Kiwiprops™

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Auto Rotation

AUTO ROTATION WHEN SAILING:

June 2013


To ensure the propeller feathers correctly once sailing:

First throttle down to an engine idle for say 20 seconds, then place the gearbox in neutral, then stop the engine.

The shaft will then slow down as the blades align themselves with the water flow and slowly come to a stop. The shaft will then remain stationary or near stationary without further attention. This may take 2 - 3 minutes. All units post April 2011 will tend to rotate slowly in reverse under the effect of the Flettner foils added to the blade root to 'bias' the rotation.

If the units are rotating slowly - you can either keep them in this condition - they are still feathered as even the smallest angle of incidence can cause rotation with uneven streamlines off the Saildrive leg, keel, shaft or bracket - but this will induce higher wear rates.

Installations with hydraulic type gearboxes that cannot be locked with the engine stopped should be fitted with paired foils on each side for enhanced stability - and will not tend to rotate in a reverse direction.

Engage reverse only once the shaft has stopped rotating, or is starting to rotate on a reverse direction - by luffing up if necessary. Engage the gearbox preferably in Ahead to lock the shaft prior to bearing away. (Reverse offers the remote possibility of turning the engine backwards).

Keeping the gearbox in neutral when sailing with foils fitted will allow slow rotation of the shaft with an attendant oscillation of the blades about their axes. This will lead to higher rates of wear on the blade mounting pins which is undesirable and to be avoided.

Do not engage reverse while the shaft is rotating in Ahead as this will engage Reverse (which is the same Ahead on the Kiwiprop™ design ) and cause high speed auto-rotation.

Engaging reverse while the shaft is rotating in Ahead will usually not work as the rotating momentum of the unit will in effect wind up the spring and in effect engage reverse - the very objective we are trying to avoid. There is no problem engaging reverse if the shaft is rotating slowly in reverse. It will simply come to a stop and remain feathered.

The prop once stopped will then remain feathered. In gear - it will still be feathered but the blades will have a very low angle of attack with very minimal increases in drag. The Kiwiprop™ approach of individually feathered blades will always have less drag than a geared feathering unit where the blades are geared together and will always present an increased projected area with consequent drag increases in real world operating conditions.

So - to place the unit in gear - do so while it is stopped or before it starts to rotate in Ahead.

All the text books say there is a lot of induced drag from a rotating prop - more than from the very high drag of a locked fixed prop. In addition this will reduce wear from the individual blade oscillations induced by the shaft angle as the unit rotates.

The best example one can use is a helicopter gliding down without power in auto rotate mode. For maximum lift - you want the blades rotating as fast as possible. This equates to maximum drag on a rotating prop. It's all about the rotating energy left in the wake.

So contrary to initial thoughts a locked prop will usually have less drag than a rotating prop. Much will depend upon the particular design of the propeller, the conditions it is operating in - ie turbulence around the keel, shaft angle, leeway, and the resistance of the shaft and bearings etc.

The above all assumes the following which would need to be checked if the unit fails to feather:

  • The Nose Cone has not moved ( eg rope wrap ) so the blades are not able to move freely about an arc from normal pitch position Ahead to trailing Astern to a further 25 deg past the trailing center position. Moving the Nose Cone moves the spring which moves the Blade Carrier and prevents the blades from moving across their full arc without contacting the reverse rollers 

  • Each blade is free to move and is not stiff or is constrained with a foreign object eg fishing line under the blade root or a plastic bag caught on a blade for example - a common cause of auto rotation 

  • The blades are not damaged - particularly on the leading edge or tips

  • There are no heavy paint runs on the blades - the effect is multiplied when these are near the leading edge as the unit depends on the correct shape of the foil to feather 

  • The shaft angle relative to the streamlines that the propeller is operating in = shaft angle + buttock lines + leeway + heel may exceed the 25 degrees design criteria in which case the blades will need to be biased to counter the forces generated by removing a wedge on one side of the blade root. All units post April 2011 have this bias.

  • The unit is correctly lubricated with the individual blades and the blade carrier boss also free to move about the boss.

    • There are 3 or 4  lubrication points - one in each blade - at the mid point, 
      plus one each at the front and rear of the boss - a total of 5 or 6 grease points depending upon whether you have a K3 or K4 unit.



FOIL DESIGN & STABILITY

BACKGROUND DESIGN ISSUES

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 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 to address the market for 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 trailing edge 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 it can be locked so preventing the unit ever adopting a motoring position where it would operate like a fixed propeller.

The 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.

 


bladecrosssection

 

 

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.

  • Use the twin wedge extensions to ensure stability of the foil at higher sailing speeds.

  • Trim one side of the wedge partially off - which will cause very slow rotation in the reverse direction. This will prevent high speed autorotation in the Ahead direction.

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 - 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.

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 all stringent the requirements of all fast sailing vessels while retaining the known all round benefits of the Kiwiprop™ unit.



SHAFT ANGLE:

Often overlooked in any evaluation of autorotation is the significant effect shaft angle can have on any propeller design including the Kiwiprop™. While shaft angle is traditionally measured relative to the waterline in use of course the water flows or streamlines around the unit are highly variable and are influenced by trim, buttock lines, leeway, heel, fore and aft trim coupled with any fore and aft pitching which will also change the streamlines about the unit in use.

Even a folding type propeller with the blades folded will have a projected area relative to these streamlines in use and makes for complicated evaluations as this alone can be the reason for autorotation of any propeller type. Our approach will always be to remove wedges from one side of the blade roots to generate a small counter rotational force against any forces generated from the projected area into the streamlines.



IMPLEMENTATION:

Blades from the now current dies will have these foils molded into both sides - we will remove the side not required. This will continue on those sizes we have in stock until all old stock has been exhausted.

We expect this conversion to be completed prior to December 2012.