Reblading propellers can pay dividends
15 February 2006
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| New propeller blades fitted to
Stena |
Fitting old controllable pitch propeller hubs with the latest design of blade can cut fuel consumption and provide other benefits.
Ships typically have a long service life, during which the operating conditions may change. At the same time propeller design moves forward. Even if the propeller was good for its time, after a few years it may be worth considering fitting new blades which profit from improved design and manufacturing techniques and take account of changes in vessel operation.
A recent Rolls-Royce success story in this type of upgrading is the propeller blade change on Stena Germanica. Together with a sister ship. Stena Scandinavica, this overnight ferry has operated between Gothenburg in Sweden and Kiel in Germany for twenty years. The original propellers were from another supplier and at intervals there had been discussions as to whether reblading would cut fuel consumption. However, only a 2 per cent or 3 per cent saving was thought to be realistically attainable. Then a detailed analysis of the situation carried out by Stena and Rolls-Royce indicated that much greater fuel savings – in the 8 per cent region – should be attainable, although there might perhaps be some increase in vibration. If this efficiency improvement were true then it was clearly an attractive investment.
To verify the calculations, Stena commissioned model tests, while Rolls-Royce accepted a penalty clause if tests showed that the expectations were unfounded.
The model tests confirmed the calculations, so Stena ordered a set of Kamewa propeller blades for Stena Germanica and also for the sister ship Stena Scandinavica. The upgrading of Stena Germanica has been extremely successful. The increase in fuel efficiency has turned out to be about 10 per cent, a massive improvement. In practice the ship can maintain its schedule using only one engine per side instead of two, the additional advantages to the customer are that the level of redundancy is increased and maintenance costs are cut. Measurement showed that the vibration level has not, in fact, increased but its character has changed, not necessarily for the worse.
Stena Scandinavica is currently undergoing the same propeller upgrading during its ordinary drydocking.
New propeller blades have also improved the performance of the double ended car ferry Rauma which operates on the west coast of Norway. To keeps its fixed schedule it operates at different speeds, depending on the volume of traffic and thus the time spent loading and discharging at each end of the route. The propulsion system includes an azimuth thruster in each end of the ferry which operate at constant revs, hence power and thrust control is by pitch adjustments only. During the crossing, the ferry is propelled only by the aft unit while the forward is inactive, the blades being adjusted to feathering position. When the ferry goes into slow speed manoeuvring mode both propellers turn at full speed, and at the link span at nominally zero pitch. An analysis of all the factors showed that fitting new blades could be worthwhile from two points of view. One, to improve the efficiency in transit. Two, to reduce the losses in the zero pitch condition.
Measurements and fuel consumption figures after conversion show that with one thruster operating at zero pitch and full revs at the terminal, the new blades required only 320kW instead of about 500kW. In the transit mode Rauma’s speed increased by about a knot, and for the same operating profile as pre-conversion the fuel saving would amount to about NOK 0.5 million per year.
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| Norwegian passenger/vehicle ferry Rauma |
Rolls-Royce carries out many other reblading operations on old propellers. The two cases described above have been highlighted because the operating conditions before and after reblading were essentially the same. In other cases the new blades may reflect major changes in the way in which a vessel is operated, so that propellers that might originally have been satisfactory are no longer operating at their design point. This was the case with the seismic vessel Seisranger. Seismic vessels normally have propellers designed for good towing power at low speed (typically around 5 knots) combined with very low levels of noise transmitted to the water or the hull. When Seisranger was converted for diving support operations involving dynamic positioning mode, the propellers spent long periods turning at full revs powered by constant speed electric motors, but zero pitch. Since the angle at which the water flow met the blades was no longer as designed, severe erosion of the pressure side was experienced, just behind the leading edge. Welded repairs did not survive long. Rolls-Royce has therefore modified the existing blades by cutting back the leading edges slightly and machining to a larger nose radius and different profile in the leading edge region At the same time some other blade tip geometry deviations were corrected. Reports from the ship indicate a speed increase of about one knot, and the former vibration at about 50 per cent pitch has been eliminated.
Fishing vessels can often profit from reblading or blade reprofiling after long service. A case illustrating this is the trawler Northern Eagle, which has twin engines geared to a single propeller turning in a nozzle. Following an engine upgrade some years ago the vessel has a total of 4,780kW available, though when trawling less power is used and the typical trawling speed is in the 4.5knot region. The nozzle is well tucked up to the hull, giving large wake field variations around the propeller disc. Heavy propeller erosion on the suction side was a problem, and repeated repairs had made the blade geometry worse and aggravated the problem. Rolls-Royce designed and made new blades and initial reports are of major improvement.
Ships typically have an economic life of 20-25 years, though some go on for much longer. Propeller designs were not as advanced 20 years ago as they are now, while machining and polishing techniques have improved radically, enabling subtle shapes to be made accurately to design. Old propellers are also likely to have been repaired and repolished from time to time, leading to an increase in waviness and departures from the design profile. As Figure 1 indicates, a variation of one or two per cent of thickness from the design shape can greatly increase the risk of cavitation. If pressures can be kept out of the cavitation range, efficiency can be increased, and this is often sufficient reason for investing in new blades. Vibration levels can usually be substantially reduced at the same time.
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| Small deviations from the designed
profile of a propeller blade in manufacture or service can reduce the cavitation margin |
Apart from improvements in open water and nozzle propellers themselves, Rolls-Royce has built up a body of knowledge and experience in the complex area of hull/propeller/rudder interaction. Using this knowledge and a combination of advanced design and testing techniques, existing vessels can be investigated to see if new propeller blades will pay a dividend in reducing fuel consumption and thus emissions, or cutting vibration levels, or both.