The Naval Architect: October 2017
A large number of energy saving devices (ESDs) have been developed and applied to ships, but only a few have been successfully utilised because of structural and economic problems, according to Dr Hanshin Seol, KRISO.
“Recently, side effects caused by energy saving devices have been frequently reported, which is a full scale performance issue including maintenance and structural problems, like propeller erosion,” says Seol.
“Some ESDs, especially those that increase propeller loading like duct and stator type ESDs, show good propulsion efficiency improvement in model tests, but sometimes [these results] do not represent their performance at full scale. Therefore, the effects of these types of ESDs are still controversial due to the scale effect — because the inflow wake is much different at model and full scales — and these types of ESDs tend to over-predict their real performance in model tests.
“Furthermore, these types of ESD can also cause structural problems. Recent results show that the propeller design to achieve high efficiency and the energy saving device are basically designed to increase the load of the propeller. As a result, the cavitation behaviour of the propeller becomes bigger or more violent, which can cause adverse effects like hull vibration. Even propeller erosion phenomena has been reported due to the abnormal flow caused by an ESD.”
In search of an answer
In response, KRISO began a research programme in 2014 to develop technologies to improve ship propulsion efficiency and to reduce the radiated noise of the ship propeller by improving the cavitation phenomena occurring around it.
“We went in with the idea to secure the rotation margin so not to overload the engine, to have as little an impact on the structure as possible — avoiding pitfalls such as propeller erosion — and to ensure the economic viability of the technologies we developed,” explains Seol, who led the project. This involved considering both wake improvement devices that would sit in front of the propeller and appendages placed behind the propeller, and centred on the idea that the developed technology would both be easily applied to new and existing vessels and be cost effective.
A preliminary study on representative techniques to improve propeller cavitation characteristics was conducted early on in the institution’s research, but in order to develop a device that satisfied both cavitation characteristics and propulsion efficiency the design team started to study the design of vortex generators and propeller caps, which it believed could be installed easily and cheaply.
Following three years of research, two types of ESD — an energy saving hybrid cap called K-CAP and an energy saving vortex generator called K-VG (KRISO Vortex Generator) — have emerged from the work conducted.
K-CAP is a departure from existing types of propeller caps. Generally, in the case of convergent type caps, the rotational flow generated from the propeller blades is gathered and strong rotational flow is generated behind the cap. In the case of divergent caps, the rotational flow caused is scattered, weakening hub vortex flow, but also decreasing propulsion efficiency — resistance is increased due to the shape of the cap.
K-CAP, however, takes advantage of both convergent and divergent shapes, simultaneously minimising cap resistance and weakening hub vortex flow. It effectively works as a new hybrid propeller cap that can control propeller hub vortex cavitation, and in doing so, improves propulsion efficiency, according to Seol.
In addition, an elaborately designed fin was created and paired with the cap in order to maximise propulsion efficiency and recover rotational flow energy that would otherwise be lost. “The K-CAP is considered to be effective for ships with large propeller loads due to the characteristics of recovering the rotational flow behind a propeller. Therefore, high speed containerships, passenger ships, LNG carriers and warships would be suitable adopters,” notes Seol.
“The K-CAP showed a 1-2% improvement in propulsion efficiency in the model test. Considering the model full-scale ship performance correlation study of existing propeller caps and the results of an International Towing Tank Conference (ITTC) study, the developed K-CAP is expected to improve the propulsion efficiency by more than 3% for full-scale ships.”
The second technology, the K-VG, acts as a wake improvement device, improving propulsion performance by 2-5% and reducing hull pressure fluctuation by 30-40%, according to KRISO studies of the technology for VLCC and tanker vessel types.
It attaches to the hull in front of the propeller, generating a strong vortex that delays the flow separation at the stern region of the ship, reducing
Vortex generators have been shown to cause propulsion efficiency losses in some cases, explains Seol, so KRISO borrowed design techniques used in the aero and auto industries where vortex generators are used as high lift and fuel-saving devices. The device subsequently accelerates the slow flow into the ship propeller, improving cavitation characteristics for the vessel. As a result, propeller inflow and thus the wake are improved. The ship’s noise and hull characteristics are also improved and the propeller erosion phenomenon
Seol emphasises that K-VG, unlike other ESDs, has the ability to improve propeller cavitation behaviour and propulsion performance at the same time, providing performance and eco-friendly credentials through the diminution of underwater noise radiation.
During the early stages of research and development, inconsistency with model test results occurred due to incomplete numerical simulation for ESDs of this design technique, according to Seol. Consequently, in order to mitigate this problem, various verification studies were conducted during the development of the K-VG and K-CAP. This involved analysing the correlation of physical phenomena that might occur in numerical simulation and model tests.
An ongoing design challenge lies in striking a functional compromise. Depending on the type of vessel to deploy the technologies, there can sometimes be a conflict between the two objectives laid out above (to improve propulsion and improve propeller cavitation characteristics) when arriving at the optimum K-VG and K-CAP design for the vessel in question. As a result, the target is set according to the performance of the ship and the demand of the customer, says Seol.
Looking forward, Seol reveals: “We are currently working on a number of full scale application projects in cooperation with shipyards and shipping companies…[and] in the near future we will verify the performance and effectiveness of the ESD’s through a full scale test.”
Research is already underway to address the impact of propeller erosion by the developed ESDs, and, on the basis of these studies, it is planned that these advanced ESDs will be applied to ships which have propeller erosion or vibration problems due to propeller cavitation, concludes Seol.