FITTING a truck-type crane to a fishing boat instead of one specially designed for marine use can cause costly accidents, according to Petrel Engineering of Cape Town.
"Truck-type cranes are designed to work on a stable and horizontal platform often secured using hydraulic outriggers to ensure stability. A marine vessel provides a very different platform: one that moves and rolls, " warns the company which has supplied big-catching hauling and lifting systems to vessels worldwide.
"Even in harbour a vessel experiences a pitching and rolling motion and, in a rough sea, this motion is more pronounced. The effect is that it induces a swing in the load being lifted."
In turn, this induces a side load on the crane structure, rather than the purely vertical load that a truck crane is designed for.
Petrel Marine Cranes are designed for such a moving platform and can operate at full capacity up to 15infinity of roll.
The crane is designed so that the structure and moving parts withstand side loads and not bend or collapse, as can happen with truck cranes.
"Marine cranes have a higher slewing torque to hold the side load imposed at the end of the boom, while a truck crane is designed to work on a level platform and side loads are not expected, " says Petrel.
"The simple rack and pinion slewing mechanism is adequate for this type of operation. At sea, however, it is woefully inadequate."
Petrel cranes use large diameter slew bearings on which the crane is mounted.
High specification hydraulic slew motors rotate the crane and are strong enough the hold the load when the crane is tilted.
Petrel crane booms are made from large steel box sections which are almost square to provide strength for lifting and, also, to withstand side loads. Truck crane booms are deep, but narrow, and so have little strength against side loads.
Petrel cranes are designed with as few working parts as possible to greatly improve corrosion protection and simplify maintenance.
However, truck cranes are generally of the telescopic type and not designed for corrosive marine conditions.
They have many working parts, such as multiple telescopic sections, which cannot be adequately protected against corrosion.
Petrel cranes have enclosed box structures treated with a high quality zinc paint system which gives life long protection against corrosion.
With its reduced number of working parts, a Petrel crane has fewer areas that the ship's engineer needs to attend to.
So, greasing and inspection demands are reduced.
In addition, Petrel can customise a crane. Customers benefit by being involved in the design process to develop a crane that suits their requirements exactly.
This can include modifying boom lengths, adding multiple lifting winches and mounting customer-specific equipment to the crane.
The crane Petrel recently manufactured for a research vessel is a good example of this, where the crane was designed with a wire compensation system mounted on the main boom. It used a system of springs to eliminate motion induced shock loads on the sensitive scientific cable that was being trailed in the sea from the crane.
Petrel Engineering says that robust marine deck cranes specifically designed for marine conditions will give the vessel operator a safe and reliable lifting system suiting a range of applications and sea conditions.
