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A knuckle boom crane is mostly used on ships or other moving installations. It's major benefit is that you get increased control over the load when the ship is in motion.

The crane is built as a normal crane, with the exception of the boom, which is hinged at the middle - creating a knuckle. The boom now looks like a finger. A hydraulic cylinder makes it possible to extend this knuckle so that it looks like a regular boom, but it's benefit comes when the boom is folded. In the folded position it is much closer to the vessel's deck, and therefore you have much less distance from the crane tip to the object you are lifting. This leads to less motion in the object, and therefore more control. Some knuckle boom cranes drop the wire altogether, relying on the increased manoeuvrability to pick up objects and drop them in the correct place.

          
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 Regular Crane          Knuckle-boom

Dynamic theory

Close your eyes !

A swinging pendulum's period is given by the equation:

T=2*π*sqrt(L/g)

So - for a regular crane L is 30 m (the crane tip's height over the deck) - giving a period of 11 seconds. This result is very close to the pitch period of a ship that is laying to the weather. In other words - when the vessel is in it's best working position, the crane is in it's worst - the motion of the vessel will make the hook swing all over the deck.

Considering a knuckle boom crane, the height from the boom to the deck can be as low as maybe 10 m. This gives a period of 6.5 seconds, which means that the hook does not get resonance due to the pitch motion of the vessel. A separate issue is that the roll-period of a vessel is around 6-8 seconds, but a vessel won't be working with weather coming in from the side due to the excessive motions. So a knuckle boom crane's worst working position is coincidental with the worst working position of the vessel. And that is a good thing.