Large Area Propeller - STREAMLINE project
In 2012-2013 SSPA carried out a number of model tests in the EU project STREAMLINE, using a new concept called ‘Large Area Propeller’ (LAP). Compared with conventional propeller designs, the LAP is placed behind the transom. This means that the size of the LAP propeller is not limited by the clearance to the hull. This will enable new propeller designs making it possible to achieve very high levels of efficiency, but runs the risk of propeller ventilation.
High levels of efficiency using large propellers
An 8,000 DWT tanker was used for this study. The main idea with the LAP concept was of course to increase the total efficiency of the ship, and the towing tank tests carried out at SSPA confirmed that this is the case. Compared with the reference vessel that used a conventional propeller location and design, the LAP concepts showed a power reduction from 6% at low speed (10 knots) up to 17% at the highest test speed of 16 knots. Of course, the increase in total efficiency mainly comes from an increase in propeller efficiency caused by the much larger LAP. The possible drawbacks using such a large propeller is that the propeller tips will fall below the base line, with a greater risk of damage in shallow waters and a greater risk of propeller ventilation.
Ventilation a possible problem
Even though propeller to hull clearance is no longer an issue, when placing the propeller aft of the transom, the distance between propeller and water surface will still be an important design aspect. Putting the propeller too close to the waterline can result in problems with ventilation and in the most extreme cases, propeller blades will even come out of the water. The tests in SSPA’s towing tank have however shown that ventilation will not be a problem in calm water. It seems that the wave pattern behind the ship will have a positive effect, where the propeller is located under a wave peak.
Extensive tests have also been carried out in SSPA’s towing tank and at the Maritime Dynamics Laboratory, to examine what will happen when wind waves are present. Propellers will experience tougher working environments at sea. The wave motion, as well as the motion of the ship will vary the flow to the propeller. The flow variation will lead to a corresponding variation in thrust and torque. From the tests, it seems that the LAP will have a higher torque variation, compared to the reference vessel using a conventional propeller location and design. This will be a structural engineering challenge with the LAP concept.
Almost cavitation free
Tests in SSPA’s cavitation tunnel revealed the expected, namely very little cavitation. At design condition only a very slight tip vortex could be detected. Still, however, pressure pulses could be detected at the transom of the ship. The pulses are created by the non-cavitation pressure variations, coming from the propeller. Showing very little cavitation, this solution is to be regarded as very environmentally-friendly, as well as having no risk to either propeller or rudder erosion damage.
Will the market agree?
We have shown that there is good potential for saving fuel using the LAP concept. We have also investigated and quantified some of the possible risks and engineering challenges. The initial cost for this concept will inevitably be higher, due to its greater size and torque. It will be very interesting to see if the market agrees to this greater initial cost, to achieve significant fuel reduction of around 15%.
The first LAP concept design. This design includes a conventional sized rudder. For the final design the rudder was prolonged to keep a straight course without using rudder angles. The structure holding the rudder is preliminary, for functionality in the model tests.
Propeller blade out of the water at seakeeping tests in the Maritime Dynamics Laboratory with 5 m significant wave height.
Only a very slight tip vortex could be detected during the cavitation test.
Tough environment for the propeller in seakeeping tests during ventilation.