Above speeds of approximately 30 knots the traditional fixed pitch propeller, an open stream machine, cannot operate due to the poor priming of its blades.
The introduction of a case which produces a radial discontinuance with the outside, allows an increase of the priming of the turbo-machinery blades and the achievement of higher speeds (40 – 60 knots depending on waterjet design).
The waterjet drive gives a boat the widest possible operating range: from open sea to shallow waters, due to the elimination of all under keel protrusions. This also contributes greatly to the reduction of night time operational risks.
The absence of open rotating parts provides complete safety for people in the water. Rescue operations by divers for survivors can be safely performed with the engine running to withstand wind or currents.
The reversing bucket may be quickly activated to stop the boat at any speed, within 1.5 times its length. It is possible to shift from full forward to full backward with no strain on the engine as the direction of the water flow is the only thing that is altered in stopping or reversing.
The absence of high speed cavitation pressure peaks, caused by proximity of the propeller blades to the hull and torque effects, drastically reduces hull vibrations and under water noise.
The waterjet, not having open rotating parts or under hull appendages, cannot cause damage to aquatic fauna.
The intense water flow and the option of directing it downward, allows the waterjet to oxygenate stagnant waters. Furthermore, as water is suctioned to the surface, suspended mucilage is dissipated.
It is impossible to damage the impeller, shafts or steering and reversing actuators in the event of grounding as nothing protrudes from the keel.
Therefore, unscheduled down-times are eliminated. The increased blade tip clearance of the axial flow impeller, caused by wear after extended use in sandy waters, does not affect speed as dramatically as mixed flow impellers do.
There is no danger of vital components (control levers, hydraulic rams) being affected by sea incrustations, by corrosion or by impact as these are all inboard mounted where they are serviceable without docking the vessel.
The impossibility of over-loading the engine leads to reduced engine wear as the characteristic of the waterjet is to maintain constant power absorption and engine speed (according to cubic law); regardless of boat speed.
The boat speed and direction can be adjusted continuously, independently from the pump speed, by setting the bucket, without the impeller losing its grip on the water, because it is never disengaged.
This is a great advantage because the maximum thrust is available at low speed for performing precise maneuvers while in severe conditions, in contrast to traditional propulsion systems in which thrust increases boat speed.
In a twin installation, using one drive in reverse, the other in neutral (and balancing their thrusts to keep the boat at a standstill), it is possible to move the boat sideways while adjusting the steering nozzles, without the use of bow thrusters or electronic control systems.
The Castoldi waterjet drive is very light due to the aluminum alloy construction of its principle parts.
The gearbox is integrated in the unit therefore the engine need not be provided with a marine transmission.
All this makes Castoldi waterjets the lightest ones on the market, even lighter than the smaller waterjets of other manufacturers, when combined with the weight of the marine transmissions.
Every Castoldi waterjet drive is equipped and delivered in a single module which includes a propulsion system with water intake, duct, steering and reversing devices, with top quality equipment required for its installation, interconnection and control, to ensure the best performance.
All this produces a pre-assembled, factory tested package; each waterjet incorporates its own self-contained hydraulic system for actuating the reversing bucket, the steering nozzle, the hydraulic clutch and the movable grid.
High efficiency at medium to high speeds (25-60 knots) due to the following: The axial flow impeller has been designed to achieve the highest efficiency and the best resistance to cavitation.
The lowering of the impeller shaft, made possible by the integrated gearbox, allows the design of a flat, straight duct enabling the water stream to flow smoothly with minimum hydrodynamic losses at high speed.
As well as the inherent waterjet characteristic all rotating parts being contained in the casing and no under keel protrusions, which allows shallow water operations, Castoldi waterjets are the best choice for boats operating in shallow and sandy waters for the following reasons:
The axial flow impeller is less sensitive than mixed flow impellers to clearance which may be formed between the impeller and the impeller housing following use in shallow and sandy waters which can greatly affect boat performance.
There is no water lubricated tail seal with support function, which would be vulnerable in sandy waters, and could induce vibrations and consequent damage to shafts, gearboxes, engines and hulls.
The impeller shaft rotates safely inside the fin oil chamber, and is therefore perfectly protected from sand or debris being suctioned into the waterjet.
The water intake is positioned as closely as possible to the stern thus limiting aeration during rough water conditions.
The “holdfast effect” of the water intake and the mass of water flowing into the duct contribute to keeping the boat in contact with the water surface and stable.
The lowering of the impeller shaft and consequently the center of thrust, allowed by the integrated gearbox, provides optimum course stability.
The integrated gearbox greatly reduces the engine room length.
The water intake is protected by the movable debris screen grid which, in most situations, blocks and rejects debris.
Furthermore, the limited number of impeller and stator blades allows the easy transmission and discharge of ingested debris which, in any case, would not damage the impeller shaft as this turns in an oil bath inside the fin.
If, in the remote event that, despite all this prevention, the waterjet does become clogged, it is possible to manually remove the debris by opening the inspection hatch, positioned close to the water intake. On request the superior Castoldi Clear-Duct unclogging system can be installed.
The Castoldi waterjet may be disassembled and accessed for inspection in just a few seconds, simply by removing the rear part.The impeller is driven by the spline shaft and it is kept in place by a single special screw which also acts as an extractor for easier removal.
Longer service life is achieved through rugged construction and full protection against corrosion.
The whole unit is protected by a hard anodizing treatment, plating all light-alloy components with 60 microns thickness of aluminum oxide (ceramic), three layers of special paint and cathodic protection by sacrificial anodes. All the main parts (impeller, impeller shaft, etc.) are manufactured with the best available materials (AISI 316 L, Duplex, Titanium etc…).