Kiwiprops – an innovative new propeller for sailing yachts

2010 U.S. OLYMPIC TEAM ANNOUNCED (reprinted from Scuttlebutt)
February 17, 2010
J/News 2-18-2010
February 18, 2010

Kiwiprops – an innovative new propeller for sailing yachts




‘Kiwiprops self-align with the streamlines, minimising drag’   

The Kiwiprop™ is a unique cost effective feathering propeller that eliminates gears, aligns the blades with the streamlines, not the shaft: minimises corrosion potential from the lightweight composite blades and offers users easy variable pitch adjustment

The Kiwiprop™ is now available as the result of a long and careful development program that was designed to bring new levels of performance and value to the sailor who wishes to retain the known benefits of a fixed three bladed propeller, for motoring and maneuverability, but at the same time minimise drag under all sailing conditions.

Traditionally feathering props have blades geared together, to ensure that they line up with the shaft when feathered. However the shaft rarely lines up with the water flow, thus increasing projected area and thus drag. With a typical inboard engine the shaft will be angled down at 10 to 15 degrees and with the hull sloping up at say 5 to 10 degrees, the shaft may be 15 to 25 degrees off the streamline of the water flow. This can cause auto-rotation and wear with the need for shaft locks. In addition there can be 5 to 10 degrees of leeway on the wind and surging sideways in big waves.

In the Kiwiprop™ the individual blades are always free to ‘weather vane’ in the actual water flow, to a minimum drag position so don’t cause autorotation

This modern light weight three bladed feathering propeller has proved much smoother and quieter in operation than equivalent two bladed units and offers the additional benefits of variable pitch, fully lubricated, minimal corrosion potential and the full reverse thrust.

A new composite propeller for yachts…

While much has been written on the very substantial performance improvements and user benefits that new materials and technologies have made available to yachting, much of it originating from our local marine industries, these have been very much focused above the waterline.

As the result of long term dissatisfaction with the performance and value propositions available to our own yachts from existing propellers, we have set out to completely rethink from a zero base the design options available today for the optimal propeller unit. This means attaching equal weight to the economics of the unit, both in initial purchase and ongoing life cycle costs as to the design.

The problems of the traditional folding propeller have been well known for some time. A lack of blade area leading to poor thrust and motoring performance, particularly in adverse conditions. A very high radius of inertia, which coupled with a high mass leads to very significant vibration problems in real world applications over time. This is driven by the need to keep sufficient mass in the tips of the blades to make them operate to any reasonable level of performance in reverse. What is often overlooked in this design approach is that the significant volume that the blades occupy generates a hydrodynamic shock as the blades sweep past the bottom of the vessel. Water is incompressible, and the volume of water that the blade displaces appears as further vibration. The only solution to this for a given clearance is to lower the speed of the blade and/or make the tip of the blade as thin as possible.

As the vibration just from the rotating mass is a function of the radius of gyration, the mass and in particular the radius of inertia are critical design parameters in seeking a smooth running unit. A design optimised for smooth operation will be as light as possible, with the mass concentrated at the centre of the unit. This is the exact opposite of the traditional folder. The benefits of this approach has already delivered dramatic improvements in stern bearing life, the bane of many yacht owners, as one would expect from the very significant reductions in moments of inertia or flywheel effect. Remember a typical stern bearing will have some 10 ~ 20 thou of clearance when new, and much more as it wears.

Our unit brings the many benefits of today’s technology to an essential device which has seen little change or improvements since Herreshoff’s first folding propeller early this century. The hydrodynamic design problems of all propellers are complex and even more so when they are required to minimise drag when sailing but at the same time deliver the maximum thrust to a vessel typically with limited power and high windage.

Thrust is a function of blade area. There is no compromise.

To increase the thrust from a given power, blade area must be increased. The obvious way to achieve this is to go to more blades for a given diameter.

In the absence of a simple cost effective three bladed feathering unit many have chosen to stick with the traditional fixed three bladed unit to obtain reasonable motoring performance but at a huge cost in terms of sailing performance.

The three bladed composite design of the KIWI FEATHER PROP captures the very significant thrust benefits available from the substantially increased blade area and at the same time addresses the well known problems of corrosion, wear, weight, vibration and poor reverse thrust inherent in the traditional two bladed folding unit. Choosing a feathering design which eliminates gears by utilising injection moulded blades of Zytel™, a DuPont composite, allows for the economical production of three bladed propellers in the under 55 horsepower ranges where the fixed three bladed and two bladed folding propeller have predominated over long periods of time.

While three bladed geared feathering designs have been the propeller of choice for all round yachting performance, they have significant design problems preventing their widespread uptake particularly in the markets of under fifty horsepower where the great volume of propellers are installed. Firstly they are expensive due to the complex machining required with each unit. Each blade requires an individual machined bevel gear in addition to the gear in the central boss.

Far worse from a sailors perspective, and which is not often recognised, the blades remain parallel to the shaft, not the water flow which introduces a significant drag penalty and causes unacceptable autorotation of the shaft when sailing. You will note that all drawings of geared feathering propellers are taken from the perspective of the shaft, and appear to show a unit with very minimal projected area. This is most misleading as with typically a shaft angle of say 10 ~ 15 degrees and a buttock line aft of say 5 ~ 10 degrees, the propeller is ‘seeing’ the water at an angle of typically 15 ~ 20 degrees. This generates a large projected blade area which translates to drag and oscillation with all it’s attendant problems of noise, wear and reduced sailing performance. Any oscillation will introducing significant wear to the internal mechanisms where it is always hard to maintain adequate lubrication in the harsh underwater environment.

Auto feathering units which self pitch should really be costed with an expensive shaft lock and all it’s attendant problems when comparing competing units. Not all gearboxes have the ability to lock the shaft if they depend upon the engine running to energise the internal clutches.

The Kiwi Feather Prop by eliminating the gears, and utilising near neutral buoyancy Zytel™ allows each individual blade to freely weather vane into the water flow that it currently sees, irrespective of shaft angle, leeway or sea motion thus always minimising drag and reducing any autorotation to very low levels.

Zytel™ lowers the total weight of the unit to under 3.5 kgs, which coupled with the thin blade tips of this design dramatically lowers the moments of rotational inertia and hydrodynamic shock for very smooth motoring while retaining full reverse thrust and eliminating blade corrosion issues. The thin blade tips also increase the thrust available by reducing the power consumed by the rotational drag.

The blades, which are the only moving parts when sailing, are filled with a high quality grease. Centrifugal force when motoring ensures the grease is retained in the blade thus addressing the important ongoing lubrication issue, which is a prerequisite for minimal wear over time.

This propeller has been developed, engineered and tested very carefully over a period of some six years. An extensive and ongoing database of every unit installed allows for ongoing monitoring and performance evaluation with accurate diameter and pitch recommendations.

Careful design and engineering has allowed us using just six standard components to cater for both shaft and saildrives, left and right hand, 15.50 to 18.50 inch diameter (nominal measure) and all pitches by virtue of the adjustable pitch feature. This allows us to cater for virtually every installation in the 15 ~ 55 horsepower range, which is the great bulk of the market volume wise, at very much lower prices than existing feathering propellers.

Not only are the Kiwi Feather Prop units cheaper and we believe virtually maintenance free but produce significantly more thrust than competing units primarily due to their careful blade design. Horsepower required approximates a function of diameter to the 5th power in these sizes. Thus very careful attention to tip design is critical, something often overlooked with competing designs.

The tradeoff for any flat blade design (required for the feathering function) as boat speed increases is a fall in relative efficiency. By starting from a much higher base however, our experience on a number of different installations of top speed in calm conditions, is equal or better depending upon the quality of the original installation. Hull displacement speed constraints will generally determine top end motoring capabilities in calm conditions.

The unit really comes into it’s own however when motoring into adverse conditions. With the very significant extra thrust available from the three blades, which perversely increases as boat speed falls, boat speed is maintained at higher levels than competing units.

We feel this technology now allows yacht owners to afford the best of both worlds. The all round motoring capability, light weight and minimum maintenance of a three bladed fixed propeller coupled with minimum drag for optimal sailing performance. Many of our users are reporting significant increases in sailing and motoring performance, particularly when punching into a head sea, coupled with very much smoother running and an excellent reverse capability.

We set out to offer three bladed performance at two bladed prices !

With more thrust, particularly in reverse, one third the weight, minimum corrosion potential, full lubrication of all moving parts and less than half the price of competing units, we feel we have achieved our objective and are confident that this propeller will over time become the propeller of choice in its target market segments.

From an ongoing service aspect the commonality, interchangeability and economics of the various components allows us to offer exchange blades for the inevitable problems that will arise.

The relatively high value and limited weight of the unit means prompt delivery worldwide via international courier services from New Zealand to anywhere at reasonable prices.

You may wish to look further at our web page where the above information and in particular testimonials from units in service on a wide range of engines and drive trains is available in more detail.

For further information on the Kiwiprop see:

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Michael James
Michael James
Michael James has been with Murray Yacht Sales since 1995 and is in the the New Orleans office.

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