A ship crane is a slewing hydraulic crane bolted to the deck of a vessel. It has about a dozen main parts: the base plate that fixes it to the deck, the slewing ring and slewing drive that rotate it, the column and booms that are moved by hydraulic cylinders, and the winch and hook at the working end, all controlled from a lever stand and supplied by a hydraulic power unit. This guide identifies each component on the technical drawing of a real marine crane, explains its function in practical terms, and shows how the components change from a 0.8 t boat crane to a 25 t deck crane.
Every figure below comes from the technical drawings and the marine crane manual of cranes built in our Xuzhou factory, mainly the 5 t knuckle boom SQ5ZA3TJ, the 1.6 t SQ1ZA3TJ+1 and the 10 t telescopic SQ10SA4T. Achord Chan, Product Operations Director at BOB-LIFT, prepared it with our engineering and after-sales teams.
Key takeaways
- The main parts of a marine crane are the base plate, slewing ring and slewing drive, column, lift cylinder, inner boom, outer boom with its cylinder and link, extension sections, winch, hook and control stand, plus a hydraulic power unit.
- A marine crane has no outriggers and no counterweight; the base plate and its bolts carry the load moment into the pedestal and the deck.
- Our smallest marine cranes, the 0.8 t and 1.6 t models, slew through 330°, while every larger model turns through the full 360°.
- The hydraulic figures on the drawing set what the power unit must deliver, from 8 L/min at 18 MPa on the 0.8 t crane to 80 L/min at 35 MPa on the 25 t SQZ500T.
What are the main parts of a ship crane?
The main parts of a ship crane fall into four functional groups. The base and slewing components attach the crane to the vessel and rotate it, whereas the booms and cylinders provide the lifting force and the horizontal reach. In addition, the winch and hook carry the load, and the controls and the power unit operate everything else. The drawing below numbers the components on our 5 t knuckle boom marine crane, the SQ5ZA3TJ, which has a horizontal reach of 4.42 m to 8.05 m and an overall height of 2,817 mm with the boom level.
The table describes the function of each numbered part, and the names in brackets are alternative terms that buyers, shipyards and crews frequently use for the same component.
| No. | Part | What it does |
|---|---|---|
| 1 | Base plate (mounting flange) | Bolts the crane to the pedestal or the deck and passes the load into the ship’s structure |
| 2 | Slewing ring and slewing drive | A large ring bearing carries the upper crane, and a hydraulic motor turns it |
| 3 | Column (king post) | The upright post that stands on the slewing ring and holds the inner boom pivot |
| 4 | Lift cylinder (main boom cylinder) | Raises and lowers the inner boom |
| 5 | Inner boom (main boom) | The first arm, hinged at the top of the column |
| 6 | Outer boom cylinder (knuckle cylinder) | Folds and unfolds the outer boom at the knuckle joint |
| 7 | Four-bar link | The linkage at the knuckle joint that lets the outer boom fold through a wide angle |
| 8 | Outer boom with extension sections | The second arm, whose sections slide out on hydraulic cylinders to add reach |
| 9 | Winch drum and wire rope | Raises and lowers the hook on a rope, independently of the booms |
| 10 | Boom head and hook | The end of the boom, with the rope sheave and the hook that takes the load |
| 11 | Control stand (lever stand, valve) | The levers and control valve that send oil to each cylinder and motor |
However, two important components do not appear on the drawing. The hydraulic power unit is the pump, motor and oil tank that supply pressurised oil to the crane, and it is usually installed below deck or in a protective cabinet next to the crane. The load moment limiter is the safety device that our marine crane manual lists as standard, and it prevents any movement that would overload the crane.
How is a marine crane different from a truck crane?
The working components above the slewing ring are essentially identical, because a marine crane uses the same type of booms, cylinders, winch and hook as a truck mounted crane. The difference lies in the structure that supports the crane and in the source of its hydraulic power, and in practice three changes matter most:
- No outriggers. A truck crane stands on outrigger legs. A marine crane is bolted to a fixed pedestal, so the base plate, the bolts and the deck structure take all of the load.
- No power take-off. A truck crane takes its oil from a pump on the truck’s gearbox. A marine crane runs from an electric or diesel hydraulic power unit, or from the ship’s own hydraulic system.
- Salt water. Every part sits in spray, so the crane needs an anti-rust finish and a fresh-water rinse after work.
Furthermore, our marine cranes have no counterweight and no lattice tower. Older articles about ship cranes often describe both, but those features belong to large cargo and offshore cranes, whereas on a hydraulic knuckle boom or telescopic crane the load moment is transferred through the slewing ring and the base bolts directly into the pedestal. If you also operate truck cranes, our guide to truck mounted crane parts explains the outriggers, the power take-off and a recommended spare parts list.
Base plate, pedestal and slewing ring
The base plate is the steel mounting flange at the bottom of the crane, and it is the first component to plan, because the deck structure must be prepared for it before the crane is delivered. Each of our technical drawings specifies the plate dimensions, the number and diameter of the bolt holes and their spacing, and these dimensions increase with the capacity of the crane.
| Crane | Rated capacity | Base plate | Bolt holes | Slewing |
|---|---|---|---|---|
| SQ1ZA3TJ+1 knuckle boom | 1,600 kg | Square, 328 × 328 mm | 12 × Ø18 mm on a 278 mm square | 330° |
| SQ5ZA3TJ knuckle boom | 5,000 kg | Square, 680 × 680 mm | 24 × Ø22 mm at 100 mm spacing | 360° |
| SQ10SA4T telescopic | 10,000 kg | Round, Ø1,000 mm | 28 × Ø25 mm on a Ø930 mm circle | 360° |
The pedestal under the plate is usually fabricated by the shipyard. Its height determines how far the hook clears the railings and the deck cargo; however, a taller pedestal also gives horizontal forces a longer lever on the deck. Our pedestal crane guide explains the flange dimensions, the foundation loads and the choice of pedestal height in detail.
The slewing ring is a large ring bearing above the base plate that carries the column, the booms and the load, while a hydraulic slewing motor rotates the crane by driving its gear. Our 0.8 t and 1.6 t models slew through 330°, and all of the larger models rotate through the full 360°. Therefore, before you fix the crane position on the deck plan, confirm that every pick-up and landing point lies within the slewing range.
Booms, cylinders and the knuckle joint
The booms are where the two main crane types differ. A knuckle boom marine crane is a crane with an inner boom and an outer boom connected by a hinge, so it folds into a compact package on deck. A telescopic marine crane is a crane with one straight boom whose sections extend hydraulically, so it reaches further for a similar weight but remains a long arm when stowed. For a detailed comparison, see our guide on knuckle boom vs telescopic marine cranes.
On a knuckle boom crane, three sets of hydraulic cylinders perform the work: the lift cylinder raises the inner boom, the outer boom cylinder folds the second arm, and the extension cylinders inside the outer boom extend its sections. In addition, our knuckle boom cranes use a dual four-bar linkage at the knuckle joint, which, according to our manual, allows the outer boom to fold beyond straight into a reverse angle. This is useful when the hook must reach down into a hold or over a bulwark.
Small boat cranes often include one additional component. The “+1” in a model code such as SQ1ZA3TJ+1 indicates a manual extension, which is a final boom section that the crew extends by hand to obtain additional reach with a light load. On the 1.6 t SQ1ZA3TJ+1, for example, the working radius ranges from 1.39 m to 4.23 m, and the rated load at maximum reach is 300 kg.
Winch, wire rope and hook
Without a winch, the hook hangs from a short fitting at the boom head, and consequently the crane can raise or lower the load only by moving its booms. A crane winch is a hydraulic motor and drum that winds the wire rope, which runs over a sheave at the boom head down to the hook. As a result, the crew can lower a load into a hold, onto a quay or into the water far below the level of the boom.
The “J” in our model codes, as in SQ5ZA3TJ, identifies a crane equipped with a winch. On our knuckle boom marine cranes the winch is an optional component, whereas our telescopic marine cranes are supplied with a winch as standard. On the 5 t SQ5ZA3TJ, the drum is mounted under the outer boom, as part 9 of the drawing shows. Because the wire rope and the hook latch are wear components, inspect them for broken wires, kinks and crushing at every inspection.
Hydraulic system and power unit
Every movement on a marine crane is hydraulic: a pump delivers pressurised oil to the control valve, the valve directs it to the cylinder or motor that the operator selects, and the oil returns to the tank through a filter. Our marine crane manual shows two types of power unit, electric and diesel, and a crane can also operate from a vessel’s existing hydraulic system, provided that the system supplies sufficient flow and pressure.
The technical drawing of each crane specifies what the power source must deliver, and the following table compares six models from our range.
| Crane | Rated capacity | Recommended power | Oil flow | Working pressure | Oil tank |
|---|---|---|---|---|---|
| SQ08ZA3TJ+1 knuckle boom | 800 kg | 3.5 kW | 8 L/min | 18 MPa | 20 L |
| SQ1ZA3TJ+1 knuckle boom | 1,600 kg | 7.5 kW | 15 L/min | 18 MPa | 25 L |
| SQ3.2ZA3TJ knuckle boom | 3,200 kg | 15 kW | 25 L/min | 25 MPa | 60 L |
| SQ5ZA3TJ knuckle boom | 5,000 kg | 22 kW | 35 L/min | 28 MPa | 100 L |
| SQ10SA4T telescopic | 10,000 kg | 37 kW | 63 L/min | 26 MPa | 200 L |
| SQZ500T knuckle boom | 25,000 kg | 55 kW | 80 L/min | 35 MPa | 260 L |
On the smallest cranes, the power unit can be very compact. For example, the unit supplied with our 0.8 t SQ08ZA3T+1 combines an electric motor and pump, an oil tank and a battery on a single frame, so a small boat can operate the crane without a separate generator.
Control stand and safety devices
On most of our marine cranes, the operator works from a lever stand mounted on the crane base, where each lever controls one function: slewing, the lift cylinder, the outer boom, the extension and the winch. A pressure gauge on the stand displays the working pressure. In addition, our manual lists an elevated operator seat and a wireless remote control as options, and a remote control is particularly helpful when the operator needs to observe the load from the railing rather than from the column.
The load moment limiter is the principal safety device: it monitors the load on the crane and stops any movement that would exceed the rated moment. Nevertheless, it does not replace the load chart, because the operator still needs to know the weight and the working radius before each lift.
Inspection requirements depend on where the crane operates. The ASME B30.22 safety standard covers articulating boom cranes, including stationary installations with a load hoist [1]. In Great Britain, lifting equipment must be thoroughly examined by a competent person every 6 or 12 months, depending on the equipment, or according to an examination scheme [2].
Stowage and corrosion care
While the vessel is under way, the crane should be stowed and secured. A knuckle boom crane folds its booms beside the column; for example, our SQ5ZA3TJ folds to 2,321 mm long, 2,261 mm high and 712 mm wide, and the 1.6 t SQ1ZA3TJ+1 folds to 1,305 mm long and 1,770 mm high. In contrast, a telescopic crane retracts its boom and lowers it to the transport position.
Salt spray attacks every component, from the base bolts to the hook. Our manual lists several anti-corrosion treatments for the marine environment; nevertheless, the paint system, the protection of cylinder rods and the sealing of electrical components are worth confirming in writing before you order. After delivery, a simple maintenance routine protects the components:
- Rinse the crane with fresh water after work in spray.
- Grease the slewing ring, the pins and the boom joints on the schedule in the manual.
- Keep the boom stowed and the hook secured at sea.
- Check the hoses, the wire rope and the base bolts at every service.
Our deck crane guide explains sea-state limitations and deck strength in more detail.
How to read a BOB-LIFT marine crane model code
Our model codes identify several of the components described above, so once you can interpret a code, you can tell how the crane is configured. Consider the SQ5ZA3TJ as an example:
- SQ5: the capacity class, here 5 t; SQ08 means 0.8 t.
- Z or S: Z is a knuckle boom, S is a straight telescopic boom.
- A3: the number of boom sections, here three.
- T: the marine version.
- J: fitted with a winch.
- +1: a manual extension at the end of the boom.
Our three largest knuckle boom cranes, the SQZ330T, SQZ430T and SQZ500T, are named according to their lifting moment rather than their capacity, and they are rated at 16 t, 20 t and 25 t respectively at short reach.
Frequently asked questions
What are the parts of a crane on a ship called?
The parts of a crane on a ship are the base plate, the slewing ring and slewing drive, the column, the lift cylinder, the inner and outer booms with their cylinders, the extension sections, the winch with its wire rope, the boom head and hook, and the control stand. A hydraulic power unit and a load moment limiter complete the crane.
What is the function of the slewing ring on a marine crane?
The slewing ring is a large ring bearing between the base and the column. It carries the weight of the upper crane and the load, and the slewing motor turns the crane by driving its gear. On our marine cranes it allows 330° on the two smallest models and 360° on all others.
Does a marine crane have a counterweight?
Our hydraulic knuckle boom and telescopic marine cranes have no counterweight. The load moment passes through the slewing ring and the base bolts into the pedestal and the deck, so the deck must be designed for it. Counterweights are found on large cargo and offshore cranes, not on this type.
How is a marine crane powered?
A marine crane is powered by hydraulic oil from an electric or diesel hydraulic power unit, or from the ship’s own hydraulic system. The drawing states the oil flow, pressure and power the crane needs, from 8 L/min at 18 MPa on our 0.8 t model to 80 L/min at 35 MPa on our 25 t SQZ500T.
Which marine crane parts wear out first?
Hydraulic hoses, cylinder seals, filters, the wire rope and the hook latch wear first, followed by pins and bushes that miss their grease. Salt water speeds up all of it, so rinse the crane after use and keep to the greasing schedule. Our factory ships spare parts within 48 hours and answers service questions within 12 hours.
How we compiled this guide
The part names, base plate sizes, slewing ranges, reach and hydraulic figures come from the BOB-LIFT technical drawings of the SQ08ZA3TJ+1, SQ1ZA3TJ+1, SQ3.2ZA3TJ, SQ5ZA3TJ, SQ10SA4T and SQZ500T, checked against our marine crane manual. The photos and the video were taken in our Xuzhou factory. Achord Chan, Product Operations Director at BOB-LIFT, wrote the guide with our engineering and after-sales teams.
Get the drawings for your vessel
If you are planning a crane for a vessel, ask for the technical drawing, the load chart and the base plate layout together, so that the shipyard can prepare the pedestal. You can browse our marine crane range or read the marine crane buying guide to choose the boom type and size. Then contact us with your vessel type, the heaviest load and the reach you need, and we will reply within 12 hours.
