The Art of Dredging

Dredging and shipping



The punaise ("thumbtack") is a dredge pump system that operates totally below the water surface.

Its distinctive shape gave it the name "Punaise" (Dutch for thumthack). The first "Punaise" was commisioned in 1990, by J.G. Nelis, Haarlem, the Netherlands, and marketed by the "Pinpoint Dredging Company", later under Ballast Nedam and Boskalis.

The first "Punaises" were built by Damen.




The contraption contains a submerged dredgepump, with rigid suction pipe, and ballasttanks. 

Its a compact, watertight unit, remotely controlled from a shore station, with an umbilical, with electrical power-, signal- and control cables. 




Once located in postion, the ballasttanks are filled and the "Punaise" settles on the sea bed and starts dredging. The suction pipe provides horizontal and vertical stability.

Since dredging production of the "Punaise" is totally depending on hydraulic erosion of the material, dredging can only be done in non-cohesive materials (sand, silt, mud), that can form unstable slopes.  And the material has to be present in thick layers.

The "Punaise" has some more obvious advantages / disadvantages:

  • no disturbance from or to shipping
  • not affected by adverse weather conditions, waves, etc...
  • relocation requires time, the device is pretty much immobile during dredging

The dredged material is pumped through a discharge line, either to discharge ashore or further underwater.

There are three models: PN 200, PN250 and PN 400 (meaning suction diameters). The two smallest models can be transported in standard containers.




Technical sketch of the "Punaise": visible are suction- and discharge line (with cardan ring), the centrifugal dredge pump and ballasttanks.


The "Punaise" performed some jobs in the ninieties, doublehandling material for beachsupply and port maintenance dredging in the Netherlands, and attempts have been made to market it in the U.S..

(Read: for the whole story.)

The "Punaise"  was a novel idea back in 1990, but is no longer on the radar today because it is very limited in its capabilities. It's a passive dredger, with can only handle a limited range of soils.


Marc Van de Velde

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