Kiwi Props - How strong are they?

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DaveW

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How strong are they - anyone know?
Would a Kiwi Prop stand getting a rope wrapped round it without breaking?

What are your views?

Cheers, Dave
 
From their web site;
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7: How strong are the composite blades?

Clearly the blades are not as strong or as stiff as bronze, but the issue is - are they strong enough for the purpose for which they have been designed. Virtually all modern aircraft have composite propellers.

Zytel contains ~ 35 % glass by weight and is thus both very strong and stiff. The black colouring is not carbon based and thus does not have a corrosion potential. DuPont have extensive technical information on their web page regarding the physical characteristics of the many different grades of Zytel they have available.

By choosing a 3 bladed design over a 2 bladed unit the forces involved are immediately reduced to 33 % per blade vs 50 % - a 33 % reduction.

Another design issue is that composites and the economics they enjoy allow a blade to be sacrificed in a catastrophic situation. With a substantial impact on the blade tips ( always the first part to hit ) they invariably tear from the mounting hole outwards leaving the castings undamaged.

We believe it is better to loose an easily replaceable blade costing $ 100 than a whole propeller or drive train when hitting the ground or a floating log or mooring chain. This can be very expensive in a Saildrive installation where the whole leg is at risk.

Ropes caught around the unit will simply stall the engine and in each of a number of cases where this has happened to date the unit has emerged undamaged. The blade form with well rounded leading edges at the root is designed for these events.

We have not yet had a blade fail in service and are confident that they are stressed correctly for the application they serve. Remember composite propellers are now freely available for outboards up to 250 hp.

So in simple terms the answer is - quite strong enough !


# 8: Should I buy spare blades to carry on board?

Our advice is that if you have a catamaran with no keels that is going to hit the props first if ever grounded - carry a spare set of blades.

If you are heading off on a world cruise with the potential to be in some out of the way places or cruise in high risk areas where there are known problems such as dead trees or coral heads - then take a spare blade or set.

In many ways the cheapest and best insurance is to carry a cheap fixed 3 blade unit to get-you-home.

For general use we see no more or less need to carry spare blades on board than you would with any other prop. We have very little demand for spare blades other than in the above situations.
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I have no experience of these props but if you do a web search it is very difficult(impossible) to find any serious problems with them.
 
From what I hear, the concept of these props is good, but the practicalities aren't.

If you hit something with a propeller, you will tend to damage several blades. If the force is hard enough to break one blade then surely it is going to do extensive damage to the next blade(s) also. So you'll need to replace all the blades at once to keep it in balance. By the time you've done that, then you've got the cost of a new bronze propeller.
 
<<If you hit something with a propeller, you will tend to damage several blades. If the force is hard enough to break one blade then surely it is going to do extensive damage to the next blade(s) also. So you'll need to replace all the blades at once to keep it in balance. By the time you've done that, then you've got the cost of a new bronze propeller.<<

Total conjecture with little evidence in theory or practice.

The blades will deform and bend, which will gently decelerate any loading thus reducing its shock value on the prop or the hub. I have an Ambassador Stripper in front of the prop, and it looks as if it has done some work this year. No apparent damage to the prop, but some scuffing to the leading edge.
Second point is that the blades are completely free to swing and are held by centripetal force in the outflung position by the shaft rotation. In the event of a big wrap or a hard object, they will deflect back to their zero value as soon as the blocking or hit forces equal max torque on the shaft, and thus put the blade in 'neutral'.
This ability to go into neutral also assists letting the blocking object past the blades and away to stern.
My guess, as an engineer, is that the angle of defllection before failure after reaching the elastic limit (i.e. the point at which the blade would not return to its original point) would be sufficient to allow the blocking object past.

You also have to remember that in the majority of installations, you have a keel to deflect large nasty lumps out of the way, or a skeg. I have actually collected more on my twin rudder setup (including a mooring buoy, but least said, soonest mended) in the 2 years of operation, as it seems that my fin/wing keel pushes the fishing nets out to the rudder path!

In 7 years of service, there have been no total failures of these props. There were some minor issues with the reverse engagement mech gumming up, but that is long sorted.

Please also bear in mind that I changed all 3 blades in 90 seconds this year when I needed to go up a size/pitch for better performance.

Testing for aircraft propellers includes max pitch/prop speed and then firing a flock of birds into them at 500MPH. Not an easy test to pass, and certainly not one that a prop made in bronze would pass.

Lastly, the prop will work with 2 blades on, but it will be a shaky ride!!!

I have other issues which I have posted elsewhere, but structural integrity is not one of them!! /forums/images/graemlins/smile.gif
 
In my experience with aluminium and bronze propellers, blades have got knocked and bent fairly evenly when coming across something solid.

I'm not saying that all the blades will be ripped off, but that if one blade is damaged, then it is unlikely that no others have escaped damage, so you'll be having to replace more than one blade.

Strongly recommend NOT running with a blade missing - gearbox damage could result.
 
I dont get your point.
I have said that the damage is unlikely to be a catastrophic failure, and wont even get bent like a bronze blade.
I bet you havent even been within 10 metres of a Kiwiprop, or had an engineering discussion in detail with the manufacturers, or done a full decision matrix for comparison to similar products, so what you are proposing is purest hearsay.

Of course you wouldn't normally run with a blade missing, but I once limped back into Ramsgate on 1 blade of a folding prop.
 
[ QUOTE ]
Clearly the blades are not as strong or as stiff as bronze, but the issue is - are they strong enough for the purpose for which they have been designed. Virtually all modern aircraft have composite propellers.

[/ QUOTE ]Yes but aircraft propellers aren't in water with tree truncks, rope and lobster pots.
I've got a Kiwiprop and find it OK what will happen with a wrap or a collision I don't know.
 
Are you categorically stating that if a blade gets severely damaged, that all the other blades will remain intact and replacing just one blade will result in a still balanced propeller, with no risk of damage to the drivetrain.

I've plenty of experience with alu and bronze props, and know what the look like after a colision or rough use.
 
Interesting and informative opinions from several respondents.
I have no doubt that the Kiwi Prop is 'fit for purpose' as a means of pushing a boat through the water, my worry is that should you become tangled in a net or lobster pot etc that the blades would break.
A bronze prop would tend to come through more or less unscathed - I'm not so sure about composite blades.
You could end up in S**t creek without a paddle...!!

Thanks for the posts, Dave
 
Sorry all, but I lost it a bit there.

Suggest that a visit to the DuPont website to get the material properties and case studies of existing high performance applications would help anyone who is researching this type of application. Although not specific to huge logs in the prop as suggested above, the material properties in the downloadable PDF suggest a high flexural modulus, and a very high yield strength at 50%RH.
As I have worked in leading edge and common plastics in automotive and aviation for some time, and seen the development of high temp and pressure use in engines and transmissions, I can only state that I have full confidence in the choice I made for a propulsive device.
A search in google for site:ybw.com kiwiprop will bring up a lot of data and experiences for those for and aganst such a design.

DuPont Zytel design case studies

Zytel Material Properties

Its been an off week so far, it will get better soon /forums/images/graemlins/smirk.gif
 
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