Boom angle of a crane

[DaveC426913]

TL;DR Summary

Is there a reason why a boom crane operator would extend the boom higher than physically necessary?

These boat hauling cranes seem to extend their booms as high as possible. Much higher than necessary for the max height of the boat lifting rig.

Does this accomplish something effort or safety-wise?

It's not like a simple torque situation - the force applied is gravity - straight down - not perpendicular to the boom as it might be in a simple setup.

 

[tech99]

It would make manoevring the boat easier for the ground crew.

 

[DaveC426913]

It would make manoevring the boat easier for the ground crew.

Why/how?

 

[tech99]

It is possible for the ground crew to move the boat around to a limited extent, which pulls the suspension wire out of vertical. When they do this, there is a restoring force equal to Weight x Sin (angle of deflection).
If the wire is very long, the angle of deflection is smaller, so the restoring force is less.
For example, a boat weighs 1t and the wire is 2 degs out of vertical. Restoring force = 1 x sin 2 = 0.03t.
By the same token, if the wire is 20m long, then the boat has been moved by 20 sin 2 = 0.7m.

 

[anorlunda]

Another factor, how high can that boat get before it could hit the boom if it pivots?

 

[Baluncore]

When they began to build booms from composites, rather than the heavy and difficult to work HT steel, all the sections were extended by an internal tensile wire. The boom had no real strength while being extended as the composite tube walls were too light to carry a large moment at part extension. The composite sections must be fully extended for the tube ends (where there are a pair of internal pulleys) to lock, and so become rigid, and able to carry a side force. The telescopic tubes are under compression, the wire is under tension.

I believe that steel booms are now designed using the same philosophy, as they must compete with composite booms. So, now all booms are operated fully extended.

The boom angle is controlled, and the safe operating envelope managed by a dedicated on-board computer, to prevent the operator from "taking the crane out-of-survey", to put it nicely.