THE PHILOSOPHY OF THE CONCEPT
A new composite propeller for sailboats...
Writings, largely from the experience of the nautical industries, abound on the subject of the very substantial performance improvements and user benefits that new materials have brought to yachting. However, these publications are mainly concentrated above the fold.
A new approach
The continuously disappointing performance/price ratio of the propellers available on the market, has led us to rethink entirely at the base the concept of optimizing the efficiency of the propellers. This approach involves paying equal attention to economic criteria in relation to design requirements, both for the initial purchase price of the propeller and for maintenance costs over the life of the product.
The problems with traditional folding propellers have been well known for some time. A blade surface that is too small producing poor thrust and engine performance, especially in adverse conditions. A large radius of inertia, which, coupled with a high mass, creates over time very significant vibration problems in real use.
This problem stems from the need to maintain enough mass at the end of the blades to maintain a reasonable level of performance in reverse. The fact that the significant volume occupied by the blades, generates a hydrodynamic shock when they pass close to the bottom of the boat, is often neglected in the very design of these propellers.
Water is incompressible and the volume of water that a blade moves, produces an additional vibration. The only solution to this problem for a given free space is to reduce the speed of the blade and/or decrease the thickness of the blade as much as possible to make it as thin as possible.
As the vibration caused by the rotating mass is a function of the radius of turn, the mass and in particular the radius of inertia are critical parameters in the search for a gentle rotation in the design of a propeller. An optimized design in the search for vibration-free operation should be as light as possible, with a concentration of mass in the center of the propeller, which is exactly the opposite of traditional folding propellers.
The benefits of this approach have already translated into dramatic improvements in the lifespan of hydrolube rings, the bête noire of many sailboat owners, as one would expect from a dramatic reduction in moments of inertia or steering wheel effect. The normal clearance between the propeller shaft and a new hydrolub ring is about 0.25 to 0.5 mm and much more when it is worn.
The advantages
Our products bring the many benefits of today’s technology to essential equipment that has undergone few modifications and improvements since Nathanael Herreshoff’s invention of the first folding propeller at the dawn of the twentieth century. The hydrodynamic properties of all propellers pose complex problems. These problems only worsen when it is necessary to reconcile the reduction of drag under sails and the search for maximum engine thrust on a boat characterized by relatively low engine power and high dunking.
The thrust depends on the surface of the propeller blades. There is no alternative.
To increase the thrust from a given power, it is necessary to increase the surface of the blades. The obvious solution to achieve this goal is to increase the number of blades for a given propeller diameter. In the absence of a simple and economical three-blade propeller, many have chosen to fall back on traditional fixed three-blade propellers. This solution allows them to obtain reasonable performance at the engine but at the cost of a huge deficit in performance under sail.
With a substantial increase in blade area, KIWIPROP’s three-bladed composite propellers provide significantly increased thrust. At the same time, they are freed from the well-known problems of corrosion, wear, weight, vibration and poor reverse performance inherent in traditional two-blade folding propellers. Kiwiprop has designed a feathered propeller that eliminates gears by using Zytel™, a patented DuPont composite, for the manufacture of injection molded blades.
Gear-feathered three-blade systems have long been the most capable propellers overall for sailboats. However, the significant problems inherent in their very design have been a hindrance to their diffusion especially for engines of less than 50 hp which represent the majority of the propeller market. In the first place, their price is high due to the complexity of the individual machining required to manufacture each propeller. The conical gear sprocket of each blade must be machined individually in addition to the hub gear.
Disadvantage much more penalizing for sailing under sail, but very often ignored, the blades remain parallel to the propeller shaft, not to the water nets which represents a significant drag and causes an autorotation of the tree under sail. Note that all diagrams depicting gear feathered propellers are made from the perspective of the shaft. They show a propeller with a very small projected area. This representation is very misleading because with a shaft inclined by 10 to 15 degrees and a rear slender line inclined by 5 to 10 degrees, the propeller receives the water nets at an angle of about 15 to 20 degrees.
This configuration generates a large projected area for each blade which results in drag and oscillation with all the correlative problems of noise, wear and decreased performance under sail. Any oscillation causes significant wear of the internal mechanisms for which maintaining effective lubrication is always a complex problem in the harsh conditions of the underwater environment.
Compared to competing models, it is necessary to add to the cost price of these propellers with gear feathering and not automatic an expensive system of locking the shaft and all the related problems. Not all different types of inverters can lock the rotation of the propeller shaft when the engine needs to be running to activate the internal clutch. By eliminating gear systems and using the Zytel’s™ neutral buoyancy, the KIWI FEATHER PROP propeller allows each blade to orient itself freely like a weather vane according to the water nets that actually run through it, regardless of the inclination of the propeller shaft, the drift or the movements of the sea. Thus, the drag and the phenomena of autorotation are reduced to the lowest levels.
The use of the Zytel™
The use of the Zytel™ reduces the weight of the whole to less than 3.5 kg. Combined with the thinness of the edge of the blades, this lightness dramatically reduces the moment of inertia of the rotating masses as well as the hydrodynamic shock. At the engine, the vibrations due to the propeller are virtually eliminated, the thrust remains intact in reverse. Finally, the problems of blade corrosion are completely eliminated. The thinness of the blade ends also increases the available thrust by reducing the rotational drag.
The blades, the only moving parts under sail, are filled with superior quality marine grease. At the engine, the centrifugal force retains the grease in the blade, thus solving the permanent problem of lubrication, an essential prerequisite to minimize wear over time.
This propeller has been very carefully developed, made and tested for more than six years. An extensive and constantly updated database of all installed propellers allows the permanent evaluation and updating of performance for an optimization of recommendations in the choice of propeller diameter and pitch.
The precision of the design and engineering has made it possible to reduce to six the number of standard components for the realization of propellers suitable for both conventional or saildrive transmissions, with rotation to the left or right, a nominal diameter of 151/2″ to 181/2″ and all steps thanks to the exclusive pitch adjustment system. This design allows us to meet the needs of almost all installations in the range of engines from 15 to 55 hp, which represents the bulk of the market in terms of number of engines, at prices much lower than those of other feathered propellers.
Kiwiprop propellers are cheaper
Kiwiprop propellers are not only cheaper and virtually maintenance-free but also provide significantly higher thrust than competing models especially due to the thinness of the blades. In these sizes the ratio between the POWER in CV and the diameter of the propeller is about 1/5. The shape of the propeller edge is a critical parameter often overlooked in competing models. The choice of a flat blade, necessary for feathering, results in a relative loss of efficiency as the boat accelerates.
However, starting from a much larger database, our experience in calm and high-speed seas on a large number of different installations, has shown equal or greater efficiency depending on the quality of the original installation. Speed limits related to hull displacement generally determine the maximum engine performance in calm seas. The propeller shows its own capabilities when conditions are bad. With the very significant additional thrust available with a three-blade propeller, which paradoxically increases as the speed of the boat decreases, this same speed is maintained at higher levels than those obtained with competing propellers.
High-performance technology
We are convinced that today this technology allows the boater to have the best of both areas. The overall driving qualities, lower weight and minimum maintenance of a fixed three-blade propeller combined with the minimum drag allow for optimal performance under sail. Many users report a significant improvement in sailing and engine performance, especially in the face of a formed sea, as well as much smoother engine operation and excellent performance in reverse.
Our goal is to always provide the performance of a three-blade propeller at the price of a two-blade !
With more thrust, especially in reverse, a weight divided by three, minimal potential corrosion, total lubrication of all moving parts and a price half that of competing products, we believe we have achieved our goal and we are sure that this propeller will take a leading position in the market segments that concern it.
From the point of view of the need for after-sales service, the versatility, interchangeability and cost control of the various components allow us to offer replacement blades to overcome the inevitable problems posed by operation in the marine environment.
The relatively high value and lightness of each piece ensures delivery just about anywhere in the world from New Zealand on time and at a reasonable price.
We invite you to consult further our website which develops in more detail the above information and in particular in the testimonials of users of our propellers on a wide range of engines and transmissions.