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,j / Gerd Marggraff (510-828-1992) f System 2: The second system in use is the deflector shield dredging vessel. It uses a specially designed shield that is lowered in the correct arc and distance behind the ships propeller to deflect water flow toward the sediment. It is extremely important that the arc and shield is designed to fit the diameter of the ship's propeller. Propulsion systems used for this dredge are preferably powered by a big diameterlarge pitch propeller fitted with a high reduction gear box. This means the propeller turns slowly but moves the water in an extremely powerful manner, not unlike a tug boat's propeller. The deflecting shield system is used by a company in northern Germany to keep cooling water channels at several power stations at an exact depth of 10-12 ft. This system is especially good for general dredging and pin pointing problem areas. The captain can work his vessel so accurately that he can back up within a few feet of cooling water intake grates to remove sediment and other debris. If the shield is lowered to its maximum depth, the water deflection will actually move the boat in reverse, as common on jet boats and planes as reverse thrusters. We all know the rumble when planes slow down on runways-this is caused by a deflector shield. System 3: The third system is known as a hydro-injecting dredging vessel. It is the latest development in dredging technology. Nozzles are mounted on a pivoting U- frame on the stern of a tug. A simple craning device lowers the rear end of the frame down to a certain height above the sediment. A la'rge volume, low pressure pump (like those used in cutter-head dredges) injects water out the nozzles approximately 1 }'z-2 ft into sediment and agitating it to saturate the water around it. The current again carries the sediment away. Several of these vessels are used on the Elbe and Weser rivers, creating channels up to 60 ft in depth. Recently, the channel at the ~Ibe had to be deepened by 10 ft to make the channel accessible to even larger Evergreen container vessels. This was completed with no problem in a short amount of time. Admittedly, this dredging depth was done with 150 ft vessel. All these dredging systems work on the fact that the sediment is carried away by tides, back to the adjacent mud fields where it came from. Around our San Leandro channel, t understand we have a cross current of up to 3 knots. Simple -z..