Hydrodynamics or pod integrity

That pods have suffered teething problems isn?t in dispute. What is causing the problems, though, and how to resolve them are still hotly debated.

Juha Hanhinen, a hydrodynamics specialist at Finnish engineering and design consultancy Deltamarin and a participant in Optipod ? the EU backed project aiming to develop design guidelines for podded ships ? believes that the pod problems experienced are a function of two factors, course stability and training.

He says that from a hydrodynamic standpoint, pod problems are not related to the ship and its aft form but to inherent features of the pod. These inherent features mean a higher steering angle is needed to maintain course than a conventional rudder requires. With a pod, steering is based on directing thrust (as with waterjets) whereas a rudder can create big lift at relatively small angles.

The greater steering angle required to maintain course with a pod means there is an increased non-uniformity of water inflow into the propeller, says Hanhinen. This causes cavitation leading the bearings to wear prematurely, he believes.

Steerable flaps

He suggests a solution could be to apply a new means of steering podded ships at higher speeds. This could take the form of steerable flaps integrated into the pod unit itself. An alternative hull design solution to the problem of course stability is to have a longer central skeg, he adds. Skegs in front of the propeller would simply heighten cavitation problems, he believes.

“Podded ships should probably be more course stable than what a designer would want a conventional ship to be,” says Hanhinen.

From a training perspective, Hanhinen says there are questions over how podded ships are treated in harbour manoeuvring situations. He explains the pods are designed for computerised operation in harbour with joystick control. This limits the pods? power consumption and angle.

Yet there has been a tendency by some captains to override the computer, says Hanhinen. The captains have put more power through the pods and set them at greater angles than they are optimally designed for. This, he says, may add to the premature wear of the bearings.

Bjorn Henriksen, a partner at Norwegian hull design firm SeaTrade, expresses a slightly different view. He says the attention focused on the manufacture and integrity of pod system components as the cause of recent problems is unwarranted. He believes those trying to resolve pod problems should focus on the underlying cause of the stresses that lead bearings and seals to fail.

Proven technology

Henriksen says the technology in the pod structure, including the bearings and seals, is all well-proven. The underlying cause of the pod problems currently being experienced, he explains, is not in the pod itself but hydrodynamics.

He explains that his thinking is based on the basic premise that the hydrodynamics of a single pulling pod are not the same as those of a propeller at the end of a shaft. With the latter conventional arrangement, water energy coming off the propeller merely drops to the bottom of the ocean.

However a pulling pod has a forward facing hub with a diameter less than that of the pod body. The speed of water increases as it leaves the propeller meaning more energy is required to get the water past the pod body. On shallow-draught flat-bottomed ships like cruise ships, this extra energy is transmitted into the plates. The plates work like a membrane sending pressure pulses through the construction and into the pod itself, he claims.

“Pods are getting a bad reputation because the shipyards haven?t done their homework,” says Henriksen. “I am quite convinced that this is a pressure pulses problem.”

His solution is the StarShip 2000 hull form, which features skegs forward of the pods to optimise the flow of water into the propeller. He says the hull form has been tested extensively at CTO Gdansk in Poland and has already drawn the interest of a number of ship owners. He adds that Uljanik Shipyard in Pula, Croatia has calculated that construction costs would be largely unaffected by building the skegs.