23rd March 2006, 14:15
The attached is a detail from a plan of a WW2 Steam Gunboat (the "Denny" Grey Class) showing upper superstructure and mast/mast stays. My question is, what are the round black objects shown halfway up each of the eight mast stays? Photos of the Class show that most of the vessels had these round 'black blobs' while a few had elongated (sausage) shapes that must have been about 2-3 feet long. I'm well on with making a model of the class and like to know that I'm getting it right. One suggestion was 'silencers' to cut down rigging noise but what do you think? I will place a pint behind the bar of my local for the most convincing answer. Cheers!
23rd March 2006, 14:48
Look as though they should be aerial stay insulators.
23rd March 2006, 14:49
I think they are insulators. had them on the plans of a Whale Catcher I built, I used split lead shot fishing wieghts, they do look the part.
23rd March 2006, 14:58
The Class had its aerials slung fore and aft via the mast. As far as available plans and photographs show, at the upper end the stays terminate at mast brackets and at the lower end to the deck via rigging screws. If it helps, there were 4 upper stays (2 fore/2aft) and 4 lower stays (2 fore/2 aft). If these are insulators, thats an awful lot of aerials?
23rd March 2006, 15:23
Take your point about a lot of aerials! But I recall one of our elderly tugs (1950s) having Ceramic insulators on the mast stays...irritating thing is I cant remember why (Cloud)
23rd March 2006, 16:55
Could they be lightning arresters.
23rd March 2006, 19:30
Many years ago, early 60s I think, I was aquainted with a radio engineer who had a shop but had done his time in the RN.
He told me about some experiments they had been involved in before WWII using more & more power to try to increase range. One day as he hit the key there was a big flash like lightning outside and he realised that they had got to the limit! Apparently some of this depended on the weather, relative humidity, etc.
From this I recall him telling me that the stay wires had 'egg' ceramic insulators fitted to stop the power on the main aerials from going to deck. I assumed that the masts were non-conductive.
Interesting point about lightning arresters - Thinking about it just how did the lightning get to ground if the main route down was insulated? Unless the insulators were fitted with lightning bypass rods and that's getting silly.
23rd March 2006, 19:51
Any metallic rigging or metal bodies near the transmitting aerials will have eddy-currents induced in them. These currents in turn react with the current in the aerials themselves causing losses in the system and also losses in received signals. To prevent these losses, metallic shrouds and rigging near the transmitting aerials are broken up with insulators. These are usually made of glazed ceramic for strength and high insulation value. They also need to be kept clean to prevent electrical leakage across them.
From the Admiralty Handbook of Wireless Telegraphy, Volume 2, R24, e, iii:
"When transmission is taking place in a ship, all the stays, etc. will be set in oscillation at a frequency depending on their length. Stays should, therefore, be divided up into sections of such a length that minimum interference is caused to reception of short waves."
Hope this helps,
John/gwzm (ex-Brocklebank R/O)
23rd March 2006, 23:10
These insulators were to break up the electrical length of the stays to minimise the re radiation of frequencies which affected the Direction Finder. that operated in the Medium Frequency band of 285-512 Khz. The DF loop can be seen clearly in the sketch.
23rd March 2006, 23:19
I go along with the idea of insulators but there is no DF loop on the plan. You must be confusing it with something else.
Thanks to all who replied, especially John/gwzm who wins the pint. I can now model away happily in the knowledge that I now know what I'm making. Cheers!
23rd March 2006, 23:27
Hmmmm, grey class. there was one commanded by sir peter scott. he used to close in shore and get pictures of ducks. think he was based in dover at that time. the grey goose was at newhaven for a while then went round to shoreham, near the lady bee boatyard. I cant remember if she had these insulators or in fact if she is still in shoreham (probably a rich mans gin palace now) perhaps one of our southern members can throw a bit light on that one for you.
24th March 2006, 06:23
Isn't that a DF loop under the lower arrow?
Looks like a swinging loop rather than Bellini-Tosi.
24th March 2006, 09:17
R651400 - Sorry, that "DF Loop" is an 18" searchlight seen in side elevation!
Billyboy - The Grey Goose outlasted all her sisters until 1955 when she was used as a floating testbed for gas turbine propulsion. As far as I know, she was scrapped on completion of trials.