Driven by a research partnership, the Holy Boss Cap is a hydrodynamic solution that delivers multiple benefits for shipowners faced with increasingly tough regulatory requirements and rising operational costs, writes Dr Batuhan Aktas
To comply with IMO’s EEDI, EEXI and CII requirements, all ships must meet specific energy requirements for their size and type. The EEDI, which became law for all ships in 2013, is not prescriptive about what methods are used to achieve compliance, creating many opportunities for innovative design.
One area of ship operations that has attracted much attention in recent years, due to the possibilities it represents for emissions reduction, is that of cavitation. During ship operations, cavitation occurs because of vortices that build up and cause bubbles to collapse under the ship’s propeller. The results of this process include increased noise, vibration and energy usage. The latter is of particular concern for ships looking to streamline their energy profile.
Existing energy saving designs for propeller hub caps, or boss caps, incorporate fins that act to improve vessel efficiency by reducing cavitation while ships are at sea. However, the finned design generates turbulence and is linked to cavitation as bubbles form underwater.
EcoMarine Innovations, a pioneering start-up supported by the University of Strathclyde, set itself the goal of addressing these issues while it was working on a project with a major European propeller manufacturer. During this collaboration, the start-up learned that cavitation is a major cause of inefficient ship operations, accounting for 3-8% of lost propulsive efficiency.
Convinced that it must be possible to develop a better way of addressing this efficiency cost and the related environmental impacts, EcoMarine Innovations studied the design of boss caps commonly used on ships. During this research, it began exploring the idea of designing a boss cap that utilises holes instead of any structure that can cause turbulence or operational risk. The result is the Holy Boss Cap (HBC).
Testing the technology
The development of the HBC demonstrates the importance of research support of the kind at Strathclyde University. The product, which was launched in February 2024, eliminates propeller hub vortex cavitation, the main source of rudder erosion, and reduces associated propeller efficiency losses. However, an early version of the innovative ‘holy’ design failed to gain traction as it was not hydrodynamically efficient, as the holes reduced the efficiency of the propeller.
The revised design of the HBC, which was implemented following extensive studies in partnership with the University, addressed these problems with carefully placed and angled holes bored into a conically shaped hub. The holes channelled into the hub affect the high pressure in the hub vortex by redirecting the flow downstream. The resulting low-pressure swirl flows in the opposite direction to conventional hubs, behind the propeller blades, reducing propulsive drag, fuel consumption and maintenance costs.
“Cavitation is a major cause of inefficient ship operations”
To assess the effectiveness of the new design, EcoMarine Innovations carried out computational fluid dynamic (CFD) tests on a typical twin-screw vessel with V-brackets and a 90-m coastal general cargo ship. Using local workstations running parametric optimisation software combined with CFD software, studies were conducted on the effects of variables such as chamber volume and profile, number of holes, and angle of the holes.
When compared with more advanced energy-efficient boss caps currently in operation, the HBC was found to be at least 3% more efficient. Overall, compared to standard propeller boss caps, the HBC improved propeller efficiency by 3.1% and thrust by 1.1%, while reducing torque by 2%, rudder cavitation by 10%, and propeller induced noise by 1-3dB. EcoMarine Innovations expects the HBC to be capable of delivering increases in propulsion efficiency of up to 5%, compared to conventional propeller boss caps.
These potential savings were further confirmed by a reputable European cavitation tunnel testing facility, where savings of more than 2.1% were found, despite challenging scale effects.
Having successfully completed validation tests at a hydrodynamic research centre in Sweden, the HBC is undergoing ship model basin trials to verify the efficiency gains on larger commercial and naval vessels. Meanwhile, the patent for the HBC is pending, with the support of Strathclyde University in the patent application process.
A simpler solution
EcoMarine Innovations believes that traditional methods of addressing ships’ energy wastage have failed to keep pace with the industry’s needs, or to consider the wider impacts of underwater noise and vibration. Zero propeller hub vortex cavitation can help towards ESG, EEXI and CII goals, improve efficiency and reduce the costs associated with cavitation-induced rudder erosion. Driven by research, the HBC is meeting this need in the market and is contributing to more sustainable ship operations.
Underpinning the start-up’s efforts in bringing this product to market is the drive to find the simplest way to address what it views as a very simple problem. EcoMarine Innovations achieved this by several metrics: installation of the HBC takes just five to six hours and the product can easily be retrofitted or installed on new vessels. Once fitted, the product can be maintained during routine drydock visits. In addition, the HBC offers reduced capex compared to existing devices. Thanks to the ease of casting and the lower amount of material required, it costs significantly less to manufacture than current conventional propeller hubs.
The business case for shipowners is compelling. A 250-m vessel with a fuel consumption of about 35 tonnes per day that operates approximately 240 days a year, can assume a 3% savings, with a return on investment of around five months.
The Holy Boss Cap has already received significant interest from shipowners and propeller manufacturers and EcoMarine Innovations is engaging with potential partners to take the concept to market.
Editor’s note: Dr Aktas is the chief executive of EcoMarine Innovations, Futureproof Ship Design Group at the University of Strathclyde, Glasgow, Scotland.
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