In prior years we advised restricting the use of Kiwiprops™ to vessel with continuous sailing speeds of less than 15 knots. After some years of development and extensive testing on a large number of fast cats and sloops for units delivered post April 2011, there are now no sailing speed restrictions on Kiwiprop™ units which will be fitted with blades having Flettner foils added to the lower rear trailing edge of each blade.
The unique Kiwiprop™ design depends upon the stability of the individual blades in the streamlines that each is currently encountering to ensure the foil shape of the blade is stable and not generating lift in either direction that would rotate the unit. This has been achieved by what is termed skewing the blade aft from it's mounting point so the center of pressure integrated down the blade is at a location aft of the pivot so ensuring the stability of the blade - ie any change in angle of attack will restore the blade to a new stable position - again with zero lift. This is no different from the design of a wind vane in principle. However here we have much smaller distances for the foil which make stability more difficult in all conditions. We have to deal with real world conditions and provide a margin of design safety such that even with fouling on the blades, maybe tip damage, poor lubrication, foreign objects - that the blades still feather ie they exhibit stability in the streamlines they are encountering. In addition on some installations the streamlines that follow the buttock lines can mean that relative to the shaft a blade can encounter an angle greater than the 25 degree design constraint. The center of pressure moves forward on all foils as speed increases. This has the effect of decreasing the stability of the foil with higher speeds. An excellent free foil simulation program from NASA shows exactly how the many variables impact on a foil: www.grc.nasa.gov/WWW/K-12/airplane/foil2.html Foils in air and water - which is incompressible - in generally behave similarly - particularly at the relatively low speeds we are dealing with. Initial versions of the Kiwiprop™ were not recommended for continuous sailing speeds in excess of 15 knots because of this and thus were not suited for installation on many catamarans - which could exceed this speed in bursts and in ideal sailing conditions. With the increased popularity of catamarans around the world and their speed requirements we spent some time on a research and development program to enhance the stability of the blades at higher speeds while retaining the known benefits of the existing design. We also needed also to address the market for older style installations with hydraulically energized gearboxes that can not be locked in gear with the engine stopped to prevent shaft rotation as is common with the relatively smaller and lower powered engines we are dealing with. These installations require an increased margin of design safety to ensure stability in all applications. An excellent paper by R. M. Freeman - 'The Rudder - A hidden cause of a common problem' presented to the National Marine Electronics Association on Feb 20th 1978 was invaluable. This paper is available on the web at http://www.woodfreeman.com/pdf/rudders1.pdf. What it shows is that in simplistic terms, as streamlines flow around any foil, initially they are laminar ( as demonstrated by the smooth flow from slow kitchen tap ) but as they approach the rear of the foil they tend to separate from the surface and become what is known as turbulent - as demonstrated by a tap at full flow. A ships rudder is in fact an exact analogy of a Kiwiprop™ blade when feathering as far as the stability issues we are trying to address go. The trailing edge of the foil ( rudder ) was thus free to move in this turbulent area without generating the appropriate feedback forces that would make the foil ( rudder ) stable. This introduces forces at the tip of the foil that may not provide the appropriate feedback to stabilize the foil. The solution to the ships rudder oscillating slightly from one side to another and causing an unstable course to be followed was to add small wedges - more correctly called Flettner foils by an aerodynamicist - to the trailing edge of the rudder. Flettner was a very famous German aircraft designer who first used these. Photos of the latest P & O transatlantic Queens show these same additions on the foils of the anti-roll stabilizers fitted to these vessels. Again - to introduce stability and prevent oscillation or 'hunting' about the zero angle of attack. In effect - in simple terms - these additions protrude out from the rear of the foil to engage the laminar streamlines away slightly from the surface and thus again stabilize the foil. Kiwiprop™ blades from ~ April 2011 onwards will now all have these small 'wedges' or Flettner foils fitted to the lower rear trailing edges of each blade. After extensive testing we are very confident that these blades will now be stable at all expected vessel speeds encountered sailing on all vessel types. The foils are just 4 mm in height. These are molded into the blade root on both sides - then one side is removed depending upon whether it is a Left or Right Handed unit to bias the unit so that it will tend to rotate in a reverse direction where the shaft can be locked so preventing the unit ever adopting a motoring position where it would operate like a fixed propeller. NB: These new blades are fully backwards compatible with all pre-existing Kiwiprops™. As these are on the very lower sections of the blades where the radial velocity is very low and that section of the foil is generating virtually no thrust anyway when motoring - they will have no effect on motoring ability. Because they remain well within the existing projected area of the foil - we believe they will not generate any additional drag when sailing.
Additional stability was provided by a small round as distinct from sharp leading edge on the outer section of the blade - which was originally designed sharp to assist in cutting seaweed and kelp - but at the risk of exposure of damage to the very sharp edges. Trimming the outer leading edge where it joined the tip of the blade to further skew the blade in line with many modern and well researched competing designs improved motoring and also further increased the stability of the foil about a zero angle of attack by moving the center of pressure aft. For those with older style hydraulic gearboxes that are only energised and can operate when the engine is running it is not possible to lock the shaft to prevent rotation - Owners have two options - which one is optimal will depend upon the sailing speed of the individual vessel.
For those with the usual mechanical gearboxes including all Saildrives - the blades will be supplied with a wedge on the appropriate side and will exhibit the enhanced stability that these foil shapes have shown under test. Any tendency to rotate will be in the reverse direction - so the unit will remain feathered with each blade at a very low 'angle of attack'. Owners can then simply engage the gearbox . usually ahead to lock the shaft while the unit remains feathered. For high speed sailing cats and high speed light weight vessels with high sailing speeds we have found the foils induce a constant force of just ~ 1 ft lb of torque on the shaft at ~ 10 knots - sufficient to ensure the stability of the foils ( ie blades ) in all conditions. Any constant rotation of the unit will involve the blades oscillating with each rotation thus generating higher levels of wear on the blade mounting pins which is undesirable. Thus the requirement to lock the unit in gear to prevent auto-rotation After extensive testing and very positive feedback from a number of fast catamaran and sloop installations over long periods of time and significant sailing distances - we are confident that these design enhancement will meet the stringent requirements of all fast sailing vessels to retain feathering stability while retaining the known all round benefits of the Kiwiprop™ unit. |
