During the development of sonar transducers, the measurement station in the Waalhaven, Rotterdam no longer met the requirements in the early ’50s:
- 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 TNO decided to create a new underwater acoustics measuring facility at the Roeleveense Plas (‘lake’) near Nootdorpnext to the A12 highway and the Hofplein line (nowadays the Randstadrail line E). The lake was a sand excavation from the ’30s. The excavated sand was deposited nearby to create a multi-level crossing with the A12, nowadays the Prins Clausplein. At that time there were already plans for a ‘Rotterdam connection’, the body of sand was never used for that purpose.
The Roeleveense Plas is a double triangular freshwater lake with sides of 300 and 400 metres (see the Google map below).
A new raft is developed at the Roeleveense Plas using two pontoons. These pontoons are former armour-clad doors that protected the German E-boat (Schnellboot) bunkers in the Rotterdam Waalhaven during the Second World War. The transport was not easy. For this purpose, the Genie had to build a Bailey bridge. The pontoons were delivered with an M19 tank transporter.
The comparted pontoons are made of half-inch steel plates and sized approximately 4 x 3 x 2 metres (lwh). Therefore, there was headroom in the pontoon. The two pontoons were connected with a gap of 1 metre in between. A hoisting device was installed on the raft. On each pontoon, there was a hut: one hut for taking measurements and one hut to house a diesel engine and power generator.
At first, the raft was anchored in the middle between the highway and the narrowest point of the lake. On May 18, 1954, the raft was moved to the centre of the right-hand triangular part of the lake.
For measurements with domes in the 1950s, the dome was lowered between the two pontoons with two pully blocks. Four metre long pipes mounted on the top of the dome gave a precise measure of how deep the dome was under the water level: 80 cm of pipe above the water surface meant that the top of the dome was 3.20 metre deep under the water level. The hydrophone was then lowered in the same way at the furthest corner of the raft. The transducer was lowered into the dome in the same manner until the pipe was 70 cm above the water surface; the top of the 60 cm high transducer was then 10 cm deep in the dome. The transducer could be rotated in the dome so that the directional deviation could be measured.
The raft was fixated near the narrowest part of the lake above its deepest point, then -18.5 metres, with four anchors. The detailed depth and underwater soil structure charts by the Nootdorps Pijnackerse Hengelsport Vereniging (NPHV) provide a good insight into the depth and soil structure of the lake. Click here for more graphs.
For experiments, the employees had to use a rowing boat and row from a jetty to the raft. This was a bit easier than in the Waalhaven with the high quay of the former German Schnellboote bunkers.
The swivel mechanism for the hydrophones and transducers under test originally came from the Hr.Ms. Paets Van Troostwijk. The swivel mechanism could be tilted in a way that the bottom side came above water to ease the mounting of a hydrophone or transducer. The swivel mechanism is still in use at the TNO Waalsdorp underwater acoustics basin.
The quiet environment and the large water depth benefited the quality of the sonar 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). The diesel-driven power generator, however, made noise and interfered with the measurements. Moreover, in the winter it was quite a challenging task to start the cold engine. It had to be cranked up by hand. In 1955, an electric power connection was installed with the shore side.
The design of the raft should have enabled measurements on sonar domes. A “dome” 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 performing measurements on domes of the Friesland class destroyers.
A new raft for sonar trails (1961)
Initially, the sonar measuring facilities station met all the requirements. However, after several years, it turned out that a replacement of the raft was required. The development of the sonar technique led from the searchlight type of sonar with a single beam to panoramic sonars. Panoramic sonars are a combination of several fixed bundles of beams in a single transducer. At the same time, the development of sonars aimed at using lower frequencies. Both factors combined resulted in transducers and domes becoming considerably larger and heavier. As a result, it became impossible to transport the equipment to the raft per rowing boat.
In 1960, TNO decided to construct a larger raft in the same lake. At first, existing pontoons would be
refurbished for fl. 55,000. After the heavily rusted pontoons were cut open, their condition was so bad that it was decided to manufacture new, larger steel pontoons with a plate thickness of 6 mm. The new raft consisted of four pontoons of 4 x 2 x 1 meter (lwh) and two pontoons of 6 x 2 x 1 meter (lwh). Inside the pontoons contain three tectylated compartments. The pontoons were delivered, floated, and assembled on 1 November 1960.
The rectangular raft of 6 x 12 metres had an opening in the middle through which the sonar equipment under test and transducers could be lowered. I-beams across a part of the opening were used to stiffen the raft structure. The wooden deck of the raft was equipped with a roller shutter above the opening. The narrow-gauge rail system ran over the hatch as well.
The raft was connected to the bank of the lake by a floating bridge of five 6 x 2 x 1 metre (lwh) sized pontoons. The location of this new raft is visible on the Google maps image above.
This new underwater acoustics facility made it possible to measure sonar equipment with maximum dimensions of 3 x 1.80 x 1.70/1.80 metres (lwh) and a weight of 5.000 kilograms. To this end, a narrow gauge lorry system with a turntable was installed on the new concrete bridge connection and the bank. The lorry system ended at a hoist construction at the Roeleveenseweg. Four plough-type anchors onshore and chains kept the raft and the floating bridge in place, 50 metres from shore and 60 metres from the lakeside. Under the raft, the water depth is 17.5 metres.
Because the new raft had steel constructions for the hoisting masts and a carpeted measuring and working hut, it lay deeper in the water than the last pontoon of the floating bridge. To achieve a smooth use of the lorry system across the connection, ballast was put in each of the bridge pontoons to ease the transport from shore to the raft and vice versa.
At the shore side of the raft, above the 2 x 10 metres gap between the pontoons, is a 13-metre high lattice construction containing a hoisting mast system. Objects up to 1.70 or 1.80 metres high (depending on the lorry used) can be lifted from the trolley with the mast. The flange of the hoisting device can be lowered to 6 metres below the water surface (5.5 metres below the raft). Two hoisting cables are connected to an equator at the bottom of the mast and an electric winch on the other side. This ensured an equal tension on each of the steel wires.
A maximum load of 5.000 kilograms could be lifted. The mast has two rotating columns. Those allowed 360 degrees rotation (independently of each other), either manually or with an electric motor. That motor is operated via a control panel in the measuring house.
A second hoisting mast is located at a distance of exactly 5.62 metres heart to heart, which relates to a 15 dB attenuation of sound underwater making it easy to process measured results. That mast is made of square profile iron. A transducer of max. 150 kilograms could be lowered. A hand winch is used to lower or lift the mast. The mast lowers the measuring hydrophone to 5.5 metres below the water surface or 6 metres below the wooden deck of the raft.
The measuring raft is 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 hose of some 150 metres runs from the raft to the drinking water mains ashore. Unfortunately, the drinking water quality 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) (MEOB), which used the raft for several years. They connected that raft with a bridge to shore. Later, the two Second World War-based pontoons were re-used as a base for a house on the other side of the lake until the end of 2019. Early 2020, the raft was moved to its original location and is in use by the Nootdorps Pijnackerse Angling club (NPHV).
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.
A separate page shows how the measurements on the bow domes of the S- or Kortenear-class frigates (destroyers) took place on this lake.
In the ’70s, the facility was used as well to carry out measurements of transducers that could not produce short pulses. In that case, the switch-on and switch-off phenomena may dominate the long-duration response signal. The inset time of the transducer may also be longer than two milliseconds. In those cases, the in-house basin on the Waalsdorp Vlakte is not suitable for the measurements. The Roeleveense Plas measurement facility is.
The measuring facility Roeleveense Plas has only one problem, especially at the end of the summer. At that time, a strong temperature gradient has built up at a depth between three and six metres. On the surface, the water temperature can rise to 20 0C. Close to the bottom, the water temperature is about 8 0C. This temperature variation can adversely affect the accuracy of the measurements. In early spring the water temperature in the entire lake is very homogeneous, around 4 0C; a temperature that is eminently suitable for calibrating measuring hydrophones.