propellers
Is blade reduction effective for slow steaming?
While slow and super slow steaming improves per mile fuel consumption, continuous vessel operation at reduced speed must come with certain warnings writes Donald MacPherson*
T
he characteristics of the hull, propeller, and engine were all developed for a particular operating speed
and
duty profile. These original ‘design point’ characteristics turn out to be detrimental when operating at slower ‘off-design’ speeds. For example, the hull has attributes (such as the shape of the bulbous bow) that were intended for a particular speed, and they typically add unnecessary drag at slower speeds. The engine will be operating at lower RPM, resulting in higher specific fuel consumption and increased maintenance concerns. The propeller will typically have far too much pitch and blade area than is needed for the reduced thrust loading. The current solution to mitigate off-design inefficiencies is to look at new engines and propellers. This is certainly appropriate as the mission of the vessel has changed and a new ‘design point’ is prescribed. However, are there ways to recover any of the ‘lost efficiency’ of operating in an ‘off-design’ condition without the full expense of replacing the engines and propellers? Can a fairly dramatic modification to the propeller be worth considering? If a propeller replacement is being considered, the existing propeller has already been sacrificed for a new model. Blade reduction may be a viable option for those willing to accept a partial measure of the full improvement that comes with new propellers. However, blade reduction is not suitable for all vessels, for example the propeller must have an even number of blades. Fortunately, many ships considering slow and super slow steaming have propellers with six blades.
The general objectives of blade reduction will be twofold: to directly make the propeller better and to indirectly make the engine better. The specific objectives are to increase propeller efficiency (and thus reduce engine power) and to increase engine RPM so that it operates with a more favorable specific fuel consumption (SFC). For this study, a variant of an existing container
64 I Marine Propulsion I February/March 2012
ship design was scaled to these fictional properties, and a resistance prediction was conducted using NavCad software. A typical engine curve shape was fit to the example specifications. For propeller design, the PropElements and PropCad software were used to design a six-bladed propeller (representative of a contemporary container ship propeller) for the predicted thrust requirement at the original 25 knot condition. A corresponding three-bladed propeller (at 0.48 EAR) was ‘reduced’ from this original propeller – keeping all individual blade properties unchanged.
Propeller performance curves were prepared in PropElements, and KT/KQ data files were exported for both propellers. These were used in a NavCad propulsion analysis. The three-bladed propeller achieves our specific objectives. It has increased propeller efficiency by 1.4 per cent to 1.5 per cent (as indicated by the lower power), lowering fuel consumption by the same amount. It also increased engine RPM by 7.6 per cent to 7.7 per cent which provides an indirect improvement in fuel consumption. Variations in specific (per-power) fuel consumption are not only related to engine loading, but also to engine RPM (engine refit ‘kits’ are used to improve the SFC for reduced engine load). There are certain ranges of power and RPM at which the engine requires less fuel to generate the same power if it is run at a higher RPM. In the RPM and power range of interest for slow and super slow steaming, it would be expected that SFC would be beneficially reduced with an increase in RPM for the same power. A review of SFC figures for a comparable contemporary engine showed that there would be approximately 0.8 per cent lower SFC for the RPM increase demonstrated here. Therefore, it would be fair to say that the cumulative effect of blade reduction for this representative containership is approximately 2.2 per cent to 2.3 per cent fuel savings as compared to the original propeller. This result would be expected to be comparable for larger ships, as well. Of course, it is important to point out that this analysis was generic in nature to demonstrate a concept. Each vessel will have its own cost-reward equation and must be evaluated individually before considering blade reduction for implementation.
How does the change in blade count affect
cavitation and hydroacoustics? It could affect it greatly, but only if the ship were put back into high
*Donald MacPherson is technical director, HydroComp, Inc
www.mpropulsion.com
Donald MacPherson: “The current solution to mitigate off-design inefficiencies is to look at new engines and propellers”
speed service (which would not be advisable). It is the substantial reduction in thrust loading at the slow and super slow
speeds that allows the
propeller blade area to be cut in half. A hydroacoustic analysis was conducted with the NavCad software (using typical clearances and wake peak figures) to determine if expected changes to blade impulse pressure and tip vortex noise would be significant. In this case it is necessary to compare the levels for the original six-bladed propeller at original speed to those of the three-bladed propeller at slow-speed. All of the indicators were favorable. Higher
blade-pass frequencies are generally preferred, but the lower blade-pass frequency with three blades is mitigated by the reduced pressures. This study did not look at the effects of blade reduction on other vibratory excitation or response. For example, torsional and axial vibration analysis calculations should be part of any design review for blade reduction. The additional gains and operational benefits that might be found with blade reduction for slow and super slow steaming are not earth-shattering. However, the value of any improvement must consider the cost of that improvement. Propeller blade reduction may be an interesting low-cost, low-risk, low-reward option for some ships being considered for operation. MP
slow or super-slow steaming
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