Experts disagree on propeller issue

If they were sure they could, they probably would. Ordering of very large containerships has resumed this year and talk has resurfaced of the potential to build vessels with unprecedented carrying capacities of 10,000 TEU and above. Terminal infrastructure is, in the most part, sufficient to accommodate and handle much larger containerships. And demand for container capacity it seems is buoyant, with China ? a principal driver of container trades ? having posted an amazing 33 per cent rise in exports in the first six months of this year. But as containership size increases speed cannot decrease, otherwise the benefits of size diminish. This, combined with a reluctance to switch to unproven propulsion solutions, means that issues of power and putting that power into the water on a single-screw are holding back orders for even bigger ships.

Today?s largest ships can carry up to 8,500 TEU (see box on pg 23). To achieve what has become accepted as a standard service speed for containerships, 25 knots, these giants use a 12-cylinder, 68.5MW main engine operating at 100-105 rev/min, which directly drives a six-bladed fixed-pitch propeller weighing about 100-110t with a diameter of about 9m. A ship with much more capacity than 8,500 TEU requires both more power and a heavier, wider propeller, to achieve 25 knots. This takes it into untried territory as far as engines are concerned and starts stretching the boundaries of what a propeller foundry can produce in a single cast.

There is always the opportunity of switching to a two-engine, two-screw solution to enable greater capacity, but this increases capital cost, operating cost and the number of running parts ? in other words it is an opportunity that operators would rather not take if a workable solution for greater capacity on a single screw is available.

Both Wärtsilä, through its Sulzer range, and MAN B&W are able to offer more power through adding more cylinders to their current large bore engine models. It is a solution that engine builders are amenable to also, according to Germanischer Lloyd. Yet a longer engine introduces some critical construction and design issues, as over-stiffening of the aft end needs to be avoided in order to prevent crankshaft misalignment and its inherent dangers. Large, fast ships with lots of power generate significant and unpredictable hydrodynamic forces on bearings, particularly when turning, which are difficult to envisage in the design. Yet, it is widely felt that these problems are not insurmountable.

The real issue is whether operators want more moving parts, therefore introducing the potential for less reliability, to achieve the desired power. That more power is required is not in dispute; it is a case of how to generate that power with minimal risk to reliability. Similarly, it is not in dispute that bigger propellers are needed. What is hotly disputed by ship designers and propeller manufacturers is which is the more critical obstacle to development ? power attainment or propeller performance?

Propellers for 10,000 TEU+

“The issue is what you put at the end of that shaft to deliver you that power from the engine,” believes Vladimir Kouzousek, a senior principal surveyor and head of engineering systems in Lloyd?s Register?s marine support group.

Dr Graham Patience, managing director of propeller design firm Stone Manganese Marine, disagrees: “You can always get over the [size] problem with propellers,” he says turning the argument back on the engine makers.

Patience points out that without investment, there are already at least three foundries in the world – Hyundai Heavy Industries in Korea, Stone Manganese Marine in the UK and Wärtsilä Propulsion (ex-Lips) in the Netherlands – with capability to produce single-cast, six-bladed, fixed-pitch propellers up to about 125t. At a stretch, with some investment, this could increase to 150t, he says. Ongoing investments in China mean this list will soon expand. Given that the largest containership propeller cast to date weighs about 105t, there remains margin for increase.

Even beyond 125-150t, there is the possibility to revert to past techniques where the hub and each blade are cast individually and bolted together. This technique ? used exclusively up to the 1930s ? will result in a loss in efficiency, but with present facilities it will enable manufacture of a six-bladed propeller up to about 350t, says Patience (although he admits that above 200t hydrodynamics become

questionable).

A foundry with a larger single-cast capacity, while logical, would be a questionable investment he thinks. Bigger casting pits and increased furnace capacity would be required ? a 100t propeller requires 140t of molten metal; already stretching furnaces to the extreme and requiring a huge amount of power (not to mention the ability to lift and pour the molten metal into the cast before it sets and the risk of failed casts). So for now, capacity of foundries to produce a single-cast propeller is likely to remain at 125-150t.

Greater weight is in large part a function of a greater propeller diameter. The 9.1m diameter propellers used on today?s biggest containerships are dimensioned at 70 per cent of the ships? design draught (13m), which is a commonly assumed rule-of-thumb maximum for acceptable cavitation performance in terms of noise and vibration. For a larger ship to achieve the same speed it would require more power, which would need a heavier propeller, which in turn would have a larger diameter.

This brings in the issue of available under keel clearance at container terminals. A Lloyd?s Register/Ocean Shipping Consultants study has identified 25 terminals that will offer a draught of at least 15m by the end of this year ? three in northern Europe, 11 in Asia, four on North America?s Pacific coast and seven wayports. The study concludes that water depth will not be a problem.

A design draught of 13m corresponds to a scantling draught of 14.5m. According to Lloyd?s Register, this means the 9.1m diameter propeller is the largest possible with 0.5m under-keel clearance and with acceptable cavitation performance. Yet many of the 25 terminals identified as having a draught of at least 15m, have deeper draughts, by 1m or more (or plans in place to dredge should customers demand it; they already have crane capacity to work three or more rows beyond the 17 rows stacked on 8,000 TEU ships).

A 16m water-depth enables a container ship to have a design draught of about 14m (maintaining 0.5m under-keel clearance on scantling draught) and load capacity of about 12,500 TEU. Using the 70 per cent rule-of-thumb, the maximum propeller diameter for acceptable cavitation performance on such a ship increases to 9.8m (weighing about 125-130t). For this to drive the ship at 25 knots it will require over 90MW. This is 16- or 17-cylinder territory on Sulzer RT96 and B&W K98 engines. Alternatively, a bigger bore, such as the proposed B&W K108 engine, could produce 90MW with fewer cylinders, but this has yet to get beyond the drawing board and further development of this, an even larger engine or a competing Sulzer product, will require big investment – something engine builders are less willing to take on than building longer engines.

Lloyd?s Register has argued that a propeller putting over 90MW into the water will have a blade area ratio (area of the blades compared to total swept area of the propeller) greater than one, which makes it more sensitive to cavitation. It says when powering is at this level the only practical solution is to increase the propeller diameter, which increases efficiency but takes the propeller weight into realms beyond foundries? current single-cast capabilities (it would also take the diameter above 70 per cent of the design draught). Hence Lloyd?s Register takes the view that 9,000 TEU is about the largest capacity possible on a single screw ship capable of 25 knots, with acceptable levels of cavitation.

Patience disagrees: “There is no magic criterion above which cavitation is any the more sensitive or susceptible.” He accepts that higher blade area ratios will reflect a higher cavitation loading but argues that sensitivity is not a direct function of blade area ratio. “The fact that the blade area ratio exceeds unity is quite irrelevant from a hydrodynamic point of view,” he says. He believes that a 9.8m diameter propeller, weighing 125t, delivering over 90MW to the water, for speeds of 25 knots with acceptable cavitation, is a realistic possibility.

Germanischer Lloyd is inclined to agree, provided draught can be increased above 14m. “If we go to deeper draught for the very big ships, like 15 or 16m, propellers with larger diameter could be built thus making a 12,000 TEU single screw containership a possibility,” it says.

It adds that such a vessel would need extensive analyses, including a forced vibration analysis induced by the propeller as well as internal engine forces transmitted to the hull as a result of the flexibility of employing very large engines. Patience warns that the higher blade area ratio can increase propeller manufacturing difficulties due to the blades overlapping, especially if CNC equipment is to be used. Yet despite these challenges, both agree that in principle 10,000 TEU and above is possible on a single screw ship capable of 25 knots.

Efficiency from single screw

If Lloyd?s Register?s analysis is correct and shipowners are determined to stick with a single-screw solution, there are ways of increasing capacity, although they go against the simplistic machinery principle that container ship operators such as P&O Nedlloyd and Hapag Lloyd abide by.

IHI Marine United in Japan, for example, has proposed adopting a contra-rotating propeller solution, which it reckons will produce enough propeller efficiency (about 12-15 per cent power saving) to enable the building of a 10,260 TEU ship, which achieves 25 knots using proven 68.5MW engine technology on a design draught of 13.5m.

IHI?s solution is based on technology developed through accumulated experience of a CRP installation on Idemitsu Tanker Co,?s 258,079 dwt VLCC Okinoshima Maru (built 1993) and the 37,000 dwt bulk carrier Juno (CRP retrofitted in 1989). Idemitsu has a second VLCC under construction at IHI?s Kure shipyard, due for delivery in July 2004, which will have a CRP system fitted. This repeat order offers strong evidence that the claimed benefits of CRP (in a VLCC application at least) can be realised without introducing

unreliability.

On a similar theme, pod designer ABB and Korean shipbuilder Samsung Heavy Industries have investigated mounting an azimuthing pulling pod unit behind the main propeller. The propeller of the pod will rotate in the opposite direction to the main propeller (contra-rotating pod). Using proven propeller and engine technology the ship will have a capacity of about 12,000 TEU, with power for the pod coming from an upsized generating set installation.

The problem for the solution is that it is untried ? the first ship applying it, a ferry for Shin Nihonkai Ferries, is due for delivery in July next year. There also remains deep scepticism about the reliability of pods, based on problems experienced with running-element bearings. However, much time and energy is being directed at this and there is confidence that these problems will shortly be solved. As a result, numerous experts view the solution as a realistic possibility.

Kouzousek, for example: “It is only a matter of time before someone installs a podded drive on a containership,” he says. And this would not necessarily just be through a CRP pod application.

Current potential

Kouzousek talks of the possibility of using combined fixed pod, rotating pod solutions, similar to those installed on the cruise ships Voyager of the Seas (one fixed, two rotating pods) and Queen Mary 2 (two fixed, two rotating pods). If 90MW plus is needed to carry 12,500 TEU, a fixed pod, rotating pod solution can put this power into the water. Kouzousek points out that podded drives on containerships would be subjected to less demanding service conditions than podded drives on passenger liners are. If pod reliability can be sorted out, given that containership operators are gaining experience of ever-larger electrical generating plants, an electric drive, pod solution is not entirely inconceivable.

The size of the generating plant could present a problem, but if trials that GE is undertaking burning heavy fuels in gas turbines (see page 4) prove successful, the gas turbine solution in combination with pods seems more attractive.

However, neat as all this technology is, it is both costly and involves the introduction of even more complexity and even more running parts than a twin slow-speed engine, twin-screw solution. The bottom line is that single-engine, single-screw is by far and away the preferred option.

It is reasonable to expect that only when all angles have been exhausted in the search for a single propeller solution with acceptable cavitation for 9,000 TEU+ ships will there be any general move towards different propulsion arrangements for powering very large container vessels. While any doubt remains there is little incentive for low-speed engine makers to push ahead with design development of models that produce more power on fewer cylinders. Foundry capacity to make bigger propellers exists. The performance of these arguably corresponds to potential for an extra 3,500 TEU capacity on a single-engine, single-screw ship. The experts need to satisfactorily resolve the propeller debate once and for all and resolve it quickly.

Load capacity threat

challenged

Korean and Japanese delegations to the IMO?s 46th SLF (Sub-Committee on Stability and Load Lines and on Fishing Vessels? Safety) meeting in London this September will table a joint agenda to block passage of revised intact stability proposals. The Korean government?s Ministry of Maritime Affairs and Fisheries (MOMAF) is concerned passage of the proposals will result in reduced loading capacity on container ships.

MOMAF fears that revisions to IMO Resolution A749, in which the intact stability code for containerships is given in Chapter 4.9, may become compulsory as a result of decisions made at the SLF meeting, rather than them being mere recommendations as things stand now. An IMO spokesman says the SLF has agreed to form a correspondence group on this matter, so that work can continue between meetings, but no decision is expected for at least a year so it is too early for it to comment further.

MOMAF says tests jointly made by the Korean Register of Shipping (KRS) and Osaka University in Japan, which applied the A749, 4.9 standard to 12 container ships of 400 TEU to 8,000 TEU capacity, showed that loading is reduced by 10 percent, while the probability of capsize in high seas is not reduced. KRS says existing stability codes, A167 and A562, suffice.