Feathered propellers for flexibility
Tobias Huuva of Berg Propulsion explains how the flexibility and efficiency of twin- and multi-screw ships can be enhanced by feathering propellers.
Many ships are today equipped with a single screw propulsion system, but with stronger demand for increased efficiency, flexibility and redundancy, designers and owners are looking towards multi screw propulsion systems. With a multi-, or usually, twin screw propulsion system the propulsion efficiency can improve, particularly with optimised hull design featuring aft body skegs. The twin skeg is used to direct the boundary layer from the hull into the propeller.
On a vessel with a single propeller the speed can only be lowered by decreasing the power from the engine. For a FP propeller the propeller speed is reduced, or for a CP propeller this is combined with lowering the pitch. This can result in both engine and propeller working in an often inefficient off-design condition. With a twin screw propulsion system the speed of the vessel can be lowered by closing one propulsion line and letting the remaining propeller drive the ship. The unused propeller can either be locked, or allowed to rotate freely.
If the propeller shaft is locked, a brake is needed on the shaft line while if the shaft rotates freely a clutch is needed to disconnect the propeller from the engine. If the shaft line cannot be locked or clutched out it is not possible to cut out one propeller so ship speed can only be lowered by reducing engine speed and letting both propellers drive the ship. Both FP and CP propellers can be either locked or clutched out but the CP propeller has an advantage since the pitch can be adjusted for lowest resistance.
Most CP propellers have a pitch range from full ahead at about 30°, which is somewhat higher than the normal pitch of an FP propeller, to full astern at about -25°. A third alternative exists if the CP hub has the capability to feather the blades, i.e. the blades can be set at 90° pitch, or parallel to the flow. Turning off a CP-propeller, with or without feathering ability, in a twin screw setup, the highest resistance would be experienced when the blades are in zero pitch position, because then the projected area is the largest. However, finding the lowest resistance involved more investigation.
A sample vessel was investigated: a 100m long tanker with 3.6m propeller driven by a 3,200 kW plant. The ship was investigated using either a single screw propulsion system with a single skeg aft-body, or a twin screw propulsion system with twin skeg aft body. The power was, for simplicity, divided between two engines of 1,600 kW each.
This type of ship would typically have a maximum speed of 13 knots at MCR and it can be calculated that it would travel at around 10.9 knots at half power (1,600 kW, one screw). The running propeller is assumed not to influence the simulated stopped propeller.
The investigations involved complex mathematics, described fully in the presentation on this subject given at the Motorship Propulsion and Emissions Conference in Hamburg, April 2010. The pitch of the blades was investigated in two conditions, feathered and at the design pitch, corresponding to 80% of full ahead pitch for a non-feathered hub. Two computations were made with the propeller locked, one for each pitch setting and one computation performed with the propeller rotating and the pitch in design condition. The rotating case is referred to as ‘self milling’ when the momentum from the water on the propeller levels the losses in the shaft line and the gear box, and as ‘driven milling’when the thrust from the propeller is zero.
A basic principle is that propeller efficiency increases if the diameter is increased and the rate of revolution is decreased. The same principle applies when the diameter of the propeller remains constant and power is decreased, as is the case when the power is split between two propellers. Splitting the power over two propellers will also imply that the loading on the propeller will be lower and consequently the blade area can be reduced without increasing the risk of cavitation. Another advantage is that manoeuvrability is improved with a twin screw setup. With a twin, or multi, screw installation speed can be varied by using one or several propulsion units, all working close to optimum performance.
A typical power demand curve shows that close to maximum speed a ship requires considerably more power to increase its speed, whereas a corresponding speed increase at lower velocities can be performed with only a minor cost in extra power. Consequently, lowering the power by 50% from full power will only decrease speed by about 20%.
Considering propulsion efficiency and overall efficiency, calculations showed that the sample ship will be about 4% more efficient in design condition with a twin screw setup compared to a single screw setup.
When the speed is reduced the power from the engines needs to be reduced correspondingly, assuming that weather and other conditions remain constant. Reducing engine power can also reduce engine efficiency and bring about a relative increase in exhaust gas emissions. A ship engine usually has the design point at around 85% MCR.
As power is reduced, engine efficiency changes. For a single screw vessel this is the only way to lower the speed of the vessel. In a twin screw setup the speed can be lowered by keeping one engine at full power and decreasing the power from the other engine. For the sample ship, speed will be about 10.9 knots instead of 13 knots at a 50% power decrease. The propulsive efficiency can be estimated when both pitch and engine speed is varied to find highest possible efficiency, and also when the engine is working on fixed rpm. Thrust deduction is also considered when a twin screw ship is driven by a single propeller, so the deduction will only affect one half of the hull; again the conference paper gives the calculations.
Resistance was calculated for a locked propeller in feathered position as well as in design position, corresponding to 80% of full pitch. The self milling and driven milling conditions were considered to determine efficiency loss and resistance. The efficiency loss due to the propeller not in use was found to vary between 2% and 16%, where the feathered propeller as expected has a much lower resistance.
A need to achieve higher flexibility, combined with redundancy and high efficiency is likely to increase interest in multi-screw propulsion. Multiple screw vessels can fit CP propellers with a feathering option, allowing variable ship speeds while maintaining high efficiency.
A twin screw vessel can have over 10% higher propulsive efficiency in comparison with a single screw ship. If, however, the propellers is fitted to a single skeg hull this gain will be lost in decreased hull efficiency. Instead a twin skeg hull should be employed where the hull boundary layer is directed into the both propellers and hull efficiency is maintained. Using a feathering propeller it is possible to vary the speed of twin screw vessels by using a single propeller to power the ship at lower speeds and feathering the other propeller. There are several options for the unused propeller, but feathering is believed to offer the safest and most efficient solution.
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