Increasing propeller efficiency with tip fins.
Propulsion efficiency of a marine propeller can be increase with tip fins. The idea is well known in aeronautics and is used very often on most of the commercial airplanes. Many modern airplanes have small fins on the end of the wings. Some fins are only bent up but some go up and down. The efficiency increase is the result of preventing flow from the pressure side of the blade to the suction side of the blade around the tip area. Practically the marine propeller tip fin can be designed similar to the winglet or a tip fin of the airplane. However some reasonable design considerations can improve the design of the marine propeller even more. A simple copy of an airplane winglet does not utilize all the opportunities in marine propeller improvement. For example the reason of bending the winglet up on the airplane was originally copied from bids. The wings of the most airplanes are flexible like the wings of bids. Facing the end fin down would require increase stiffness of the wing. Also bending airplane wing or the tip fin up increases the passive stability of the airplane. Some airplanes have two tip fins going up and down but the upper fin is still significantly larger.
On a marine propeller there are no difficulties affiliated with airplane wing design. A marine propeller does not have flatter, it is very stiff and it does not need to provide passive stability. So it is possible to take a full advantage of the tip fins. The correct design of the fin tip on a marine propeller would be bent towards to the pressure side of the blade. It provides significant advantage versus proposed by Kappel winglet.
Adding fins on the tips of the blades can modify practically any propeller. The picture below shows a modified aluminum propeller. The tips of the blades were cut to a smaller diameter to provide space for attachment of aluminum fins. The attachments were shaped to provide an optimal hydrodynamics.
Propeller was tested at speed of 40 knots with slippage of 14%. The improvement of the propulsion efficiency was 10%, or increase from 75% without the tip fins to 85% with the tip fins. The acceleration of the boat also improved from 14 seconds to 11 seconds.
The author based on previous experience and experiments selected the size and geometry of the tip fins on the experimental propellers. Additional optimization of size and geometry can possibly add additional 3-5% bringing the final improvement to 15%. However in previous experiments the authorís intuition was always close to the optimal. Extra modifications added maximum of 5% on high slippage propellers.
Here are some considerations how the fin tips can improve your boat or a ship.
Practically the tip fin is recommended for any propeller including a small fishing boat and a large oil tanker. For example, a small modification of a propeller on 2000HP recreational motor yacht increased maximum speed from 38 to 40 knots and saved about 200 gallons of fuel on each trip.
The tip fins can be used also for increase of propulsion efficiency on cavitating propellers or surface piercing propellers on racing boats. However some additional design modifications needs to be done on the propeller blade geometry. It may be reduction of the blade chord and pitch, or trailing edge modification because the tip fins make propellers heavier.
The tip fins can be used with other improvement option like reduced trailing edge cupping, surface laminators, ventilation, diffuser ring modification and other described on our web site. But adding the tip fins provides the highest up to 40% increase of the efficiency.
When considering a method for increase of the efficiency it is necessary to find out why the efficiency is low. The reason of existed efficiency reduction will determine the best method for the improvement method.
Often people give me some propeller parameters and ask if it possible to increase speed of a boat. The answer is always yes. In most situations the propeller can be improved with simple modification, very often incorporated the modification of the trailing edge cupping and adding the tip fins. Very often it may take 5 seconds to hit a prop with a hammer to add 3% of efficiency, particularly on racing propellers with hub exhaust. You can e-mail a photo of a prop so I can show where to apply a hammer and for how much. If the problem is more complex, I would need a CAD model of the propeller or the real propeller in my hands.
Dr. Stepan V. Lunin