Debating hull efficiency

Posted on:

One of the most interesting discussions at the ShipInsight Conference (February 2020) was that devoted to hull efficiency. This is a topic that has been a perennial matter of debate and argument in shipping for centuries.

It has taken on a lot more relevance in recent years both because of the EEDI which requires significant improvements in ship efficiency at regular intervals and also because the IMO has moved the debate on invasive species from ballast water to biofouling.

Today hull efficiency as initially measured by the design of the ship is affected in practice more by the condition of the coating than anything else. A ship may be designed to sail with minimal resistance and its performance can to some extent be measured at design but once in operation it is the accumulation of biofouling that can within a very short time make all of the designer’s calculations meaningless.

Stein Kjølberg – Global Concept Director, Jotun made the point that the path to improving hull efficiency through coating choice is not the easiest to travel. Despite the new ISO 19030 Standard, the choice of coating will always be dictated by ship type and service profile and by money – making the point that any money spent on installing a scrubber cannot be spent on choosing the best coating for the task. Kjølberg also mentioned the benefits of performance based monitoring and working with cleaning companies to ensure optimal performance.

We now know that Kjølberg was holding something back and would very likely have loved to reveal details of a development that Jotun had planned a major event to announce just days after the ShipInsight conference. By now the shipping world will know that the development Kjølberg couldn’t speak about was the new Jotun Hull Skating Solution which involves the installation of an inspection and cleaning robot when applying its latest coatings. More details of that system can be found at https://shipinsight.com/articles/jotuns-hullskater-to-revolutionise-hull-cleaning

From a ship manager’s point of view, Nick Topham – Managing Director, Bernhard Schulte Shipmanagement, although not totally dismissive of performance management tools made the point that there has been one calculation – propeller slip – that has historically been able to determine the deterioration in performance over time. Topham also admitted to being a little confused over a modern obsession with charterers’ demands for propeller cleaning asking the question how does one split out hull efficiency from propeller efficiency to any meaningful degree.

On the question of selecting a suitable coating, Topham made the point that the service a ship is employed on can vary tremendously over the life of a coating so every choice is some sort of compromise. So too is when hull cleaning will be most effective which begs the question of why some charter parties specify fixed intervals.

Topham’s viewpoint is one that is shared by more than a few traditionalists and his point that including set intervals in charter parties can mean that on occasions money may well be wasted cleaning a hull which has suffered little fouling while on other occasions an earlier clean may pay dividends. Perhaps there are better ways to predict cleaning intervals that combine the old and the new and by making comparisons of predictions and actual condition may pay dividends.

Simon Doran – Managing Director, HullWiper mentioned the problem of squaring the circle around hull cleaning with it becoming both more important in meeting ship efficiency targets but all too often prohibited by ports because of the potential to release invasive species. Fortunately some of the new systems recover all removed growth and hopefully will become more acceptable. Doran also said that manual cleaning as has been practised for many years, can help keep hulls clean but often, poor practices actually worsen the situation with damage to coatings allowing for regrowth to occur in a very short space of time.

These are views that probably resonate with many shipowners and having spent large sums on premium coatings are wary of the damage that can be inflicted by cleaning services. Hopefully the more modern improved cleaning systems will help allay some of those fears as experience with them grows.

On the question of how frequently should hull clean and propeller polishing be carried out in a five year period between dockings for optimum hull efficiency and how effective in hull efficiency gains is propeller polishing Vs hull clean? The general view was that hull cleaning is more effective than propeller cleaning and generally proactive cleaning pays better dividends than waiting until a hull and indeed the propeller are both heavily fouled. One wonders how that debate may have changed direction if details of systems such as that revealed by Jotun in mid-March had been introduced at the time.

Inno Gatin – CFD developer and consulting engineer, Wikki approaches the matter of efficiency not so much from hull cleaning but design using CFD as a new tool that was not previously available to ship designers. CFD gives a much better indication of efficiency but can also identify areas of the hull that are more problematic than others. Thus CFD can be used for optimising maintenance.

When the discussion opened out to audience questions one of the most popular was How do current coating systems compare with those of10 years ago? How much further can they go? Kjølberg answered by reminding that TBT was probably still better at protecting against fouling than many modern products he also felt that self-polishing systems have probably gone as far as they can go. Even so, with expected new regulation R&D continues at all coatings makers to attempt to produce what is needed.

Answering the same question, Topham pointed out that over the last ten years the chartering model has switched and many vessels are now switching operating profiles much more frequently making choice of the optimum coating more difficult. Doran said that from hull cleaning point of view silicon coatings are more popular and getting better allowing longer periods between cleaning.

Moving to the design side of hulls and the improved efficiency offered here, another questioner asked if CFD will put test tanks out of business? According to Gatin probably not. The two complement each other and the tank test provides the base line against which actual operation is measured. However, CFD allows designs to be checked and adapted without the need for a towing tank which is not available to all designers.

Innovative technology also raised its head in this discussion with a question asking are hull air lubrication systems potentially further enhanced or affected by the choice of hull coating system? The panel discussion covered the point that ALS is still uncommon in commercial vessels but was found on many military ships of much older vintage. Those ships still needed hull cleaning even if the ALS was used frequently however the ALS on military ships was not installed for efficiency but to mask the sound signal of the vessel.

Chris Millman, an audience member from Carnival Corporation which does have ALS on three ships did suggest that in his company’s experience there is a net benefit around 5% when ships operate at 18kt but this disappears below speeds of around 12kt. The ALS has not shown to have any effect on fouling.

Alert:

We're expanding our content. Please take 60 seconds to complete an anonymous survey.