98° gearbox tilt: the right angle?

Tugs built with azimuth stern drive (ASD) and thruster in-line configurations could save fuel if these propulsors each had a 98° gearbox, according to extensive studies.

Schottel undertook research into optimising the hydrodynamic performance of propulsion thrusters, both individually and in combination with the vessel’s hull, discovering this is a crucial factor in enhancing tugboat operations.

This study used computer modelling and computational fluid dynamics (CFD) analysis to demonstrate the recognised benefits of the 98° water gear for vessels can also be applied to tugs.  

“The 98° gearbox has shown an improvement in thrust performance in a variety of tug designs, as well as thruster-hull and thruster-thruster interactions during routine operations at bollard pull conditions,” says Schottel sales manager for tugs and offshore support vessels Erwin Boijmans.

“This improvement can result in reduced fuel consumption and lower greenhouse gas emissions due to increased efficiency,” he says. “The 98° gearbox has shown a positive effect on the effective thrust of the propulsion unit during free sailing for offshore vessels, although further research is required for tugs.”

Tugs and offshore vessels all benefit significantly from maximising thrust with minimal losses, especially when vessels are in dynamic positioning (DP) mode. Schottel had found during studies that the combination of an increased steering speed (5 rpm), faster ramp-up time (4 seconds) and the lower gearbox of 98° resulted in a 25% reduction in DP footprint for offshore vessels.

“The 98° lower gearbox was found to be the optimum angle to keep the downstream of the propulsion unit away from the hull, which is confirmed by full-scale simulation and model tests,” says Mr Boijmans.

“With a 90° lower gearbox the flow tends to adhere to the ship’s hull. The thrust is reduced due to friction losses between the hull and the propeller wash, possibly enforced by the Coanda effect, a phenomenon where a fluid jet, such as water, remains attached to a surface.”

The main advantage of the tilted lower gearbox is the change in flow direction produced by the propulsion system. This way, the downstream flow is kept away from the ship’s hull, reducing hydrodynamic losses.  

Thus, any negative interactions between thruster and hull or thruster with thruster is reduced. The shape and orientation of the propeller jet has a direct influence on energy losses, with the amount dependent on vessel speed and propulsion power.

Benefits of installing a 98° gearbox in a tug include simplifying the design of the shaft line. “The offset in height can be compensated by inclining the azimuth thrusters and combustion engine,” says Mr Boijmans. “Now a straight shaft line can be selected with a reduced weight, the number of bearings in the shaft line can be reduced, simplifying the overall structure and reducing the need for multiple bearing foundations.” This also lowers maintenance and alignment issues and structural noise.

With a 5° tilted nozzle there could be an increase in thruster efficiency of 16%, but a 98° lower gearbox is still required to minimise thruster-hull interactions, according to CFD simulations. However, there is also a considerable increase of the cavitation area with a tilted 5° nozzle, regardless of the gearbox angle, so “there is a higher risk for cavitation erosion, noise and vibration,” says Mr Boijmans.

Schottel undertook CFD analysis using a model of a 28-m ASD tug with two of its rudderpropellers of SRP 460 LE type with a fixed-pitch propeller in a nozzle.

The Germany-headquartered manufacturer found a 98° lower gearbox leads to “enhanced thrust deductions, improving the efficiency, safety, and manoeuvrability of tugboats.”

“Reduced thruster-hull interaction provides a stable separation between the propeller wash and the hull, resulting in reduced noise and vibration throughout the structure, and minimised thruster-thruster interaction, ensuring more thrust due to less disturbed inflow to the opposite thruster and a decreased interaction zone, allowing for a wider azimuth operation range.”

There was superior efficiency of the 98° gearbox compared with a 5° tilted nozzle combined with a 90° gearbox, while tractor tugs benefit primarily from the 98° gearbox in terms of improved thruster-hull interaction and their operating profile.

“Factors such as inclination angle, the distance between thrusters, trim, the gap between thrusters and hull, dynamic conditions, steering angles and the distance between tug and assisted vessel and its design can all impact the thrust performance,” says Mr Boijmans. “In various ASD tug manoeuvres, the 98° lower gearbox is advantageous in ahead, astern, and traverse modes.”