Purchasing a grab can be more important for the calculation of vessels or contracts than it first appears.
For example, many years ago a shipping company won a contract, which factored in four vessels to handle the given bulk volume (cement-clinker) within the given time-schedule.
After a while it appeared that the grabs on these four vessels were working so efficiently and quickly, that charge and discharge time was much shorter than first calculated.
The shipping company did new calculations and discovered that they were able to fulfil the time-schedule and the necessary bulk-volume using only three vessels, due to the unexpected short charge/discharge times in the harbours.
They took out one vessel from this contract, but earned the money for the calculated four vessels. This story, of course, is a special case, but it shows the impact which grabs can have.
Grabs are the link between the vessel and shore. It depends on the grabs (and on the crane-drivers) how long the vessel will stay in harbour (if there is no limit set from shore-site).
The advantages of a single-rope grab are that they can be operated on every kind of crane and they are quiet robust. They are thus fit for areas or circumstances where only low maintenance is possible.
Disadvantages are that these types of grabs are the ‘slowest’ solution. Additional problems can happen when working high, because of the length of the closing rope.
This means in harbours with tide it can happen that during the time when the water is at its lowest point the grabs can not reach the top of the hoppers. At the very least the necessary time per cycle goes up, because the jib has to be lifted more to reach the hopper on the shore-site.
Some manufacturers offer their mechanical single-rope grabs with a remote-control for opening. However, this is not a big advantage and it does not really increase the discharge rate.
On the other hand with a remote, the grab now contains some electrical and hydraulic parts which increase the possibility of a malfunction. The advantage of the single-rope grab, which is fully mechanical and easy to maintain, is partly lost.
Diesel-hydraulic grabs have a diesel-engine installed instead of the electric motor, so they can operate independent from any external power-supply. They are controlled over a remote-control.
The advantages of these grabs are that they can operate on every kind of crane without any further equipment such as single-rope grabs, but they are also fast like electro-hydraulic grabs. These grabs are a kind of mixture of the advantages of the mechanical single-rope grabs and electro-hydraulic grabs.