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THE G.P. FLOTILLA

BY HERC MEMBER MAC.

04 Nov 2014

Most of my war service was spent aboard HMS Mercury, at one time known as the Admiralty Signal and Radar Establishment (ASRE) - not as some of our friends in the other services were wont to address us - the Admiralty Radar and Signal Establishment (….?) and it was there that I met up with one of the best kept secrets of the war. In fact very little publicity has been given to it to this date.

Apart from communications equipment generally, ASRE was engaged in the development of RADAR and HF/DF arid it was, as a result of advances made in these two fields, particularly the latter, that the whole idea evolved.

From back before 1938 the Germans, with their Teutonic logic had decided on modularisation of radio equipment and the rationalization of valve end component types. This was evident from the domestic radio equipment displayed at the 1938 Berlin Radio Exhibition and, although few of us realised at the time, it was part of the build-up for the war which was to follow. The same modules could be used in military equipment without too much disturbance of the production lines.

Be that as it may, one result was that, with the restrictions imposed, the German boffins were well behind the Admiralty in HF/DF and were apparently unaware of the progress made here. In consequence, the U-Boats were allowed to continue their practice of radioing home their day’s report when they surfaced at night to charge their batteries.

With the latest developments it was possible to get a quite accurate fix on the U-Boats when they broke W/T silence and the Admiralty decided to take full advantage of the situation. They accordingly commissioned a special destroyer flotilla. The five ships were built for high speed and maneuverability and were equipped with the latest ASDIC (now SONAR) gear, anti-aircraft and surface radar and, of course, the latest HF/DF. Their armament was director- controlled 5.7 inch high-angle guns. The only other unusual feature was the two large tanks below dock.

The contents of these two tanks was the real secret and, being still subject to the Official Secrets Act I am a little scary of how mach I may safely disclose even at this late date. However, to return to the story, these tanks hold a specially formulated, thin oily fluid the colour spectrograph of which had to be accurate to within a few Angstroms. The fluid was composed of two principal elements, one .highly volatile and the other of a specified viscosity which lied to remain constant irrespective of the ambient temperature. This requirement caused considerable problems since multi-grade or viscostatic oils were not available at the time and, in the end, a compromise had to be made by providing four separate graded - G.P. (North Atlantic), G P (Mediterranean) etc.

When the Flotilla was finally ready for sea it took up positions across the convoy routes where the U-Boats used to lie-in-wait and where contacts could be expected. They kept WT contact with one another; Captain D on the flag ship acting as coordinator. At dusk the radio watch was doubled and as soon as a likely transmission was found, the HF/DF equipment was brought into use and the bearing and the frequency passed to Captain D. The other ships swept the indicated area on the same frequency with their HF/DF, reporting the bearings which they obtained. One bearing provides direction, two a rough fix, three the “Cocked hat” or triangle of uncertainty as navigators call it, any more bearings are merely gilding the lily.

Having obtained a good fix, the destroyers steamed full ahead for the U-Boat’s position. Once they were within range, the surface radar was switched on and it was not long before the U-Boat captain realised he was the object of some unwelcome attention. He crash dived, but by then the ASDIC gear had him and, with their superior speed, the destroyers were able to encircle him. The U-That could only cut engines and go down to the bottom hoping that the uneven sea bed would fool the ASDIC and disguise its position.

Meanwhile, on top, the destroyers formed a. circle over the U-Boat, steaming round in a line astern end releasing into the inside circle some of the fluid from their tanks. When an adequate film had been distributed, two of the destroyers .stopped their engines and hove to at silent stations (no noise of any kind aboard - not even speech). The other three, after two or three more circles, withdrew to take up again their stations on the convoy routes.

Down below, the U-Boat, lying doggo on the bottom heard the activity and, finally, the retreating screws of the three destroyers as they returned to their stations. After a period of tense silence the U-Boat captain decided to make a move and very cautiously bring the boat up, listening all of the time but unaware of the two destroyers silently waiting. Nearing the surface he ordered “Up periscope” so as to have a look around and this is where the final trap was sprung. The periscope, on being raised, passed through the film of fluid the destroyers had left on the surface and some of it adhered to the lens, setting rapidly as the volatile element evaporated once it was out of contact with the water. Due to the accuracy of the colouring in the film, the U-Boat Captain could only see a pale green light through the periscope and, misled into thinking that his depth gauges had been damaged in the crash dive, ordered the helmsman to bring her up a bit, and a bit more, and a bit more....

This is what the two remaining destroyers were waiting for. As soon us the U-Boat broke surface they had her on their surface radar, carefully tracking her movements until she reached an altitude of 1,000 feet when they switched to the anti-aircraft radar and shot her down with the 5.7 high angle guns.

It still puzzles me why so little mention has been made of the Green Paint Flotilla.

Mac. 1BRS40651 February 1981


Re-published from the Vital Spark, the clubs monthly magazine.
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To read Amateur Radio related articles written by the Hastings Electronics and Radio Club Members, please visit the clubs magazine archive on our official site, the Vital Spark archive containing many interesting, educational and technical pieces:

http://herc-hastings.org.uk/vital-spark-articles/

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