Will my mast clear it? - invention needed

[jdc]

I have a desire to revisit an old haunt, Cothele up the Tamar past Weir Quay.

Problem is my current boat is bigger and my mast quite a bit taller than my previous one and won't fit under the HV electricity cable. Or will it?...

The lowest point of the catenary is not centred over the channel, and its charted clearance is above HWS (or HAT in new charts I believe), so maybe is pessimistic. Hence I want to go and actually measure the height of the cable: anyone got a good idea how best to do this?

Two ideas are:
- Put a TV camera, either self levelling or with an artificial horizon, at the top of the mast and approach slowly. The relative elevation of the cable as a function of horizontal distance will allow me to calculate clearance. Problem: complex and expensive. Advantage: directly determines if the cable is above or below the mast; an error in horizontal distance giving only a proportional error in clearance.

- Anchor some distance off and measure vertical sextant angle, then calculate height. Problems: (i) indirect measurement, the result is a difference between two large numbers (height of cable and height of mast) so very sensitive to error, and (ii) what do I use as the horizon? Advantage: low tech and I have all necessary equipment already.

But does anyone have better and more ingenious ideas? Go up the mast with a theodolite, use an estate agent's ultrasonic 'tape measure' from the dinghy, Impulse radar, laser range finder or ...

PS: I already know it's insane...

 

[Twister_Ken]

Heel the boat as you go under the electrickery.

 

[Fr J Hackett]

Be very carefull there is a very great danger particularly in damp weather that you will act as a path to earth you will need a significant gap between cable and mast rather than just clearance.

 

[savageseadog]

Remember that the power can flash over a distance. Rule of thumb is 1mm per KV but you want to be as far away at least as much as the length of the insulators. Seriously, your boat will explode if it goes wrong.

Assuming you really have to do this use a theodolite/Abney level or sextant and do the geometry. Don't forget you have a radio aerial as well.

You could always load the boat up with a few tons of lead to lower it

 

[rob2]

All grand ideas, but how close to do have to get before it arcs? I had a mate who lived under a pylon. If you stood in his kitchen and held a neon tube by one end, thus earthing it, and pointed the other end skyward, it would light.

Like some of the ideas for measuring a bridge clearnace maybe. The false horizon you're looking for used to be created by oil in a basin and sighting across the surface, I believe.

Rob.

 

[saxonpirate]

B****r that.....

I'd be thinking of hiring a 'Pedalo' for the day.....

 

[Thistle]

Ask the guys who own the cable?

 

[nimbusgb]

If it's a 375 kv cable any closer than 4m and you are looking for a flashover! I'd be swimming away from your boat If it got closer than 6m!

Laser rangefinder may not be able to target the cable.

The depth of the catenary will vary with air temperature, power demand all sorts of things. If there's any doubt, drop the mast!

 

[charles_reed]

I have a desire to revisit an old haunt, Cothele up the Tamar past Weir Quay.

Problem is my current boat is bigger and my mast quite a bit taller than my previous one and won't fit under the HV electricity cable. Or will it?...

The lowest point of the catenary is not centred over the channel, and its charted clearance is above HWS (or HAT in new charts I believe), so maybe is pessimistic. Hence I want to go and actually measure the height of the cable: anyone got a good idea how best to do this?

Two ideas are:
- Put a TV camera, either self levelling or with an artificial horizon, at the top of the mast and approach slowly. The relative elevation of the cable as a function of horizontal distance will allow me to calculate clearance. Problem: complex and expensive. Advantage: directly determines if the cable is above or below the mast; an error in horizontal distance giving only a proportional error in clearance.

- Anchor some distance off and measure vertical sextant angle, then calculate height. Problems: (i) indirect measurement, the result is a difference between two large numbers (height of cable and height of mast) so very sensitive to error, and (ii) what do I use as the horizon? Advantage: low tech and I have all necessary equipment already.

But does anyone have better and more ingenious ideas? Go up the mast with a theodolite, use an estate agent's ultrasonic 'tape measure' from the dinghy, Impulse radar, laser range finder or ...

PS: I already know it's insane...

Go to Cothele in the rubber dinghy, anchoring the other side of the power cable.

I've had to do the same on a couple of occasions.

 

[Seajet]

Definitely a job where Kirk/Archer leaves the Enterprise in orbit and uses the rubber Shuttlecraft, carrying extra fuel & lifejackets, torch...even if they don't seem necessary.

 

[Ex-SolentBoy]

Lots of interesting comments on how to measure the height of the cable but none on how to calculate the height of tide. Just as you are concerned about the dip of the cable and it's variance in the centre from LAT, so should you be concerned about the innacuracy of tide tables at any given time..

Sure, you could do some maths, and there are many better on the forum than I who could help you with that, but tide heights and cable heights are just estimates. I am sure the people that chart these things take measures to ensure they err on the safe side, but is that any consolation when you fry your boat and it's occupants?

The only really accurate way would be someone at the top of the mast with an accurate sighting level. But, how much can the charge from the cable jump? Another estimate.

Unless I had professional advice, fully insured, and the professional advisor and the insurance man onboard, I think I would be rowing my way upriver

 

[sarabande]

which boat please and what mast length ? My sailing partner took a Rival 34, air draft approx 40ft to Cotehele a couple of weeks ago.

 

[LadyInBed]

Phone / mail the Electricity board and ask them the SAFE clearance height Left, Right and Center of the river for the day of intended passage

 

[john_morris_uk]

What sort of boat and what is the height of the mast? We go under those cable regularly and our mast height is just over 17 metres (on a 39 footer) The cables are a lot higher than you think and the channel is well to one side anyway - and so not at the lowest point of the catenary. I know the charted heights say what they are - although I am never sure whether that is 'safe height' or actual height. A surveyor told me that he asks the electricity board and then takes some sights with a sextant to check. Why don't you ask the electricity board as well for peace of mind?

 

[saltwater_gypsy]

Hmmmmm.... Cable voltage 375kV, breakdown in DRY air is 3MV/metre. Divide one by t'other and.........we get about 8metres. I would reckon on a factor of safety of two so wear your doc martins choose a dry day and make sure there is 16metres clearance, ... no...... make it 20.... no ....don't go at all!!!!

 

[VicS]

I know the charted heights say what they are - although I am never sure whether that is 'safe height' or actual height.

Clearances marked on the chart in magenta are safe clearances. Those in black are actual clearances.

Above MHWS ... I think

 

[john_morris_uk]

Clearances marked on the chart in magenta are safe clearances. Those in black are actual clearances.

Above MHWS ... I think

Thanks Vics - I asked a senior surveyor that question and he didn't know the answer - and we were in the middle of surveying data to update a chart. Just shows that even professionals don't know everything - although one of the definitions of professional might be something about approaching work in a certain way knowing a fair bit, but also realising what you DON'T know.

 

[AngusMcDoon]

Hmmmmm.... Cable voltage 375kV, breakdown in DRY air is 3MV/metre. Divide one by t'other and.........we get about 8metres. I would reckon on a factor of safety of two so wear your doc martins choose a dry day and make sure there is 16metres clearance, ... no...... make it 20.... no ....don't go at all!!!!

I'm not an expert, but the maths looks questionable here. From Wikipeeeedea...

The dielectric breakdown strength of dry air, at Standard Temperature and Pressure (STP), between spherical electrodes is approximately 33 kV/cm.

which agrees with your 3MV/m.

So for 375kV the breakdown distance is approximately 11cm. I'm not saying how close is safe as I don't know, but I don't think the breakdown distance is 8m according to the my simplistic calculation.

 

[lw395]

I'm not an expert, but the maths looks questionable here. From Wikipeeeedea...

The dielectric breakdown strength of dry air, at Standard Temperature and Pressure (STP), between spherical electrodes is approximately 33 kV/cm.

which agrees with your 3MV/m.

So for 375kV the breakdown distance is approximately 11cm. I'm not saying how close is safe as I don't know, but I don't think the breakdown distance is 8m according to the my simplistic calculation.

At risk of being pedantic it's the peak voltage that will flash over not the RMS...
And the pointy tip of the VHF aerial will act like a lightning conductor.
I would allow at least a metre for tidal 'uncertainty' too. Especially if there has been a lot of rain coming down the river.
My own experience is that approaching such things is very, very scary, from the cockpit it is totally convincing that the mast will hit somewhere around the upper spreaders, when in reality there is a 10m clearance. (That was the Itchen Bridge, but the principle is the same).
There is a lot to be said for following something taller!
Ask at the boatyards up river.

 

[AngusMcDoon]

At risk of being pedantic it's the peak voltage that will flash over not the RMS...
And the pointy tip of the VHF aerial will act like a lightning conductor.
I would allow at least a metre for tidal 'uncertainty' too. Especially if there has been a lot of rain coming down the river.
My own experience is that approaching such things is very, very scary, from the cockpit it is totally convincing that the mast will hit somewhere around the upper spreaders, when in reality there is a 10m clearance. (That was the Itchen Bridge, but the principle is the same).
There is a lot to be said for following something taller!
Ask at the boatyards up river.

Would an earthenware mast help?