Trigonometry and obfuscation

Every so often Waterways Ireland (whom god bless and preserve) sends out a Marine Notice to warn people of sporting activities, so that they can avoid them, and of one or other of the hazards of the navigations. One such, No 68 of 2016, issued on 9 June 2016, says (inter alia):

The attention of all is drawn to the dangers associated with overhead power lines in particular sailing vessels, sailing dinghys and workboats with cranes or large airdrafts.

Vigilance is required especially in the vicinity of slipways and dinghy parks, while voyage planning is a necessity in order to identify the location of overhead lines crossing the navigations.

ESB Networks emergency number is (353)1850 372 999 and Northern Ireland Electricity Networks is (44) 0800 616 817.

Then on 21 July 2016 Waterways Ireland issued Notice No 89 of 2016, which was specific (as far as WI is concerned) to the Northern Ireland navigations. Extracts:

Many sailboats have masts of 9m (30ft) or more and, as most of these masts are made of aluminium, they are an excellent conductor of electricity. If an aluminium mast or rigging come into contact with or too close to power lines, it could result in a fatality.

NIE Networks advises all boat owners to take some simple precautions to stay safe.

  • Plan your route carefully when transporting your boat to or from where it is being launched, making sure you have adequate clearance under overhead power lines. When you are stepping the mast or erecting long aerials, be sure to do so in an area totally clear of overhead power lines.
  • Once out on the water, if you are sailing on inland waterways or near islands or headlands, you should still look for overhead lines as they do cross over waterways. You must ensure that your mast or aerial has proper clearance from any power lines.
  • Always check your charts when underway to ensure you are aware of the location of overhead power lines.

In order to avoid hitting an overhead power line with a mast, it would be useful to know how high off the water, perhaps at Ordinary Summer Level, the power line was. Then you could subtract from that the height of your mast, and some safety factor, and decide whether you could safely sail under the line.

If you don’t know how high the power line is, you have to resort to trigonometry. My recollection of these matters has perhaps faded a little, but as I recall you stand a small boy of known height on your cabin roof and, with your eye level with his feet, you measure (a) the horizontal distance from his feet to your eye and (b) the angle from the horizontal to a line from your eye that touches the top of the boy’s head and the bottom of the power line. Then the small boy falls off the roof, you have to tack, your pencil falls overboard and it all seems much more difficult than finding the height of a tree. And of course you can’t measure the distance to the power line without sailing up to it, which is what you’re trying to avoid until you can decide whether you can safely do so. [Note: this paragraph is not to be used for navigation.]

Life would, therefore, be much easier (and safer) if you knew the minimum clearance under the power lines. Happily, NIE Networks is happy to tell people what it is; I emailed and a speedy response including this:

The clearance on all NIE Networks overhead power lines which cross over navigable waters is 10.5m (lower bank to line or earth). All overhead power lines are marked on navigation charts for the Erne and Lower Bann waters.

However, it should be noted that this is the clearance for NIE Networks overhead power lines and applicable to Northern Ireland only. Clearances for overhead power lines in the Republic of Ireland may be different and would require confirmation from the Electricity Supply Board (ESB).

Big it up for NIE Networks, then. But over several years I tried unsuccessfully to get the ESB to give me the equivalent information for lines crossing navigable waterways in the republic; it would not do so. Without that information, boaters cannot “ensure that [their] mast or aerial has proper clearance from any power lines”, so ESB’s advice is useless blather.

It is some time since I last asked ESB for information, and perhaps it has since been made available, but I see no sign of it on the ESB website. I would be glad to hear from anyone who can supply reliable information.

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