## Crane calculations to load a cargo on a vessel. How to know the most efficient crane needed for the work.

Is well known in the crane industry that the calculations and the election of the crane needed to load a cargo on a vessel is done by the crane provider.

It is also, a common practice from the crane’s providers to propose a “over” capacity crane in order to be completely sure that the crane will be enough for the cargo that will be lifted. As heaviest is the crane, more expensive for the client and much more profit to the crane provider.

Above option from crane providers is OK, but in the other hand, and unfortunately in this sector, there are some providers that do not pay enough attention to all the factors involved (and relevants) to calculate the crane needed to lift a cargo and bad consequencies happen during the lifting operations.

Now we can start considering the most important points to calculate the minimum mobile crane needed to load a cargo on a vessel.

1.- Vessel data.

If we are going to load a cargo on a vessel, we need to know the most important info from the vessel:

LOA: “length over all”

Beam.

Freeboard

Draft

In the following image you can check this 4 data points of the vessel:

2. Port data.

As we are loading the cargo in a Port, we need to know following items:

First of all: Draft of the Port > Draft of the vessel.

Distance from water level to quay side (in red color in following picture):

Other very important point (and many times forgotten) is the distance of the defenses in the port:

Distance from quay to crane legs (sometime there are rails of port cranes that don’t allow to position the steel plates of the mobile cranes on top of these rails). Normaly this distance should be around 2 or 3 meters. Please find a picture showing this points; in red the steel plates of the crane and in green the port rails.

3. Cargo data.

The last info we have consider is the distances, dimmensions and weight of the cargo to be loaded.

Cargo weight/length/height.

Height of the cargo where we want to load on a vessel.

Distance from cargo lifting point to crane’s hook.

Distance from crane’s hook to crane’s top boom.

With all above info clear, in order to know which crane do we need to load a cargo on a vessel, following info is needed to check the outreach table of mobile cranes:

Tons to be lifted.

Horizontal distance from crane’s cog (center of gravity) to cargo lifting point.

Vertical distance from crane’s cog to crane’s top boom

With pitagoras and with horizontal and vertical info, we could know the boom extension needed:

boom extension (yellow color) = sqrt (blue^2 + green^2)

Once we know the weight to be loaded, the horizontal distance and the boom extension, we can check in the outreach of the mobile crane if this is OK:

In the following example, we can reach 45,5 tons with an horizontal distance of 11 meters and boom extension of 39,1 meters.

I created a google sheet to help you to calculate the minimum crane needed to lift a cargo on a vessel. Just fill up the blue cells and the sheet will calculate everything, the link as follows and choose the tan named “Crane to load a vessel”:

Please feel free to write a comment for anny doubt/assistance.

## Tandem lifting calculations

Tandem lifting calculations – WikiLogistics

When you are considering to lift an special piece in tandem lift with 2 cranes, some quick calculations are needed to know in advance what will be the weight at each crane will be working.

Knowing the length, weight and position of the cog (center of gravity) we can easy calculate the weight soported by each crane at diferent positions where the slings are placed.

General example:

Tandem lifting calculations – WikiLogistics

P is the weight of the piece (Kgs)
F1 is the weight soported by crane number 1 (Kgs)
F2 is the weight soported by crane number 2 (Kgs)
l is the length of the piece (mm)
a is the distance between the leftmost to the position of the sling from crane number 1 (mm)
b is the distance between the rightmost to the position of the sling from crane number 2 (mm)
c is the distance between the leftmost to the position of the cog (center of gravity) (mm)

Equilibrium condition says that:
1.- The resultant force acting on the object is zero.
F1 + F2 = P so F2 = P – F1

2.- The sum of the moments acting on an object must be zero.
F1 x (c-a) = F2 x (l-b-c)
F1 x (c-a) = (P – F1) x (l-b-c)
F1 x (c-a) = (P x (l-b-c)) – (F1 x (l-b-c))
F1 x (c-a+l-b-c) = P x (l-b-c)
F1 = P x (l-b-c) / (-a+l-b)

Weblink if you want to calculate F1 and F2 automatically: