The Art of Dredging

Dredging and shipping

Manoeuvring twinscrew vessels, with bowthruster(s)

 
Hopper dredgers usually manoeuvre in tight quarters such as in harbours, near quaywalls or underwater slopes.

Manoeuvring and positioning is an inherent part of dredgers` cycle, but can be time consuming. Swift manoeuvring can take the dredge production to the last few yards of perfection.  

In this article we will give some extra notes on manoeuvring and mooring the modern hopperdredge setup: two controllable pitch props  (CPP), one (or more) bowthruster(s); often with high-lift rudders ("flap" rudders, Becker-, etc...). Some high lift rudders change the direction of the propeller wash 90 degrees, on max rudder angle.

 

 Most textbooks on ship's manoeuvring do not get past the basics.

 

Inboard rudder/Rotating

When rotating the vessel -intuitively- the operator will create a torque with the flow of the main propellers. To assist the rotational torque, the rudder set on the prop wash.

This setup is useful for short turning manoeuvres, especially with a weak bowtruster.

 

The inboard rudder implies to the flow of water directed inboard.

The reverse prop will then suck in the flow of the other prop. In shallow situations this might cause a lot of debris being dragged through this propeller.

On one such occasion I  got a truck tyre between propeller and propeller nozzle. Since that event, I use propellers in a more demure way, alongside quay walls. Especially old quays are mere dump sites.

The forward trusting propeller will create a low pressure area on the ship`s side, which may assist to move into a (closed) quaywall (given that the bowthrusters force is inverted).

 

Outboard rudder/Sidestepping

Many operators will always use inboard for every manoeuvre. However physics show us that this is not a balanced set-up.

 

With outboard rudder, the flow of the forward-thrusting prop is directed away from the hull.

The rotational torque from the props counterbalances the the bowthruster`s torque on the vessel.

The residual power goes into parallel translation also known as "sidestepping".

 The maximum force that can be generated to move the ship aside is either determined by the bowthruster or the max reverse thrust of the main thrusters. 

 If the rudders are out of center with the propellers, the rudder-lift will be decreased.

For docking scenarios, the forward-trusting propeller will create relative high pressure area on the side and will cushion the vessel when moving towards a fixed object.
 
 

Mooring twinscrew vessels, with bowthruster(s)

 

A twinscrew vessel, with bowthrust-assist, is a breeze to moor !

There are a few guiding principles, to keep it on the safe side:

a. Move parallel towards the quay wall. Single screw vessels require to sail towards the quay under an angle. This unnecessary with twinscrews (with bowthruster). Just keep the same heading as the quay wall, from 30-20 meter out, and it's almost impossible to make contact  damage to ship or quay.

b. Once the vessel is appx. 10 m from the quay, manoeuvre in "safe mode"; this is: use minimal power on the thrusters, if you have more than one bowthruster, switch off one BT, etc... Go slow and keep full control. In this stage, it must be possible to keep control, even when one main engine shuts down. Some twinscrew vessels have absurd amounts of powe, but you may need only 10-20% of that power for mooring.

c. Don't use a spring line to slow down the vessel.... this is a relict from single-prop-mooring, and highly dangerous for the deck crew. Just position the ship exactly alongside the quay, and start mooring once postioned, in relative safety for the crew fore and aft.

d.  If you approach the quay, check out the quay fenders. Some  fenders, with high friction, will "glue" or "grip" the vessel in position, other fenders will allow the vessel to glide fore or aft, and it's difficult to keep the vessel steady in one position.

 

These fenders will easily "grip" a vessel and keep it steady, in position,

...and so will these, piece of cake,

 

while these fenders are 'slippery" and allow fore-and aft movement of the vessel.

 

e.  While giving out mooring lines, use minimal power; you can clutch out the prop close to the quay, etc... because mooring lines and props are NOT compatible.

f. And keep the props running until the last mooring line is taut, no matter the nitty-gritty phone calls from engineroom. Just don't rush.

Mooring is a game of patience and precision.

 

 

 

Marc Van de Velde

february 2016

 

 

 

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