandDevelopment/DDC/devnearohl_final/appendixIII/appIII-part2/
http://www.nationalgrid.com/uk/LandandDevelopment/DDC/devnearohl_final/appendixIII/appIII-part2/
fisherman
Well-Known Member
I wonder how well the engine functions at that angle, re water and oil pickups for instance...
prv
Well-Known Member
Ex-SolentBoy
Well-Known Member
Stunning boat. Anyone know what it is?
Leighb
Well-Known Member
Interestingly the manual for my Beta 20 mentions 25degrees as the maximum heeling angle. Fair enough for motor sailing if you are going further over you should be reefed more.
However I would say the yacht in the video was over more than that 30-35 perhaps?
fisherman
Well-Known Member
Forgive my stupidity, but if there's that much wind, why motorsail?
prv
Well-Known Member
Thistle
Well-Known Member
You can go quite a lot closer to the wind.
Leighb
Well-Known Member
Another reason is that if you are tending to get stopped by short steep seas the engine keeps you going.
prv
Well-Known Member
If going to windward and it looks like they're not quite going to pass that headland (or pass it too close for comfort), some people will stick the donk on for a few minutes to cheat a slightly higher course, rather than tack away and back. I'm not usually such a person, but my Dad often is.
In Hand, Reef and Steer (book on handling old gaffers) there is a section on staying off a lee shore in heavy weather. After discussing various traditional approaches, even the traditionalist Mr Cunliffe points out that most boats these days have an engine, and if you're going to suffer the drag of a propellor all the time, you might as well make use of it when needed to help punch up to windward.
Pete
prv
Well-Known Member
It's a good point - I ought to check what the specified angle is for my Yanmar. 25 degrees seems small - perhaps because the Beta is an industrial-based engine rather than a ground-up marine one?
I know that the old-fashioned (though still made) Listers used for pumps and site machinery can do 40 degrees of tilt.
Pete
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