Propeller dynamics: blowout etc

[ForbesZ]

Wow, you must be an expert. You probably only need on blade on your prop then. Lol. So now you knew all along why blowout happens, but were just pretending not to know before?? Very strange.

Not an expert but do have experience with cracked propeller blades, surfacing props, blowout, as well as a degree in physics which included semesters about fluid dynamics (among many other things). This is why I appreciate the responses from Gimmie Fuel, who evidently has a strong engineering background.

Anyway, the original question asked was what can cause a propeller blade to crack when the prop hasn't hit anything. I mentioned that blades often crack as a result of the constant loading and unloading of water on the blades when surfacing a prop. All the same things that I've said in this thread. I also suggested that where the crack develops is approximately the location where the blade pierces the water's surface. You suggested it was ridiculous to think the location of the crack was indicative of how much the propeller blade is piercing the surface of the water, and then brought up blowout for some reason.

So in the end, I never mentioned anything about blowout - you brought it up. Further up in this thread you suggest that it was to debunk the theory that the blades cracking is a result of surfacing the prop, but I'm not seeing anywhere in this thread where you have successfully made the connection between blowout and cracked blades. Perhaps you could point it out or something. I have always thought and continue to think you are 100% correct about everything you've said about blowout, it's just that blowout in and of itself doesn't explain what the original person asked, which is what can cause a blade to crack when the propeller has never hit anything. So feel free to continue with reporting back your research on blowout. I've offered my opinion on what causes propeller blades to crack when they haven't hit anything and still see no evidence of it being directly related to blowout. When blowout happens, the driver and everyone within earshot of the engine knows it. And none of the blowout incidents I've seen or heard of have resulted in a cracked blade. I'm not saying it can't happen, all I'm saying is that the props involved in the blowout incidents that I've seen or heard of have been fine.

 

[karzrus]

The comments in ForbesZ post above conform to my understanding as well.

Cavitation can and will occur on the intake side of an impeller/propeller fully submerged in a liquid as soon as the conditions exceed the performance capability of that propeller. Surfacing and/or exhaust gases do not need to be present for it to occur. But while there will be a performance loss and there can be destruction of the impeller/propeller, that destruction appears differently in the form of pitting and erosion on the blades.

Surfacing damage where one or more blades break due to work hardening from repeated flexing beyond the parent materials ability endure, does not seem to appear the same or conform to the cavitation damage. I have seen a lot of cavitation damage in my work environment. None of it looks like the damage seen on a stainless steel propeller that looks like new except for the blade broken off, a second blade with a crack starting and a 3rd and 4th blade that look perfect.

The technical information posted at the beginning of this thread, confirms what I understand, at least as I read it.

 

[karzrus]

I explained what forces will fatigue a blade causing breakage which was asked. Prop blades can break on a well setup PERFORMANCE boat even if it doesn't experience cavitation or blowout events. That can happen regardless if cavitation is involved. Cavitation is different than blow out as the article says. Can cavitation lead to blow out? Yes if severe enough. Gearcase shape also plays a big role. Many surfacing setups do not have below waterline exhaust either (outboards centered in this discussion do for the most part however)Most sustained cavitation will cause erosion of the blade and/or gearcase and can be seen as "cavitation burn." Unbalanced loading situations (such as a surfacing propeller) create noise, vibration, and harmonics within the system (NVH) which cause fatigue of the blades. Remember, everything is a spring and does have deflection under load. It is just a matter of how much or how little.

I didn't notice this post before, but it also conforms to my understanding and experience of the differences between cavitation damage and the form of fatigue stress blade failure seen by some.

 

[karzrus]

blacktruck, I'd like to hear your explanation for what is happening to Chuck. I just don't read about any drastic blowout incidents in any of his posts even though he is obviously running hard each time he dumps a blade.

see attached thread
http://checkmate-boats.com/forums/showthread.php?t=31566

Even though I'm sure Chuck won't mind a good answer and some needed help, please answer here first rather then polluting Chucks thread. Maybe we can all learn and Chuck's friends on the forum can get him out of winter hibernation mode to look up your answer.

 

[Jimway]

A little fuel for the fire, as it were. There are several forms of cavitation, not to mention ventilation, blowout, loading, unloading, noise, vibration, harmonics, and even heat. Cavitation alone could be rounded into several different groups including vortex, bubble, face, sheet, cloud, root and tip. The cavitation bubbles collapsing can cause increadible hammer effect to the propellor. In addition, the temperature in the immediate area of the collapse can be much higher than one might think. Enough to cause the adjacent layers of material in the prop to become brittle....

 

[DropTop5.0]

I can not confirm or deny Chuck has ever surfaced his props.

 

[blacktruck]

Not an expert but do have experience with cracked propeller blades, surfacing props, blowout, as well as a degree in physics which included semesters about fluid dynamics (among many other things). This is why I appreciate the responses from Gimmie Fuel, who evidently has a strong engineering background.

Anyway, the original question asked was what can cause a propeller blade to crack when the prop hasn't hit anything. I mentioned that blades often crack as a result of the constant loading and unloading of water on the blades when surfacing a prop. All the same things that I've said in this thread. I also suggested that where the crack develops is approximately the location where the blade pierces the water's surface. You suggested it was ridiculous to think the location of the crack was indicative of how much the propeller blade is piercing the surface of the water, and then brought up blowout for some reason.

So in the end, I never mentioned anything about blowout - you brought it up. Further up in this thread you suggest that it was to debunk the theory that the blades cracking is a result of surfacing the prop, but I'm not seeing anywhere in this thread where you have successfully made the connection between blowout and cracked blades. Perhaps you could point it out or something. I have always thought and continue to think you are 100% correct about everything you've said about blowout, it's just that blowout in and of itself doesn't explain what the original person asked, which is what can cause a blade to crack when the propeller has never hit anything. So feel free to continue with reporting back your research on blowout. I've offered my opinion on what causes propeller blades to crack when they haven't hit anything and still see no evidence of it being directly related to blowout. When blowout happens, the driver and everyone within earshot of the engine knows it. And none of the blowout incidents I've seen or heard of have resulted in a cracked blade. I'm not saying it can't happen, all I'm saying is that the props involved in the blowout incidents that I've seen or heard of have been fine.

No it was never about cracked blades or hitting anything originally, it was debunking the load / unload statement. I clearly stated that many times. I can't believe you have any experience because the phenomenon you've described, load > unload, causes blowout (the whole point that discredits that idea), which is clearly discussed in the article by someone who is an expert. The fact that you keep repeating that there's a full load / unload event continuously, demonstrates you don't know what you're talking about. Furthermore, a blade breaking would absolutely cause blowout.

To the question about blades cracking and breaking, I already stated, it is extremely rare for a conventional performance boat, like the boat that was discussed, with a stock high performance prop from a reputable manufacturer that is setup properly, at an elevated transom height, to crack or brake a blade; they are designed exactly for that purpose. That is a fact.

There is nothing left to debate, the article, my responses are all crystal clear and factual. I have more experience than you and it seems like you are trying to create arguments that are not there.

Just for fun, which props are you seeing crack so often? And please post pictures and describe the setup. This should be really interesting. I hope you don't kill anyone with your setup that breaks and cracks props so frequently.

 

[blacktruck]

blacktruck, I'd like to hear your explanation for what is happening to Chuck. I just don't read about any drastic blowout incidents in any of his posts even though he is obviously running hard each time he dumps a blade.

see attached thread
http://checkmate-boats.com/forums/showthread.php?t=31566

Even though I'm sure Chuck won't mind a good answer and some needed help, please answer here first rather then polluting Chucks thread. Maybe we can all learn and Chuck's friends on the forum can get him out of winter hibernation mode to look up your answer.

I took a quick glance and (please correct me if I'm wrong) but those props have been modified, so all bets are off. They shouldn't be modified at all. In fact, Mercury racing specifically adjusted the cup height on the Bravo 1 XS props to create the odd pitches, aimed at the new big outboards, like the 400. So you should be able to dial in a stock prop, no problem.
You can read about it on the Mercury Racing website and from Mercury Racing themselves they say from a customer question:
"There is no need to have the XS props worked. They are good to go right out of the box. Leading edges are matched, trailing edges are matched and cupped and props are zero balanced. The shortened exhaust barrel minimizes stern lift."

Mercury Racing is super helpful, so definitely lean on them if you have to. They have always been super helpful to me for everything.

I was just in an MTI like this one a few weeks ago, can run over 100 all day, no broken blades, no cracking

 

[karzrus]

I took a quick glance and (please correct me if I'm wrong) but those props have been modified, so all bets are off. They shouldn't be modified at all.

Uncertain about props one and two( considering the source, likely modified)Prop four was a rebuild.

But prop three was out of the box new. Most interesting to me is that four different props all broke the same way and the surviving blades look like new.

 

[karzrus]

No it was never about cracked blades or hitting anything originally, it was debunking the load / unload statement. I clearly stated that many times. I can't believe you have any experience because the phenomenon you've described, load > unload, causes blowout (the whole point that discredits that idea), which is clearly discussed in the article by someone who is an expert.

For me it was about cracked blades like Chuck is seeing. I don't see cavitation burn, I see failure that looks like work hardening. Like what happens to coat hanger wire you bend back and forth till it breaks.

But also I don't think you or the article have factually drawn a direct correlation between the loading on each individual blade as it enters and leaves the water in a surfacing application and blowout. The blowout described is for the propeller as a whole, rather then each blade. There is no mention of the propeller running half in and half out of the water. There is no statement that in those situations the boat or propeller unify or fully mix the air and water to create a homogenous mixture that evenly loads each blade the same at all times.

Yes, the whole propeller is constantly loaded and there is no blowout, but not each blade.

This is a little like in a car with an open differential(non posi) one tire spins and the other does not. The two tires are not evenly loaded, but the motor, transmission, drive shaft and ring and pinion are. They don't care that one tire is doing all the work.

The whole rest of the boat does not care if only one or two blades are doing the work at any given moment or that the blades are taking turns doing the work. The boat only cares that at least one blade is working at any given moment.

 

[ForbesZ]

No it was never about cracked blades or hitting anything originally, it was debunking the load / unload statement. I clearly stated that many times. I can't believe you have any experience because the phenomenon you've described, load > unload, causes blowout (the whole point that discredits that idea), which is clearly discussed in the article by someone who is an expert. The fact that you keep repeating that there's a full load / unload event continuously, demonstrates you don't know what you're talking about. Furthermore, a blade breaking would absolutely cause blowout.

To the question about blades cracking and breaking, I already stated, it is extremely rare for a conventional performance boat, like the boat that was discussed, with a stock high performance prop from a reputable manufacturer that is setup properly, at an elevated transom height, to crack or brake a blade; they are designed exactly for that purpose. That is a fact.

There is nothing left to debate, the article, my responses are all crystal clear and factual. I have more experience than you and it seems like you are trying to create arguments that are not there.

Just for fun, which props are you seeing crack so often? And please post pictures and describe the setup. This should be really interesting. I hope you don't kill anyone with your setup that breaks and cracks props so frequently.

Blah blah blah ... whatever dude. You want to see someone that doesn't know what he's talking about take a look in the mirror. Like karzrus said, the discussion has always been about cracked blades and it was for Chuck. You started off attempting to tie it to blowout. I've never mentioned blowout despite your suggestions to the contrary and never wrote anything like what you say above. Karzrus and Sam both know what I wrote so you're not fooling anyone. Anyway it's clear that responding to this thread is pointless so I'm out. I have better things to do than this. You're the king and I'm a dumb idiot with no experience. Feel better??

 

[karzrus]

Well, I don't like the reality that maybe I'm wrong and don't know it yet.

I keep waiting for either myself or blacktruck to have an epiphany and type "Oh I get it now"

Maybe it won't happen for either of us, I'm glad we have a forum to discuss it and hopefully learn without getting frustrated with the other guy or getting personnel. That's the hard part.

 

[blacktruck]

Blah blah blah ... whatever dude. You want to see someone that doesn't know what he's talking about take a look in the mirror. Like karzrus said, the discussion has always been about cracked blades and it was for Chuck. You started off attempting to tie it to blowout. I've never mentioned blowout despite your suggestions to the contrary and never wrote anything like what you say above. Karzrus and Sam both know what I wrote so you're not fooling anyone. Anyway it's clear that responding to this thread is pointless so I'm out. I have better things to do than this. You're the king and I'm a dumb idiot with no experience. Feel better??

Sorry for the misunderstanding, I started the thread, so I know what it is about. You must be confusing it with a different thread. The original started with the Starliner, full load / unload on props. That's it. This thread, which I started clearly states what it is about from the onset. Sorry if you got mixed up.

 

[blacktruck]

For me it was about cracked blades like Chuck is seeing. I don't see cavitation burn, I see failure that looks like work hardening. Like what happens to coat hanger wire you bend back and forth till it breaks.

But also I don't think you or the article have factually drawn a direct correlation between the loading on each individual blade as it enters and leaves the water in a surfacing application and blowout. The blowout described is for the propeller as a whole, rather then each blade. There is no mention of the propeller running half in and half out of the water. There is no statement that in those situations the boat or propeller unify or fully mix the air and water to create a homogenous mixture that evenly loads each blade the same at all times.

Yes, the whole propeller is constantly loaded and there is no blowout, but not each blade.

This is a little like in a car with an open differential(non posi) one tire spins and the other does not. The two tires are not evenly loaded, but the motor, transmission, drive shaft and ring and pinion are. They don't care that one tire is doing all the work.

The whole rest of the boat does not care if only one or two blades are doing the work at any given moment or that the blades are taking turns doing the work. The boat only cares that at least one blade is working at any given moment.

I appreciate the input but ya I started the thread and never mentioned anything about cracked blades, it was specifically about the unload / load comment that came from the Starliner discussion. Seems people might be confusing it with another thread about a 24?

Anyway, nothing personal. A little frustrating when fairly clear evidence is presented, yet still people want to argue about it, or maybe just don't understand it, which is totally fine.

The differential posi / non-posi is not the same. Once you understand why it isn't, you will be closer to understanding the article and how there is not a continuous load / unload event on the prop.

 

[blacktruck]

Well, I don't like the reality that maybe I'm wrong and don't know it yet.

I keep waiting for either myself or blacktruck to have an epiphany and type "Oh I get it now"

Maybe it won't happen for either of us, I'm glad we have a forum to discuss it and hopefully learn without getting frustrated with the other guy or getting personnel. That's the hard part.

I wasn't trying to make it personal. I think it's just hard when people seem to be mixing another thread and different things, confusing what is being talked about. The article is very clear, it supports what I am saying very clearly. Ultimately, people can and will believe what they want to believe. And, some will set there boats up in a dangerous way as a result.

The goal really is to get the most out of a setup as safely as possible.

 

[blacktruck]

Uncertain about props one and two( considering the source, likely modified)Prop four was a rebuild.

But prop three was out of the box new. Most interesting to me is that four different props all broke the same way and the surviving blades look like new.

What kind of prop was #3?

 

[Jimway]

I think one must be careful if one is to come amongst the Checkmate family here and state something to the effect that 'You don't know what the blank you are talking about". Lots of knowledge here. Lots of experience here also. Impressive vessels, impressive engines and impressive and proven performance. I find propellor 'dynamics' interesting, and hopefully we may find out what happened to the props in question. Interesting also, what happens as we increase power and speeds to the edge of (and sometimes far beyond) the capabilities of the equipment, which many of us have done, some of us may have been doing this possibly longer than some others have been alive. A little respect still goes a long ways...

 

[keith4216]

https://youtu.be/Hj-VmZYcUhQ

 

[karzrus]

What kind of prop was #3?

New Bravo 1 FS 24P, broke after 10 hrs of use.

 

[karzrus]

The differential posi / non-posi is not the same. Once you understand why it isn't, you will be closer to understanding the article and how there is not a continuous load / unload event on the prop.

I don't think there is a continuous load / unload event on the prop, I think it's on the blades separately.