Propeller dynamics: blowout etc

[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.

OK, so this comment got me thinking why you and I see this so different.

In a surfacing application and using extremes to make a point.

Regarding the differential, I think the hub of the prop is acting like what would happen if you had a solid spool piece in a differential. (each tire has no choice and must spin in the same direction and speed as the other tire at all times)

When running a prop at the surface, it's like a paddle wheel where the paddles spend time in the water pushing and time out of the water doing little work.

In the most extreme case of this a boat may prop walk sideways on the propeller.

 

[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.

Ya, that is interesting. I would definitely have Merc Racing look at that un-modified prop.

Hard to diagnose over the internet but I'd want to know if all 4 props experienced this under the same conditions? ie. full throttle, maxed right out for example. How high was the engine running, what RPM etc.

Aside from the cracked props, have you worked off a baseline? You can find your theoretical max speed:
(Tach RPM * Pitch) / (1056 * Gear ratio) = TMS

Work from your last run and take your real world speed divided by TMS. Or use the Merc Racing slip calc

I suspect you're running the engine too high. Every boat is different but I would guess the 24 runs wet, it's not super light so get a baseline with the propshaft at a very conservative height, and mark off your jack plate gauge so you know where you are all the time. You might find you're going faster, more efficiently running it lower. And when setup properly, you won't be breaking blades.

Another factor is the blade or blades was damaged in shipping, at the boat ramp, when you're swapping it or something, any imperfection could cause a weakness. Just trying to think of anything is it's very unusual.

And maybe acquire your props from someone else, if they've all come from the same place.

Hope that helps.

 

[blacktruck]

OK, so this comment got me thinking why you and I see this so different.

In a surfacing application and using extremes to make a point.

Regarding the differential, I think the hub of the prop is acting like what would happen if you had a solid spool piece in a differential. (each tire has no choice and must spin in the same direction and speed as the other tire at all times) - Ya, the blades are directly connected to the driveline equally, unlike a non posi diff. It's not a great analogy though.

When running a prop at the surface, it's like a paddle wheel where the paddles spend time in the water pushing and time out of the water doing little work. - They are doing tons of work, when a prop(s) loses pressure "doing nothing" = blowout, that's why the article is important. Remember, when the gearcase goes through the water at high speeds, the water flow is disrupted to all the blades. The rake, cupping and aspects of the prop are designed to harness that mess as best it can.

Imagine if you were running a prop, fully submerged, and one blade just broke off. What would happen to your RPM?

In the most extreme case of this a boat may prop walk sideways on the propeller.

Sorry if the cracked blade got mixed but it's cool. I think we aren't far apart, I think my notes above make decent sense. If not, let's just move on and try to get the most out of our boats as "safely" as we can and enjoy them.

 

[DropTop5.0]

I'm dumb as a stump but I know how to move on. Prop design with regards to a cleaver and a chopper. I thought both were surfacing props but it appears to me that you could surface a cleaver a lot longer than a chopper without taking any damage. Meaning the proper design of the cleaver looks a lot stronger. Isn't a cleaver intended to give us stern lift?

 

[Instigator]

Ur killin me Clark.

Blacktruck, you need to go to a boat race, any boat race, and educate them on propellers.
Based on what youre writing, they are uninformed and need your help.

The opening post of this thread, quoting Jim Russel, a genius and friend of mine is good but not the whole story.

There needs to be a propeller terminology 101 mandated that all hi perf boat owners read.

99.9% time when I hear some one saying their boat "blew out", at 55 mph, they're almost always describing cavitation.
As in, they were running 55 and dialed in a bit more up trim trying trying for 56 and it suddenly "blew out".
No it didnt.
You got that prop too close to the surface and it aerated, i.e. cavitaed.

Blow out on the other hand, is caused by the gear case travelling through the water faster than what it can reform at behind it.
This just happens to happen, right at the propeller.
Hence, nose cones. Their job is to extend the running (core) length of the gear case to get that vacuum (which is what it is) to behind the propeller.
They do other things but that is their designed purpose in life.
I've run numerous, stock lower unit boats well into the 80's w/o blowing out, and know many that have been into the 90's.


As to breaking blades......, the thinner they are, the faster they are. The thinner they are, the more fragile they are.
Again, go to a race, any race, find a prop w/thick blades and post a picture.
That is all I will post on that subject as no one I know will even argue about it.

PS, next time you copy and paste Jim Russles works, tell him you're arguing with me.
He will laugh.

PS, PS, please got here and do a post educating them on propellers.
They know nothing.
screamandfly.com

 

[blacktruck]

Ur killin me Clark.

Blacktruck, you need to go to a boat race, any boat race, and educate them on propellers.
Based on what youre writing, they are uninformed and need your help.

The opening post of this thread, quoting Jim Russel, a genius and friend of mine is good but not the whole story.

There needs to be a propeller terminology 101 mandated that all hi perf boat owners read.

99.9% time when I hear some one saying their boat "blew out", at 55 mph, they're almost always describing cavitation.
As in, they were running 55 and dialed in a bit more up trim trying trying for 56 and it suddenly "blew out".
No it didnt.
You got that prop too close to the surface and it aerated, i.e. cavitaed.

Blow out on the other hand, is caused by the gear case travelling through the water faster than what it can reform at behind it.
This just happens to happen, right at the propeller.
Hence, nose cones. Their job is to extend the running (core) length of the gear case to get that vacuum (which is what it is) to behind the propeller.
They do other things but that is their designed purpose in life.
I've run numerous, stock lower unit boats well into the 80's w/o blowing out, and know many that have been into the 90's.


As to breaking blades......, the thinner they are, the faster they are. -It actually depends on the prop design more, most cleaver style and other racing props have thick trailing edges for added strength. The thinner they are, the more fragile they are. That's why you don't want them to thin, then they break, then you don't go anywhere. lol.
Again, go to a race, any race, find a prop w/thick blades and post a picture.
That is all I will post on that subject as no one I know will even argue about it.

PS, next time you copy and paste Jim Russles works, tell him you're arguing with me.
He will laugh.

PS, PS, please got here and do a post educating them on propellers.
They know nothing.
screamandfly.com

Why would I need to go educate them on stuff they already know? It's very standard knowledge, they would agree with everything I said. I cited Jim Russel at the beginning, so it was very clear it was his work. But if you think he is wrong, take it up with him yourself, especially if he's your good friend

Of the 2 points, 1 point of only 1 blade doing all the work (full load / unload): you are never going to win that argument, it's just impossible.

2nd point, a well setup "non-race boat" with stock blades on an appropriate prop should not be breaking blades all the time, it would be very rare. You will never win that argument.

I clearly stated it was not about race boats with worked props. Not sure why you have to keep going back to that. That's great you're a professional racer that breaks props all the time. Glad you're not setting up my boat. And it must be really expensive busting your custom props every time you go out

Just for fun. Which race team do you race with? How long have you been a professional racer?

Sorry man, you are wrong about those points. Keep breaking your blades and have fun.

PS, I"ve been on Scream and Fly for a very long time. It's a really good forum, most are more knowledgeable or are looking to learn, it's more productive.

 

[Jimway]

Fiberglass question, anyone?

 

[keith4216]

Number of beers question anyone/ lol

 

[Problem Child]

Moral of the story is the less you surface your prop the longer it should last. I know many guys running 1000+ tq and hp with no blade breaking issues. Ill keep my $1000 prop buried !

 

[Instigator]

Blktrk, I would not touch your boat.
You already know everything.

I no longer race and never claimed to be "professional or work for one".

Ever heard of Mazzco, Hoss, Spinelli propellers?

On the other hand, I have modified 200 hp OMC fishing motors to near 400 hp ,9K RPM and well over 100 mph.
Been all over the country w/them.

At 19 yrs old I moved to Houston to build world champion Butts Aerowing raceboats and did race for him.

Raced kneel down for 9 yrs, designed/built my boats and motors and beat on my own propellers.
Started at 17 and paid my own way.

Sold Checkmates in '89/90 and worked for a dealer w/13 boats in stock, a wall of propellers and free use of all.
I took a Starliner w/a 300 hp V-8 Evinrude to a race and used it for a patrol boat all weekend. And did laps around the course.

Have owned/restored two twin o/b 24' Sonics. Second one I built both motors for free, from scrap parts I collected.

Have experimented w/set back, motor height, propeller rotation, toe in/out etc.
I have a wall of propellers for my Starliner.

Other than, I don't know a damn thing.

Have fun educating the troops.

 

[Instigator]

projects

 

[Instigator]

projects and the bosses boat

 

[blacktruck]

Blktrk, I would not touch your boat.
You already know everything.

I no longer race and never claimed to be "professional or work for one".

Ever heard of Mazzco, Hoss, Spinelli propellers? Ya, those are the most popular prop brands other than the big one, I think everyone has heard of them.

On the other hand, I have modified 200 hp OMC fishing motors to near 400 hp ,9K RPM and well over 100 mph.
Been all over the country w/them.

At 19 yrs old I moved to Houston to build world champion Butts Aerowing raceboats and did race for him.

Raced kneel down for 9 yrs, designed/built my boats and motors and beat on my own propellers.
Started at 17 and paid my own way.

Sold Checkmates in '89/90 and worked for a dealer w/13 boats in stock, a wall of propellers and free use of all.
I took a Starliner w/a 300 hp V-8 Evinrude to a race and used it for a patrol boat all weekend. And did laps around the course.

Have owned/restored two twin o/b 24' Sonics. Second one I built both motors for free, from scrap parts I collected.

Have experimented w/set back, motor height, propeller rotation, toe in/out etc.
I have a wall of propellers for my Starliner.

Other than, I don't know a damn thing.

Have fun educating the troops.

Great. Was only trying to be helpful and I think the responses got way off of the original point. Sounds like you've run many setups and although I never claimed to be an expert, I am right about the points outlined in my previous post, which was the whole point. And I backed it up with an article from an expert, your good friend

Thanks for sharing.