Kiwi prop fail

[kenbo]

Keys do not keep a prop, Kiwi or others, on the shaft. The taper of the shaft and prop are what hold it on. Did you mate the prop's taper to the shaft, did you check that you had contact between the prop and shaft over 70% of the taper's surface with machinist's blueing? That shaft and prop were milled on two very different machines by two different machinists. The installer of the prop needs to do the final mating of the two surfaces

Keys are supposed to be loose fitted on the top and bottom of the keyways. They only serve to prevent rotation during shaft nut tightening.

Ken

[Captsteve53]

Originally Posted by kenbo

Keys do not keep a prop, Kiwi or others, on the shaft. The taper of the shaft and prop are what hold it on. Did you mate the prop's taper to the shaft, did you check that you had contact between the prop and shaft over 70% of the taper's surface with machinist's blueing? That shaft and prop were milled on two very different machines by two different machinists. The installer of the prop needs to do the final mating of the two surfaces

Keys are supposed to be loose fitted on the top and bottom of the keyways. They only serve to prevent rotation during shaft nut tightening.

Ken

Ken, While i agree with most of your above statements (especially the mating tapers and use of bearing blue) and it's importance in keeping the prop in place, your follow on final statements regarding the key way and keys is way off the mark ,and would like to set the record straight! Before Novice prop fitting yachties get the wrong idea.

Here's a few facts re key ways and there fitting:The key is a critical element in successful transmission of torque:

The fit of the key is critical in assuring sufficient capacity of the shaft-to-coupling hub interface.
1)A loosely fitted key can roll or shear when heavily loaded causing the loss of the coupling/boss/propeller
2)The fit of the key is critical in assuring sufficient capacity of the shaft-to-coupling hub interface. Be sure to check that the key fits tightly in the shaft key way. This is normally a "Tap fit"
3)the key has a sliding fit (but not be too loose) in the coupling hub key way; and that the key has a clearance of 0.003 to 0.020 in (0.08 to 0.51 mm) with the hub key way at the top of the key.

A lot of the above is quoted directly from the "Machinist Handbook"which is recognized as the "Bible" for Engineers being in publication since 1914, for reference any thing regarding Engineering AT ALL is in this publication,

Cheers Steve (MIIMS-Lloyd's Maritime) NZ Marine Chief Eng 1

[kenbo]

Originally Posted by Captsteve53

Ken, While i agree with most of your above statements (especially the mating tapers and use of bearing blue) and it's importance in keeping the prop in place, your follow on final statements regarding the key way and keys is way off the mark ,and would like to set the record straight! Before Novice prop fitting yachties get the wrong idea.

Here's a few facts re key ways and there fitting:The key is a critical element in successful transmission of torque:

The fit of the key is critical in assuring sufficient capacity of the shaft-to-coupling hub interface.
1)A loosely fitted key can roll or shear when heavily loaded causing the loss of the coupling/boss/propeller
2)The fit of the key is critical in assuring sufficient capacity of the shaft-to-coupling hub interface. Be sure to check that the key fits tightly in the shaft key way. This is normally a "Tap fit"
3)the key has a sliding fit (but not be too loose) in the coupling hub key way; and that the key has a clearance of 0.003 to 0.020 in (0.08 to 0.51 mm) with the hub key way at the top of the key.

A lot of the above is quoted directly from the "Machinist Handbook"which is recognized as the "Bible" for Engineers being in publication since 1914, for reference any thing regarding Engineering AT ALL is in this publication,

Cheers Steve (MIIMS-Lloyd's Maritime) NZ Marine Chief Eng 1

Steve thanks for clarifying my post. I was clearly too parsimonious with my words regarding fitting a key into a keyway. I assumed, and apparently erroneously, that most would know the key should not be a sloppy fit in the shaft and should be a close, sliding fit through the prop hub. The loose fit I referred to was the space between the top of the key and the prop hub. Otherwise how could the prop be inserted entirely home on the shaft without binding.

In my experience it is this lack of clearance of 0.003 to 0.020 in at the top of the key that causes most prop fitment failures. But I believe it's beyond most yachtsman to accurately measure that clearance without using Plastigauge or it's equivalent. That's why I stated "loose fit' at the top and bottom.

Thanks again for clarifying it's very helpful.

Ken

[barnakiel]

I did some research on Kiwiprops online and offline and then I took it off my list. They seem to have more problems than some alternatives. I know they are less expensive too, but I am into the more expensive and less troublesome options.

Now on a fixed 3-blade. I wish I could have a feathering something and sail a bit faster.

Perhaps Kiwiprop wants to send me a unit for testing on the Atlantic loop? I can return it in September 2018 with about 10k miles on it.

Cheers,
b.

[daletournier]

Originally Posted by barnakiel

I did some research on Kiwiprops online and offline and then I took it off my list. They seem to have more problems than some alternatives. I know they are less expensive too, but I am into the more expensive and less troublesome options.

Now on a fixed 3-blade. I wish I could have a feathering something and sail a bit faster.

Perhaps Kiwiprop wants to send me a unit for testing on the Atlantic loop? I can return it in September 2018 with about 10k miles on it.

Cheers,
b.

Barnakiel, I dont have one on my current boat and most likely won't have one again but as mentioned earlier ive done many miles with one on my last boat therefore can speak from experience.

When you say more problems than alternatives thats just not true and damaging to a mans business. On the internet you get a timy percentage of users commenting and even then there isnt that many bad reports.

The only issue I have experienced and know others have also is, you must keep the prop greased, if you dont they won't open well in reverse and will overload smaller engines like the yanmar 3gm30f. Its easy to grease them in the water. Keep in mind my very expensive maxprop (which I like) also needs greasing or it can lock up.

Also the blades can get dints, small ones you can file out. It is possible to break a blade but you really need to hit something very solid very hard, to me this sacraficial breakage (which ive never had) is better than possibly something else more serious. Blades are easy to change in the water.

They also have a sweet spot where they are abit smoother, meaning a certain rpm.

That's about it, really its pretty good for the money, not perfect but good.

Feathering and folding props are more complicated and come with possible troubles because of this, fixed props are less trouble but don't have the benefits.

[barnakiel]

Fully agreed. I am not bashing Kiwiprop just saying I took it off my potentials list after an extensive online and offline research. This is about my personal preferences for price/potential problems ratio, not about a product being bad. Every client / boat owner is different.

I am aware how many 'product problems' are in fact installation problems, etc. Been there done that (the costly mistakes) myself.

Probably there is a way for the prop to be fitted and locked on so that it cannot be lost. Our 3-blade fixed VP one has a key and then a slotted nut with a pin doing thru a hole in the shaft. I have never heard of one lost and there are hundreds of thousands of these in our home waters. If such archaic designs done without CAD and computers could be done well back in the 60'ies (and I guess ages before too) then what stops modern businesses from designing equally reliable props today will likely remain an eternal enigma.

So I am still into something that feathers. And stays on the shaft.

Cheers,
b.

[mikereed100]

Originally Posted by S/V_Surya

I fitted the prop. I fitted the prop with complete manufacturer's specifications liberal use of red Loctite. All screws nuts Etc were torqued to manufacturer's specifications.
I want to stick my neck out and say that there is a design issue.
Prop depends on a chemical bond hold it in place and exposed to air seems to break down. It needs to have a mechanical connection as a Fail-Safe.

If that's the case, I am at a loss. Unfortunately the prop is no longer available for post-mortum. It would be interesting to have a look at the boss nut and set screws.

[Captsteve53]

Just re visiting Barnakiels post re the keyway , it is one area where I agree and have seen numerous mistakes made with the height of the key way itself, where it binds on the boss rather than allowing a full mating of the shaft taper, (Not trying to be confrontational but seeking a probability answer for all that may be using these prop's now or in the future),

Bottom line is that to come off (leaving no trace or damage) then the lock screws need to come out first/then the securing custom nut un screws and the prop complete is lost!,

A failure with say a "bad boss casting"and a crack then falling apart is about the only other feasible situation, if you had a GPS track perhaps a under water survey along the track with a metal detector?? (would love to find the real situation!!)

Cheers Steve

[mikereed100]

Originally Posted by Captsteve53

Just re visiting Barnakiels post re the keyway , it is one area where I agree and have seen numerous mistakes made with the height of the key way itself, where it binds on the boss rather than allowing a full mating of the shaft taper, (Not trying to be confrontational but seeking a probability answer for all that may be using these prop's now or in the future),
Cheers Steve

While this makes sense, I don't see how it could cause the set screws (both of them) to back out, allowing the nut to unscrew itself, unless the vibration it caused was enough to overcome the pressure of the set screws?

I remove my props in the water at least twice yearly so that I can clean them well and perform routine maintenance in the cockpit. When I re-install them I sometimes use blue Locktite (never red) and sometimes use nothing. I have nearly 4,000 combined hours on my Kiwis and have never had an issue with the set screws backing out.

By the way, this system of set screws on the prop nut is one of my favorite features of Kiwiprops. Instead of using a prop puller I simply back off the set screws just enough to allow the nut to turn. In this way, as you back off the prop nut the prop is pulled off the shaft by the set screws without the use of violence or profanity.

[lateral]

Have you every greased prop in the water, Mike, without removing prop?

[GILow]

This is an old-ish thread, reawakened at an interesting time.

I have just fitted a Kiwiprop to my boat, and, being me, did all the work myself so I got to know the prop pretty well.

Regarding the OP’s original problem, I make the following observations based on my own installation experience:

1. The failed key way explanation is plausible. I admit it is rare for keyways to fail, but if the key had sheared, the prop would easily spin on the shaft and in doing so undo the retaining nut in very short order. (Assuming shaft rotation was clockwise when viewed from the stern). This is because the weird locknut is positively linked to the prop body to prevent it from undoing with the grub screws, which in my model use one extended grub screw that engages in a hole on the retaining nut, thus eliminating the need for friction. I would not expect to see any scoring from this as it would be practically instantaneous, maybe four or five revolutions of the shaft in relation to the prop, certainly less than ten.

Second, and this is a bit dark, but given the boat had been on the slips, what’s the chance someone had actually tried to steal the prop while it was accessible and had been either interrupted or had failed to figure out to remove it after undoing a few bits, such as the grub screws. The prop is a smart looking bit of kit, it does not look cheap, someone may have been tempted.

In summary, having installed mine, I see no design flaws that would lead to this sort of failure, so I would be looking for something well out of the ordinary to cause it to fall off.

[Wotname]

Originally Posted by GILow

This is an old-ish thread, reawakened at an interesting time.

I have just fitted a Kiwiprop to my boat, and, being me, did all the work myself so I got to know the prop pretty well.

Regarding the OP’s original problem, I make the following observations based on my own installation experience:

1. The failed key way explanation is plausible. I admit it is rare for keyways to fail, but if the key had sheared, the prop would easily spin on the shaft and in doing so undo the retaining nut in very short order. (Assuming shaft rotation was clockwise when viewed from the stern). ............

This comment surprises me and while I admit I am not familiar with the Kiwiprop, I understood it used a conventional taper arrangement when fitted to a conventional shaft drive.

If so, the retaining nut should supply sufficient pressure on the taper to ensure the torque is transmitted by the friction of the taper surfaces rather than the key alone. If going forwarded, the prop blades are increasing the load on the taper surfaces. Going astern is a different matter although I have seen props where the retaining nut is clearly loose (i.e. daylight between the nut and the prop) and the prop was still sufficiently tight on the taper for the problem not to be noticed (forward or reverse) until a visual inspection showed it up the on the slip.

[Jim Cate]

If so, the retaining nut should supply sufficient pressure on the taper to ensure the torque is transmitted by the friction of the taper surfaces rather than the key alone. If going forwarded, the prop blades are increasing the load on the taper surfaces. Going astern is a different matter although I have seen props where the retaining nut is clearly loose (i.e. daylight between the nut and the prop) and the prop was still sufficiently tight on the taper for the problem not to be noticed (forward or reverse) until a visual inspection showed it up the on the slip.

Quite agreed, Wottie. It is the taper that should be absorbing the torque, not the key. CF thrashed this out quite recently in a thread about the need for lapping a prop onto the taper in a new assembly.

Anyone who has struggled to remove a prop from a tapered shaft should recognize the truth of this idea!

And Merry/Happy to you both, Wottie and Matt, from the Insatiables!

Jim

[GILow]

Yes, I agree the taper should take the torque, but things go wrong sometimes. Maybe it was never tightened properly. There are lots of traps with the job as we all know.

And an interesting coincidence that it failed so close to when the boat had been out of the water. Something dried out perhaps?

Anyway, my point about the failed key way theory was more to say that the way the prop is attached means that IF the key had failed, it is practically guaranteed that the prop will come off immediately on a right hand rotation shaft because of the way the retaining nut is attached to the prop hub.

By contrast, my old fixed three blade prop would just have freewheeled as the nut was prevented from turning by a good solid cotter pin.

I actually reckon my dark theory about someone trying to pinch it is the most credible.