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, fitting 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. High centers of gravity on the blades, which coupled with a high mass leads to very significant vibration problems in real world applications. 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 and the mass, an identical mass and center of gravity retained over time for each blade 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 center 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 the radius and the overall mass. Remember a typical stern bearing will have some 10 ~ 20 thou of clearance when new, and much more as it wears, which immediately creates an out of balance condition and thus vibration. This is one of the reasons Saildrives will always run so much smoother, as they have no shaft tolerances.

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 propeller 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 and by utilising injection moulded blades of Zytel™, a DuPont composite, allows for the economical production of three bladed units in the under 50 horsepower ranges where the fixed three bladed and two bladed folder 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 units 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 central boss. Blades are cantilevered off washer type bearings and the stress and wear increases as small gears magnify the forces.

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 auto rotation with all it’s attendant problems of noise, wear and reduced sailing performance. As they rotate, or even when the shaft is locked, the blades oscillate introducing significant wear to the internal mechanisms where it is always hard to maintain adequate lubrication in the harsh underwater environment.

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 eliminating or reducing autorotation to very low levels.

Zytel™ lowers the total weight of the unit to just 3.3 kgs, which coupled with the thin blade tips of this design dramatically lowers the vibration 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 lubricated with a high density silicone 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 unit has been developed, engineered and tested very carefully over a period of some six years. We have each had units on our own yachts, a 36 ft Townson and a 37 ft Jim Young design for over 4 years. We have just started actively advertising in recent issues of "Breeze" — the magazine of the Royal New Zealand Yacht Squadron and on the Internet. The Web page address is listed below.

Careful design and engineering has allowed us to use just six standard components to cater for both shaft and saildrives, left and right hand, 15 to 18 inch diameter, and all pitches by virtue of the adjustable pitch feature. We believe this will allow us to cater for virtually every installation in the 20 ~ 50 horsepower range, which is the great bulk of the market volume wise, at very much lower prices than existing units.

Not only are the units cheaper and we believe virtually maintenance free but produce significantly more static thrust than competing units primarily due to their careful blade design. Thrust approximates a function of diameter to the 5^th power in these sizes. Very careful attention to tip design is therefore absolutely critical, something often overlooked with competing designs. Static thrust of the 16 inch unit at 1100 RPM from a 30HP Watermota with 2:1 reduction is measured at over 425 lbs. Compare this with the thrust obtained from 12 competing units detailed in the attached article from "Cruising World" of June 1996 where the maximum was 235 lbs and the average of all the units around 200 lbs. A shaft speed of 1100 RPM used in our tests equates to a comparable engine speed of 2900 RPM with the 2.6 : 1 reduction gear for the 30 HP Yanmar, about the average for the Cruising World tests.

The tradeoff for any flat blade design, 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 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, which perversely increases as boat speed falls, boat speed is maintained at higher levels than competing units.

A wooden Cavalier 39 of around 9 tons went from under 4 knots to 5 _ knots when punching home into a stiff breeze when the new propeller was installed.

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 twice the static thrust, one third the weight and less than half the price of competing units we feel we have achieved our objective and are confident that this unit 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 units for the inevitable problems that will arise. Ropes around props! Hitting flotsam! etc etc

he relatively high value and limited weight of the unit means three working day delivery via DHL from New Zealand to anywhere at reasonable prices.