What a fabulous idea. A feathering prop with plastic blades. Makes a lot of sense really. The most obvious advantages are less weight and no electrolysis but it also means that the blades can be painted with antifouling, something that can’t be done effectively with metal blades. You can paint them of course but the paint never stays on for long and once there’s growth on the blades, the prop becomes hideously ineffective but paint does stay on the Kiwiprop’s plastic blades so they stay clean all season.

Each blade is made of a special plastic and is mounted independently from the others. This is where the Kiwiprop differs from most other feathering or folding props. The advantage of this system is that each blade is free to follow the path of least resistance when in ‘sailing’ mode. Other feathering props like the Max prop for example, have blades that are linked to one another and because most marine engines are installed at a bit of an angle, there’s always at least one blade causing some drag.

Another advantage of the Kiwiprop is that the pitch can be adjusted easily, even with the boat in the water. Each blade has a small Allen key bolt. Simply turn each bolt half a turn to change the pitch by a degree and a half. Most other props require dismantling if you want to change the pitch. However the Kiwiprop has no adjustment for pitch in astern.

The Kiwiprop is made for boats with engines from 15 to 55 hp. It has a central hub made of stainless steel and because there are no dissimilar metals in it’s construction it means that no anode need is fitted.

Enough of the technical stuff. Fitting was straight forward with the prop using the existing taper on the shaft. The pitch was preset at the factory to match my engine and gearbox. My Dana 24 was originally fitted with a fixed two blade prop which worked fine in forwards but lacked bite in astern. This made manoeuvring in port on windy days a lottery. In theory the Kiwiprop would perform better when motoring and sailing.

Under power the Kiwiprop has bite. The boat accelerates well, especially considering that it weighs over 4 tons but what is most impressive is that way it stops. Slam it into astern and the boat stops from 5 knots in it’s own length. If you’re not holding on, the sudden deceleration will knock you down. A massive improvement on the old two blader.

The Kiwiprop has made also made a big difference to the boat’s sailing performance. In light airs with a bit of a chop the Dana now sails through it. Before with the drag of the prop it would be enough to stop the boat’s progress. The boat’s wake is cleaner too. The difference is quite noticeable. If you were coming from a 3 blade fixed prop you will really feel the difference

Where’s the catch?

Now for the not so good stuff. Right from the start the Kiwiprop made a lot of noise at lower revs. At 2000 rpm the prop was very noisy. If I upped the revs to about 2500 it went quiet but perhaps I just couldn’t hear it over the increased noise of the engine.

So I tried a finer and courser pitch. Sadly it didn’t make any difference to the cavitation noise. It is something I have to live with apparently. Strangely, a mate has a Kiwiprop on his steel boat and it is absolutely silent which tends to suggest that it’s more to do with the hull shape of my boat and prop aperture than the prop itself although it must be said that the original 2 blader didn’t make any noise!

On a recent canal trip this problem was really intrusive. As there is a 3 knot speed limit it means running the engine at well below 2000 rpm and at that speed the prop makes a right old noise. Obviously I have been in touch with Kiwiprop in New Zealand but apart from suggesting changing the pitch and higher revs for the prop they have not been much help.

Feathering props are said to reduce prop walk but the Kiwiprop still has a healthy kick to port but this may be more to do with the shape of the hull than the prop itself. In a recent test of folding and feathering props done by Yachting Monthly, the Kiwiprop came out about average for propwalk.

Then after just a year the prop started to become difficult to get into astern. The blades were not engaging properly and the extreme pitch was so much that the engine could not pick up. The only way to make it work was to go direct from forwards to astern which as you can imagine is a right pain.

Many emails have passed between me and Kiwiprop in an attempt to get to the bottom of this. Everything has been tried, from upping the tickover,  changing the gearbox oil to checking the exhaust is not blocked but all to no avail. Eventually I was told that the reason why it won’t work properly is most probably that the base of the blades are scored from where they run against the rollers. Every year I have checked the prop and every year the three rollers are all loose and it is this, I believe, which has scored the blades.

So I am glad that we have at last discovered the reason why the prop won’t go into astern but knowing why does not help much. It would seem that the problem can be cured by a nice set of new blades. With duty and shipping it comes to about $600. Even if I were to change the blades, I can see no reason why it won’t happen again.

Conclusion:

I am disappointed in the clattering cavitation noise the prop makes, clearly audible over the noise of the engine and I am annoyed that I have to buy new blades for it so soon.

The prop cost about 1200 Euro which as feathering props go is a very good price but if the blades need replacing every few years then perhaps it’s not such good value after all.

The bolts that hold the rollers on the latest versions of the Kiwiprop are apparently now held more firmly in place using a punch and a hammer to crush the threads slightly but this seems to me a crude way to solve an engineering problem.

Doing research online I discovered that I am not alone with these problems. There are many references to loose or even missing bolts, reversing problems and even the odd missing blade!

In my opinion the Kiwiprop just isn’t robust enough for the marine environment, it has far too many issues and simply can’t be trusted to work when needed. It was worth a try because the advantages seemed many but at the end of the day reliability is far more important to me.

Like most technology, fabulous when it works but hopeless when it doesn’t. Having a 4 ton boat that won’t stop is a worrying and potentially dangerous situation. It’s hard enough trying to manoeuvre in most marinas these days without wondering if the prop will engage before you smash into a big power boat.

It’s true that we probably do much more motoring than most being in the Med and often away for months at a time but I did not expect problems after just a year of use no matter how many hours I used it for. It will be interesting to see what happens to those 4000 props out there once they have as many hours on them as I have on mine.

Overall a disappointment.

UPDATE: June 2011

Kiwiprop finally sent me a set of slightly larger blades and a set of new rollers (free of charge) in an attempt to solve the noise and ensure that it engages astern every time.

The blades are just half an inch larger in diameter which is not very much and this takes their tips to within a quarter of an inch of the hull. The idea is that the larger blades will need a finer pitch to work. The finer the pitch, the less chance of cavitation noise.

New 16.5” blades. It leaves the tips very close to the hull aperture but these bigger blades make this prop much quieter and smoother than before.

The rollers are now pentagon shaped. This will hopefully make them more inclined to rotate even if they are a little seized up by marine growth unlike the original round ones which seized up quickly. This didn’t stop the prop from working but it’s my belief that it contributed to the excess of wear on the bottom of the blades which then led to the prop not engaging astern correctly.

New Pentagonal blade rollers. This should keep them turning all season and in theory do less damage to the base of the blades where they touch.

The prop is still a bit clattery at low revs but is massively improved after that. At 2000 rpm the prop still has a little cavitation noise but it is slight and certainly not half as annoying and intrusive as it was before. By 2200 the prop makes no noise at all. This is great news as 2200 is a good amount of revs for the engine. Just enough to get it working but not so much that it is too noisy or uses an excessive amount of fuel.

Now at 2200 rpm the boat can manage almost 5.5 knots which is truly excellent. Even at lower revs the prop works well but makes some noise. As far as I am concerned, so long as the prop is quiet at cruising revs I can live with some slight noise at lower revs. There has to be compromise somewhere after all.

With new blades and rollers, the engine now engages astern perfectly but then it used to when it was new too. So this post will need to be updated regularly to report on whether the new pentagonal rollers are working. Will they wear a similar groove in the base of the blades? If so will that groove affect the prop going into astern? In theory, with the pentagonal rollers always able to turn, it shouldn’t matter if the blades do get scoured. In any case I’ll keep you posted.

Many thanks to John at Kiwiprops who came through in the end.

Update Jan 2012

The new blades have certainly reduced cavitation noise but the old problem of not going properly into astern is back after just a few months. I wrote to Kiwiprop who told me that it is a question of spring tension. So now I have to take the unit apart again to sort it out. There have been a few comments on this post recently that point at poor reliability and frankly I am not impressed.

The conclusion that I have now come to is that the Kiwiprop is not for me. I want something that simply works. I can no longer trust the Kiwiprop to go into astern and once I have lost faith and confidence in a product it has to go.

Update Jun 2012

When I hauled the boat I discovered that all three rollers were loose again. Lucky they didn’t fall out.

This is just a final confirmation for me that the Kiwiprop is likely to fail at some point so I have now removed it and replaced it with a Variprop 4 blade bronze prop. You can read about it here. It’s expensive, almost twice the price of the Kiwiprop but it’s what I should have bought in the first place. I never seem to learn that you generally get what you pay for in life. It’s a shame as the Kiwiprop has a lot going for it and the company are constantly improving the design but from what I have experienced it just doesn’t offer the kind of reliability I look for in a propeller.

Update Feb 2021

I just received the following from John at Kiwi Prop. They are constantly updating and improving their design and he asked that I add the following to this post, so here it is!

It is now some years since an update was made to this website and in the intervening period Kiwiprops has continue to prosper and now has an installed base of some 8,000 units in virtually every country dating back to 1998.

We maintain a database with a build record of every installation and thus able to monitor ongoing performance and functionality issues.

Today we have over 65,000 propeller years of units in service and consequently over 200,000  blade / reverse screw  years of service upon which to analyze and generate feedback and modifications.

Rather than dramatic design changes to the unit we have operated a continuous improvement program with successive small changes that have been that have been rigorously tested to the extent that is possible.

We have also adopted a design constraint making all components backward compatible with all previous units.

Marine engines come with a whole host of power ranges, maximum engine rpm capability and in addition a wide range of reduction options which are not always the same in ahead and astern. In fact this variation is the norm and one needs to recognize that a Yanmar shaft installation for example with say 2.2:1 reduction in ahead, like all small Yanmar’s will have a reduction ratio of 3.0:1 in reverse.

Another variable on marine engines is the type of clutch they employ which leads to very differing engagement speeds with consequent differences in the force involved with the reverse rollers contacting the blade root surface. At one extreme we have the dog clutch of the Yanmar SD 20 Saildrive which is not actually a clutch at all as it is either fully in a fully out and leads to huge shock loads on the Propeller during Reverse’s engagement.

Many smaller gearboxes today will have what is termed a cone clutch, consisting of a bronze cone into a metal cup and energized by mounting on a spiral spline so that as torque increases, the force on the bronze cone into the cup also increases. These can present difficulties getting them back into neutral with high idle speed, or any glazing of the surfaces of the cone.

Virtually all manufacturers today are phasing out cone clutches and reverting to the normal multipack clutch which has a much smoother engagement and no difficulty engaging neutral. The loading forces on a blade root with these clutches are much lower and lead to significantly lower wear rates.

Today  – Saildrive’s comprise in excess of half the market under about 80 hp and more so in new build production vessels. Yet all Saildrive’s driven by the nature of their drivetrain will have exactly the same reduction ratio in ahead and a stern.

While it is not possible to optimize a particular propeller design for all these very varying constraints, it is important to recognize they exist and make appropriate trade-offs including economic to provide what one considers as an optimal solution.

An optimal solution for sailing vessel will generally attach equal weight to motor and capability and the reduce drag from the feathering function when sailing.

The improvements undertaken over the years can be summarized as follows:

In every instance these changes have been very well documented on our extensive website: www.kiwiprops.co.nz

A full database search function on the top right hand corner of our homepage will bring up the information on any keyword entered.

A switch to 50 % glass content blades.

Post mid 2009 we became aware of the existence of a new blade material:

DuPont™    Zytel® HTN53G50HSLR NC010

In simple chemistry terms this constituted a long chain molecule, rather than the previous 35 % glass product we were using which was an aromatic or ring molecule.

The great advantages of this new product were contained 50% glass fibre by weight, was impervious to both hydrocarbons and water meaning it was stable over a very long time frames when immersed. It also of course had much higher strength and stiffness but retaining the obvious zero corrosion potential of the previous product.

The trade-off was a much higher moulding temperature which influenced production and die cooling and of course came at a significantly higher price. It also required a switch to carbide tipped tooling due to the highly abrasive nature with the high glass content in the material.

The development of ogival foiled blades:

The increased strength of the new material allowed for thinner blades which has two benefits – they will generally be more efficient and can in principle be made quieter. This was a comment from above but caution is appropriate as in any aperture situation, particularly on this type of vessel with a very broad keel, it is always going to be a challenge when the propeller blades simply are not going to see continuous smooth streamlines entering the unit.

To retain design flexibility and minimize stockholdings as well as very expensive die costs,

we elected to maintain the existing symmetric foil shape which allows for both left and right hand rotation and then mill off either side of the blade in jig  so that in simple terms it more closely resembles a traditional Propeller with a flat face aft and an ogival foil on the forward face. This has been determined over many years as optimal for motoring functionality – but of course we were restricted as more removal would have a negative affect on feathering stability.

We conducted extensive testing on this new foil shape using a friends catamaran fitted with both and old and new foil on each side this negating any variation from hull condition, currently loading and sea state.

Results for this trial are on our website under Ogival Foils.

In addition – we had a very helpful engineer who had had a Kiwiprop and was motoring the inland waterway from New York to Florida and return. His considered feedback and analysis with a popular 3GM30 on 2.61:1 – which we do value – was that the new ogival foils delivered between 0.3 and 0.5 knots additional motoring speed over the course of that voyage at the same engine rpm.

This information has been replicated on many situations now and we are very confident that the foil shapes we are using are both optimal for motoring, yet retain adequate strength with a high margin of safety and the shape change has not affected feathering functionality.

There is a very extensive analysis of the testing and design undertaken using computational fluid design (CFD) beginning with the profile of the actual blade die shape and progressing to illustrating the effect of this particular shape and ogival modification on power and thrust. Actual vessel speed versus derived agreed to within 5%. This is all available on our website under Ogival Foils. CFD will deliver the results from a particular foil shape under analysis – it will not tell you what is optimal which has to be carried out using hydro dynamics and trial and error to an extent.

The development of V foils on the lower trailing blade edge:

To ensure feathering functionality, and recognizing that in the real world propellers are subject to fouling, we added two small the foils extension to the lower trailing edge of each blade. This then allowed us to easily grind off during assembly the appropriate side leaving a small extension that when Sailing had the effect of biasing the blade such that the tip favoured movement in the head direction that’s preventing any winding up of the internal torsion spring which could lead to reverse engagement.

We needed to deal with growth on the blades, such as barnacles, oysters and also the rarer situation of for example seaweed or some other flotsam, such as a plastic bag fouling the blade while sailing.

Having this for an extension only on the base of the blade had no effect on motoring performance as the speed of advance at this lower section of the blade was really only matching the forward speed of the vessel so generating no forward thrust.

The analysis of these small foil extensions was undertaken by Flettner – a very early and highly respected German helicopter design engineer – who added a very small piece of metal sheet to the trailing edge of the rudder of an ME109 World War II fighter that could be easily bent with a simple spanner so biasing the rudder to remove any imbalance on the control stick.

Reverse screw switched from UNC ¼” to M8:

From approximately mid 2008 we increased the thread size to M8 which was a heavier screw more appropriate to the higher powered engines and larger blades e.g. 19.50” that were coming into service.

All Reverse rollers, either conical or Tri-roller design will fit over either screw, as the bearing dimensions have not altered.

The hexagonally head remains unchanged on both designs. It is a simple task to bore the existing thread with a 7.3 mm drill and re-tap using an M8 x 1.25 or standard M8 taper tap and stainless lubricant for those wishing to upgrade.

Reverse screw attachment:

It is important to ensure that each of the three M8 threaded Reverse screws ex SS 316  that hold the Tri-Rollers are retained securely in the boss of the unit. The screws are machined with a landing above the thread consisting of the 9.0 Ø Tri-roller bearing and when tightened pull down flush onto this flat.

Any side force on the screw thus generates a tension in the M8 screw as it attempts to roll up about the axis of the flat on the Tri-roller and the flat on the boss.

Each screw thread(s) is coated with a red high strength grade MIL spec Loctite™ 277 and torqued down using a torque wrench.

We then had two options to provide a margin of safety with a second level of security to ensure these do not come loose.

One obvious option is a spot of weld from the inside to the boss, but this excludes any potential removal for any reason at a future date. In addition this would introduce a different grade of SS 316 with the inevitable possibility of generating an electro potential across the joint and consequent corrosion.

The approach we use is simply to pin punch the underside of the mushroom headed boss near where the screw exits. This provides a slight distortion and tightens the boss down onto the thread of the screw making any removal very difficult – as all the normal tolerances between the thread of the screw and the thread tapped on the boss have been removed.

This still allows for removal of the screw, normally requiring the addition of heat to soften the Loctite™, but does require a much increased torque to undo the M8 screws.

Our experience from the over 200,000 screw years of service is that unless we have an environment experiencing extreme and abnormal corrosion with electron flows from an external source to the sharp thread edges, this mounting method has proved to be 100% reliable and excludes any possibility of an electro potential being generated from an additional grade of SS 316.

Four bladed K4 unit for  larger 50 – 75 hp installations:

With the advent and increasing popularity of higher horsepower installations, particularly units such as the Volvo D2–75 and similar Yanmar units required for the ever larger vessels becoming more popular we undertook a development program utilizing as many of the standard components as we could to address this market.

Due to the larger shaft sizes required for these higher power levels a new larger boss was required to accommodate up to 40 mm ISO shaft mountings or 1.500” shaft in SAE mounting.

Blade area to displacement is a critical design ratio for any propeller and the higher displacement typical of these large vessels required a full bladed unit. These are typically smooth running and meant that stress levels per blade were at the 20 hp level typical of the existing K3 3 bladed unit where 60 horsepower over three blades produced the same stress levels per blade.

Developments undertaken on the Tri-roller concept and mounting of the reverse screw was able to be completely duplicated on these larger units as was the Titanium blade mounting pins. The same blades were trimmed to a larger radius at the base to fit the larger boss. Thus a large portion of the components were able to be used on this K4 unit providing positive commonality and economic benefits and reduced component stockholdings.

The first unit was installed for trial in 2011 – there are now some 200 units installed since 2012.

Threaded Titanium Blade Attachment Pins:

For the initial years our units were produced using simple quarter inch pins and nickel silver that were pressed/tapped into a hole in the blade that had been drilled 0.004” under size.

We were not able to use SS316, as it is prone to crevice corrosion which was likely to be experienced in this application.

We have seen many units over the years where these pins have been 100% successful with no design issues emerging.

However if for some reason, which we did not recommend, the pins had been removed -each time this tended to drag material from the hole and they would become progressively less tight in the blade.

To offer a solution where we could eliminate corrosion with confidence and also ensure that the blade mounting pin was secure under any circumstance we designed a new blade retention pin ex 8 mm Titanium rod stock whilst retaining the quarter inch undersize hole used previously.

These pins turned from titanium rod stock have a slotted head on one end and a female thread on the other which will except a small male threaded and slotted cap. Both the headed end and the capped end require a standard 45° countersink leaving 25 mm in the blade.

Mounted with a blue medium grade Loctite™ on the thread we have yet to see a scenario in many tens of thousands of operating years of a single failure of this mounting system.

This is extensively documented on our webpage under:  Blade Mounting

TRI Roller – Reversing roller modification(s):

Coupled with the advent of the stiffer and stronger blade material and where rates on the blade roots which contacted the reverse rollers, we undertook an extensive research program to offer an approved solution to the simple conical roller that we had progressed to.

In addition we had found that despite extensive instructions to the contrary, the fact that the antifouling was often carried out in a yard and not by the owner, we had to assume that the whole unit would be antifouled and this would invariably see the reverse rollers, whose function was to rotate upon contact with the blade root during a reverse function seized up with antifouling paint or Prop-Speed.

Using a sliding motion, rather than a rolling motion, would very dramatically reduce the point pressure on the blade root and consequently reduce the wear rate.

After extensive trial and error we found that what we term a Tri-Roller, which was basically a conical roller with three flats machined on it would generate a sliding motion with low contact pressure per unit area during a reverse function.

However to ensure it did not seize up from antifoul application, these three flats would allow the reverse roller to be rotated through 120° for each reverse function engagement using the mechanical force of engagement.

Any addition we designed a small press fit polypropylene cap that could be simply tapped into the upper surface of this or the previous version conical roller – as an additional insurance to keep antifouling and any growth deposits away from the bearing area of the roller and mounting screw.

We have been using these for many years now and have yet to see a Tri Roller that has frozen and regard this as the optimal approach to the design requirements involved.

In addition – to further reduce anywhere on the blade root we have machined a small tapered cylindrical surface between the conical surface and the flat. We also linish this transition to ensure that the leading edge of the sliding surface does not dig in or scrape the composite material during reverse engagement.

Given the multitude of clutch types that exist in the market we have also added what we term and “ Impact Screw “ to the blade root at the point of maximum pressure experienced during a reverse engagement function, which occurs approximately when the blade is in a 45° pitch position, on its way to the normal 24° maximum pitch of the Kiwiprop design.

This provides a metal on metal contact from the Tri-roller to the blade root and has virtually eliminated wear at this contact point.

This is well documented on our website under the heading: Impact Screws

Blade root  V Seals:

The very early units we produced did not have a seal in the blade root, but depended upon the low tolerances and shape of the blade extending over the spherical blade carrier which prevented high-pressure water forcing into the blade / blade carrier and removing grease over time.

To minimize the grease removal we then added an O-ring to the base of the blade which provided improved sealing. These readily available and low cost seals did provide an improved level of ceiling and a small amount of flexibility to accommodate the inevitable tolerances which can change over time between the blade route and its mounting.

To provide a further improved level of sealing, we designed a carbide cutter to machine a stepped recess in the blade root and then made a matching die to produce a softer V – Seal with the ability to accommodate the inevitable wider range of tolerances from assembly variations and wear over time between the blade root and blade carrier casting and leg.

We are confident that these seals do provide a higher level of grease retention, and by the very small quantities required when greasing the blades post haul out. Care must be taken at this stage, as carefully described in our video and manual, not to over pressure when greasing as the seals are so effective they can be distorted from the very high pressures that can be generated with a normal grease gun.

Material changes – Glass reinforced poly-propylene Nose Cones:

The first units we produced in both Shaft and Saildrive configuration used a white Acetyl Nose Cone for some years. We needed a material available in rod format for machining purposes.

Acetyl has many attributes for this role, it is widely available, is very tough and not prone to cracking so accepting of the four cap screws that hold the two halves of the Nose Cone(s) together. It does however expand over time when continually immersed as this component inevitably is in service. This could be accommodated by simply providing slightly greater tolerances when new – but being an un-necessary variable – in a perfect world it would not be present.

On the advice of our plastics engineer suppliers we switched to a much harder PETP which is stable underwater but more prone to cracking – particularly if overstressed. Low temperatures for example which shrink the length of the cap screws holding the two halves provides additional stress. Overtightening without a torque wrench also could lead to overstressing. A small percentage of these displayed cracking after some years of service, but continued to deliver the required functionality of transferring forward thrust to the boss and accepting the tail of the internal torsion spring to pre-tension required for the feathering function.

In 2008 we found that we could obtain in rod stock format – a glass reinforced polypropylene product from the US which was not available in New Zealand that met all our material design requirements, very tough, very strong and totally impervious to and dimensionally stable under water. We have used this product exclusively now for nearly 13 years and have yet to experience a failure.

Internal sleeve and aft washer switch to Vesconite:

The first units we produced were from nickel aluminium bronze castings which provide an excellent bearing surface between similar metals. Over time – to cater for increased volume production we switched initially the blade carrier casting, followed by the boss that fits to the shaft taper or spline in the case of the Saildrive to a lost wax investment casting in SS 316.

SS 316 has many admirable properties for continuous immersion in salt water but is prone to what is termed  “ galling “ whereby two moving soft surfaces  “ gall “ or catch and freeze when used in a moving bearing type situation as we had with the 100º of movement between the boss and blade carrier during a reverse engagement function.

To address this we needed the material that was impervious to both saltwater and hydrocarbons as we would be lubricating the bearing and retaining grease inside the unit.

A fibre reinforced composite product from South Africa used extensively in marine and heavy industry labelled Vesconite was selected.

We inserted a sleeve between these two components on the bearing service, and a washer with an L-shaped profile to assist in grease retention between the boss and blade carrier aft joint contact surfaces.

These have proved very resilient over long periods of time and have delivered the functionality required. The larger K4 four bladed unit also has a washer at the forward end as the Nose Cone for this unit is also from SS 316.

Web site development:

Over the years we have developed a very extensive website containing many hundreds of pages of what we believe to be relevant information that is useful to a Kiwiprops or potential Kiwiprops user.

The website has been maintained on a very regular basis to always be current and provide an authoritative source of information relating to the unit.

To assist visitors to the website, on the upper right hand side of the homepage there is a keyword search function that covers the entire database.

Simply entering a keyword will bring up every reference in our database containing that keyword – a very useful function.