New hydrofoil breaks 100% efficiency barrier of a regular propeller.

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About the author:

Dr. Stepan V. Lunin was graduated from Moscow Aviation Institute in 1986 as an engineer in manufacturing of helicopters with Master Degree. In 1989 completed second Master Degree in Applied Mathematics from Moscow State University. In 2000 received Ph.D. in Computer Science from Izhevsk Institute of Mechanics. Worked in airspace, automotive and marine industry on advanced gear drives, composite materials and propulsion systems.

In this article the author is proposing to apply a well-known aeronautical idea to a marine propeller.

Volvo Penta developed and successfully marketed counter rotating propellers on stern drives. The counter rotating propellers provide higher propulsion efficiency for two major reasons:

  1. The losses of efficiency from spinning of water is less because of the two propellers rotate in opposite directions.
  2. The extra increase of the propulsion efficiency is coming from the reduction of the hub diameter because the engine power is split between two gears making them smaller in diameter. The smaller size hub has lower hydrodynamic drag.

However the counter rotating propeller stern drive are more expensive in manufacturing and less reliable. For safety reasons it is much better to have two engines and two stern drives. Most of the recreational marine engines and stern drives are designed for life of 400 hours or less. It means that the engine or the stern drive has a high probability to fail when the boat if out way from shore. Two smaller engines are not only more reliable but also less expensive than a single big engine.

The current article is presenting an alternative to counter rotating propeller stern drive. The presented method allows to increase the propulsion efficiency of two single propellers, increase speed and reduce fuel consumption much more than the counter rotating propellers.

The method can be applied to new and used boats, inboards, outboards and stern drive boats. The main condition is to have two or more single rotating propellers. The optimal and the simplest option would be two single inboards.

The combined propulsion efficiency increase may be 100% and higher. It sounds impossible if to consider the efficiency of a regular propeller of 70%, so adding another 100% would kick the total number to 140%, which is violating the law of physics. The explanation of the braking increase of the efficiency is that the combined efficiency is calculated as total saving of cost of fuel required to go for the same distance with the same speed. Or the reduction of the total propeller shaft power to go on the same speed. Or increase of the maximum speed of the boat. For example 100% or doubling of the efficiency means that the modification of the propulsion system would cut the fuel consumption and required engine power for a half or may increase the maximum speed with the same power for 35%. The presented modification does not brake the law of physics, but it reduces the combined drag of the boat so the boat can go much faster and with lower power. The real efficiency of the propellers will increase higher than on a comparable double counter rotating propeller but it will stay below 100%. But the boat owner will see much higher saving and increase of boat performance. The boat owner will notice much bigger reduction in operating cost and more gain of speed than the increase of the efficiency of each individual propeller.

This efficiency improvement method is based on NASA patent for high efficiency propulsion system for airplanes.

 

NASA proposed to install propellers on the tips of the wing and make them rotate against the circulation generated by the lifting surface of the wing. Both the propellers and the wing gain efficiency. The each single propeller works as a counter rotating propeller and each has increased propulsion efficiency. The wing has lower lift losses because of the air circulation on the tips is reduced by the propellers rotating in opposite direction.

The marine application of this patent would have higher increase of the efficiency because the wing/hydrofoil can be installed in front of the propellers. The airplane wing generates the most intensive circulation on some distance behind the wings. The installation of the propellers in that area is not practical on airplanes. But on the boats it would be easy to do. The lifting hydrofoil would have to be installed around the center of the gravity and the propellers are normally behind on a distance from that point.

The most increase of speed and fuel saving will be from the reduction of the wet area of the boat because the lifting force from the hydrofoil will partially displace the hull from water reducing the wet hull area. The propulsion efficiency of each single propeller will also increased because the propellers will be in counter circulating flow behind the wing tips.

If designed correctly the hydrofoil and two single propeller combination will have a much higher performance compare to Volvo Penta double propeller drive. For example the fuel economy may be reduced for 50% or more on the same speed or the maximum speed can be 25% more or better.

 

Basic considerations:

  1. The improvement will be higher on displacement of semi planing boats.
  2. The size of the hydrofoil needs to be selected to displace up to 60% of the boat in order to maintain dynamic stability.
  3. The hydrofoil can be made with controllable angle to optimize performance for different speed and for different loads.
  4. Install the hydrofoil in front of the center of the gravity.
  5. The hydrofoil span has to be 10-20% higher than the distance between propeller shafts because of the location of the center of circulation behind the hydrofoil.

 

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Dr. Stepan V. Lunin

January 2005.