Great Lakes Self unloaders
Great Lakes Self Unloader
One of the most efficient cargo boats is the Great Lakes self unloader.
This is a specially designed boat that has the ability to discharge its bulk cargo without the use of shore facilities and does it without the need of any shore based personnel.
It is usually loaded with a variety of cargo such as coal,iron ore pellets,stone,cement clinker,grain of different types, potash and salt and almost anything which can be classed as bulk.
Most have several holds so a mixture of cargo can be carried and it is when the receiver wishes a blend this option is available.
Most loading is done from shore based facilities but it is not unusual to load from another selfunloader.
It is the discharge when this special boat comes to its own. As Mentioned previously this is all done by the ships crew.
At arrival at the discharge place (I don't say berth for reasons mentioned later) the mooring is done by landing crew members on the dock. Unlike deep sea vessels there are only four wires (occasionally six) used and they are from for'd one and two, and aft three and four. These wires are attached to self tension winches so after mooring they are switched to automatic.
The unloading boom is swung ashore, This is about 250feet long. To do this it is necessary to start ballast going in to an offshore tank so the boat stays upright. To assist this there are indicator lights positioned for'd, aft and in the engine room control room.
These lights are arranged as follows. There are five lights on each display. White light n the middle and two red light for port and two green light for starboard. When the boat is upright a white light is on. If a list to port then a red light comes on. Red flashing indicates ˝ degree to port, a solid red light one degree one fixed red and one flashing indicates one and a half degree and two solid red is two degree list. Hope this is a sufficient explanation. It is important as the boom is very heavy and itsessential to make sure it does not come loose.
One mate is stationed at the bottom of the boom at all times and he has an emergency stop bottom in his hand and should he see anything untoward he can shut the whole system down immediately. There are numerious walkie talkies for constant communication. At the base of the boom is the control room where the electrician sits and he is the person who controls the rate of discharge by monitoring the load on the electric motors. There are several depending on if there is a two belt system in the tunnel or three. Then there is the loop belt which brings the cargo from the lower hopper to the hopper at the base of the boom and last the boom belt motor.
Before discharge starts the mate gives the electrician the sequence of discharge and then the tunnel men on the bowls of the boat open the gates accordingly. These hydraulic gates can be open fully or partially depending on how fast the cargo flows. If if flows too fast and loads the belt too much the electrician indicates to tell the tunnel men of throttle the gates in. It takes lots f cooperation to unload the cargo and one of the worst problems is if the belt is loaded too heavily and the electric motor stalls. Then its a matter of crew shoveling the excess cargo off the belt till it can go again. If a tunnel man does this he is not a popular guy.
The unloading sequence is very important as done wrongly it puts massive stresses on the hull and there have been occasions when the boat has broken its back.
While this is going on there is another mate monitoring the ballast and watching the cargo holds to see that there are no hang ups with the cargo. The modern lakers have a coating on the hopper sides of the hold so the cargo does not stick. There are a series of vibrators which can be turned on to shake the sides if by any chance cargo sticks.
When the hold is getting close to being empty deckhands are sent down to clear up any ago on the walkways. When they are down the hold one person delegated to watch and has another emergency stop button.
The rate of discharge depends on the type of cargo but with iron ore pellets is is in the region of 6000 tonnes per hour. It varies with other cargoes and I have seen in the fall when the temperature falls below freezing coal cargoes which are wet, taking days as it freezes in the holds and when on the boom belt the moisture coats the belt an the coal slips choking the spout feeding from the hopper.
It is a very coordinated operation and if the crew are experienced it is problem free.
Each cargo has its own special handling , as with potash you can't unload it too fast as it breaks the granules and the receiver is most upset. Clinker is slow as you don,t wish too cause too much dust or the environmental people will shut you down.
On some of the boats you get to berths or should I say discharge places where it requires what is referred to as a “Punt job”. The boat gets near as possible to the place and the punt is sent to go ashore with four deckhands and four heaving lines. There is usually a shore person there to tell out where to dump the cargo and if he is willing to help pulling mooring wires ashore with his truck or pay loader. Depending on the water levels it could happen the boat can't get close enough for the boom to reach. This can mean waiting till the water level rises or the mate loads the belts very lightly and shoots the cargo off. As the boat lightens it is winched closer. I will tell a story about a time we did a “punt job” later.
When discharge is complete the boom is slowly swung inboard and the balancing with ballast is done. Once the boom is in its cradle the power its then switched to the bow thruster and the crew sent ashore to cast off and away you go to the next port.
To give you an idea of how busy we are ,we carried in one season 126 cargoes and some of those were two or three berth discharge points.
I hope my explanation is understood giving you an idea of these special boats.