Measurement station Roeleveense Plas (1953 – 1961)
TNO used its measurement station Roeleveense Plas in Nootdorp both for underwater acoustic measurements (sonar measurements) and research in making submarines ‘invisible’ to radar waves (stealth) from 1953 until the mid of the nineties:
Underwater acoustics (sonar)
During the development of sonar transducers, the measurement station in the Waalhaven, Rotterdam no longer met the requirements in the early 50’s:
- the water was not deep enough on the spot,
- the background noise was too high due to the proximity of ship traffic and ports, and
- the distance between the laboratory in The Hague and the port in Rotterdam was impractically large.
In 1953, a new raft was constructed in the Roeleveense Plas using the pontoons from the earlier sonar measurement facilities in the Rotterdam Waalhaven. The Roeleveense Plas is a triangular artificial freshwater lake with sides of 300 and 400 metres located near Nootdorp next to highway A12.
The pontoons originally were the doors which protected the German Schnellboot bunker in the Waalhaven during the Second World war. The compartmentalised pontoons were made of half inch steel plate and sized approximately (lbh) 4 x 3 x 2 metres. There was therefore headroom in the pontoon. Both pontoons were connected to each other with a gap of one meter in between. A hoisting device was installed. On each pontoon there was a hut: on one pontoon a hut for taking measurements and on the other a hut with a diesel engine and dynamo for the power supply.
The raft was stationed with four anchors near the narrow part of the lake above the deepest point which is -18.5 m (at the right hand side of the left white coloured deep area). The detailed depth charts and under water soil structure charts by the Nootdorps Pijnackerse Hengelsport Vereniging (NPHV) provide a good insight in the depth and structure of the lake; for more graphs, visit their webpagina.
For measurements, the employees had to row with a rowing boat from a jetty to the raft. This was a bit easier than in the Waalhaven with the high quay of the old Schnellbooten bunker.
The tilting and turning mechanism for the hydrophones and transducers under test originally came from the Hr.Ms. Paets Van Troostwijk. The mechanism could be tilted in a way that the bottom side came above water to ease the mounting of a hydrophone or transducer. This mechanism is still in use in the TNO Waalsdorp underwater acoustics basin.
The quiet environment and the large depth (18.5 metres) benefited the quality of measurements, especially for frequencies below 500 Hz. This raft was also made suitable for measurements on sonar domes (a “dome” is a streamlined envelope of the transducer that serves to reduce the noise of the water flow). However, the diesel engine for generating electricity made noise and interfered with the measurements. It was quite a challenge in the winter to get the cold engine working: it had to be cranked up by hand. Reason for installing an electric power box on shore in 1955.
The design of the raft should enable measurements on sonardoms. A “dom” is a streamlined enclosure of the transducer that serves to reduce the flow of noise. Unfortunately, it turned out that the measurement set-up was unsuitable for perform measurements on doms of the Friesland class destroyers.
A new raft for sonar trails since 1961
Although this measuring station initially met all the requirements, after a number of years it turned out that a replacement was needed.
The development of the sonar technique led from the searchlight sonar with a single beam to the panoramic sonar with a combination of a number of fixed beams in a single transducer as described above. The second element in development was the use of lower frequencies. Both factors combined resulted in transducers and domes becoming considerably larger and heavier. Transporting it to the raft could therefore no longer be done per rowing boat as before.
That is why it was decided in 1960 to construct a larger raft in the same lake. The raft consisted of two pontoons of 10 x 2 metres and two pontoons of 6 x 2 metres. The resulting rectangular raft of 6 x 14 metres therefore had an opening of 10 x 2 meters through which the equipment under test could be lowered. The raft was connected to the bank with a floating bridge of five connected pontoons of 6 x 2 meters each. This made it possible to test objects with maximum dimensions of 2 x 2 x 4 meters and a weight of 5 tonnes. To this end, a narrow gauge rail was installed on the bridge and on the bank ending at a hoisting device at the Roeleveenseweg. Ground tackles ashore and chains kept the raft and the bridge in place.
Because the raft had built-up structures such as steel constructions for the hoisting masts and a measuring and working hut, it lay deeper in the water due to the weight than the last pontoon of the pontoon bridge. Ballast was put in all of the bridge pontoons to achieve a levelled transport route over the bridge pontoons.
At the shore side of the raft and above the gap is a 13-meter-high lattice-construction hoist mast. It can lift up objects to 1.80 m above the trolley. The flange of the hoisting device could be brought down to 6 metres below the surface of the water. Two hoisting cables were connected to an equator at the bottom of the mast via an electric winch. This ensured an equal load on both of the steel wires. A maximum load of 5 tonnes could be lifted. This mast also has two rotating columns that independently of each other could rotate 360 degrees. This rotation could be done manually or by using an electric motor that was operated from the control panel in the measuring hut.
A second mast was placed at a distance of six meters of the main mast. That mast was made of square profile iron and was operated with a hand winch. It could be lowered to 6 metres below the water surface.
The measuring raft was equipped with a three-phase power connection with sufficient power for the hoisting motor, a welding machine and the 12 kW transmitter for the 216TP5R transducer and other (measuring) equipment. A plastic tube of about 80 metres ran from the raft to the drinking water mains ashore. Unfortunately, the quality of the drinking water worsened over time.
The new raft was put into use in 1961. The old, Second World War-based, pontoon raft was transferred to the Marine Elektronisch en Optisch Bedrijf (MEOB) – the Navy electronic and optics department (MEOB), which used the raft for several years. They connected the raft with a bridge to shore. The Second World War-based pontoons were later re-used as a base for a house at the other side of the lake.
TNO used the new raft and shore facilities for sonar experiments until the mid-1990s. The Netherlands Royal Navy became the new owner of the raft and related facilities.
Invisibility of submarines in the radar domain (‘stealth’)
This story is currently under review. Publication will follow in Autumn 2019.