Monday, 1 February 2016

Storm Gertrude damage - not too bad, thankfully

I seem to have got my yagis in a twist.
Oh dear, what a stoochie. They are both supposed to point the same way!

This Winter the UK Met Office and Met Eireann jointly started to name storm-force events. This one, being the seventh to be named got a name beginning with G, so it was titled "Gertrude".
This is Gertrude passing north of Shetland, dragging winds across Scotland (BBC News Scotland website).

All the usual signs were there - the railway ground to to a standstill due to the overhead lines being down, and trucks were blown over on the A1, closing it. As so often, Ayton was "cut off". Well, not really, despite blown down trees you can usually work your way round it. Blown down trees are not such a problem here as further South - most of the weak ones have already been blown over. We get so many events that we do not get the "once in a generation" tree felling which occurs elsewhere. What gets blown over are trucks and vans.
Peak winds noted in Shetland were well over 140km/h, here we had gusts of about 105km/h (photo BBC News Scotland website).

I did not actually realise that the 2m antenna had become loose until I went out to wind up the tower for some meteor scatter. It was still pretty windy and I decided not to risk raising the antennas above the ridge tiles of the house. The antennas are quite well protected when the mast is wound down - but still one worked loose.

Next day I tilted the tower over and managed to get the antennas back in line. The antenna bracket was well tightened and it had ripped a gouge out of the aluminium pole where it had torn loose. It had fallen about 100mm as it twisted round. I decided to leave it at that height and tighten it there rather than move it back to where it had left tear marks.

At this point the bracket supplied by Wimo was showing its weaknesses.
I remarked at the time I put it up that this antenna was rather lightly built, (link click here) when I was daft enough to say --
it is beautifully made and I doubt if I will have any problems with it in the gales.
Well my defence is that the antenna was OK, it was the bracket which gave me the trouble. The photo above shows that it is a single U-bolt design, which I never really like (the Vine 4m beams also have this type of bracket and I have remarked before how poor they are).

After I had fixed it back in line I noticed that the 2m beam was now pointing too much skywards. It is often said that a couple of degree upwards mounting is good for meteor scatter but this was too much. So even though I had tightened the wing nuts fairly tight I tried to straighten the boom. And once I did that that, the mounting loosened again. I got about two more turns on the wing nuts. That might explain how it came loose in the wind.

Anyway, both Wimo and Diamond use wing nut fixings. I am not absolutely against them, provided that the nuts are well made. Even though Diamond say "do not use tools to install this antenna", I do give the wing nuts extra tightening using a pair of pliers. I did the same with the Wimo, both last time and this time. You run the risk of sheering off one side of the wing nut, but you can feel happy that the antenna is well tightened up. I have a box full of wing nuts; they are not hard to replace. I could replace them with conventional nuts too.

So maybe I need to fit a better bracket to the Wimo. They do not seem to quote a maximum recommended wind speed, but they do quote a wind loading figure for 160k/h (!!). What I do not want is for the antenna to "windmill", in other words work loose and start turning round on the mast. Not only does this cut into the mast, the co-ax winds up and is usually damaged.

Perhaps I need to think again about why I mounted the antenna at the weight balance point. That means that the wind-free load on the rotator is fairly evenly balanced, but it probably makes the wind act more strongly on the front of the antenna. If I moved it to the wind balance point there would be less turning force in a gale at the cost of more strain on the rotator. But I do not know where the wind balance point is (I would have to guess).

The boom catches more wind than the elements as it is much thicker square section. The wind balance is probably nearer half way down the boom. In the photo above the joint in the boom is about half way, so I could easily move the bracket 200mm or so, which might make all the difference, and still leave the weight more balanced than putting the support right at the centre. Would that stop it whistling in the wind too?

Now that Gertrude has passed, say hello to Storm Henry.
The black areas represent 14m (48ft) swells in the Atlantic (magicseaweed on the BBC New Scotland website). Fortunately it hits the West Coast of Scotland first so our swell should not be so bad, but the winds will still be quite something.

It was only yesterday when I fixed the 2m beam straight, and Henry is due overnight tonight. Whilst Gertrude brought 105km/h wind here, Henry has been revised down (!) to 113km/h.

Several rail routes are to close after 16:00 and road closures have already been announced.

Let us see what happens next!!!

73

Jim
GM4FVM

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