Metal versus plastic prop instability

[jetnfast]

Hey guys,

I have been running between the stock plastic prop and an octura x442 on my Traxxas Spartan. I am fairly experienced with this boat, with about 90 runs on the hull. With the stock prop I have managed a best of 49.7 mph with the batteries centered. In this configuration the boat is very stable, even at WOT with little to no chine or torque roll. It seems dialed in at this point....

The last few runs I have switched back to an x442 and its a whole different boat, managing a best of 48.5 mph and lots of instability, chine and torque roll at WOT. The batteries were centered and I made no change to the trim tabs (4 of them). I am really surprised at how unstable the boat is with the x442. Its a handful to drive and loves hopping out of the water with this prop as well. I didn't think the prop change would make such a huge difference on stability but it seems that way. I am wondering if I should dial some down trim on the starboard side to help counter the roll? Anything else to get it dialed in with the x442?

Both props were running a neutral strut angle.

...At this point besides the looks, the plastic prop has run much more stable at WOT, and apparently a little faster, odd huh?

[Heaving Earth]

Not really. Just because you use a metal prop doesn't automatically mean its gonna be faster or more stable.

[Heaving Earth]

Also did you sharpen and balance your prop?

[jetnfast]

Sure did, sharpen balance and polish. I realize that just because its metal it's going to better, just surprised it led to much more instability. I guess I need to fine tune the trim tabs to see if I can dial it in, otherwise I will go back to the plastic prop.

[J.W. Pepper]

Hey guys,

I have been running between the stock plastic prop and an octura x442 on my Traxxas Spartan. I am fairly experienced with this boat, with about 90 runs on the hull. With the stock prop I have managed a best of 49.7 mph with the batteries centered. In this configuration the boat is very stable, even at WOT with little to no chine or torque roll. It seems dialed in at this point....

The last few runs I have switched back to an x442 and its a whole different boat, managing a best of 48.5 mph and lots of instability, chine and torque roll at WOT. The batteries were centered and I made no change to the trim tabs (4 of them). I am really surprised at how unstable the boat is with the x442. Its a handful to drive and loves hopping out of the water with this prop as well. I didn't think the prop change would make such a huge difference on stability but it seems that way. I am wondering if I should dial some down trim on the starboard side to help counter the roll? Anything else to get it dialed in with the x442?

Both props were running a neutral strut angle.

...At this point besides the looks, the plastic prop has run much more stable at WOT, and apparently a little faster, odd huh?

Hey Guy,
There is an outstanding thread on the subject of props that was done recently... http://forums.offshoreelectrics.com/...-Octura-M-or-X - EVERYBODY stayed civil, as a result, an enormous amount of information was presented & exchanged.

Check out posts #8 & #18, both contain links to a couple websites with a bewildering amount of knowledge on the subject of propping & setting up FE boats in general. The subject of chine walking was discussed as were the comparisons between the "X" & "M" style of Octura props.

I urge you to read through that thread before trying any other tuning adjustments with your boat. IF, I remember correctly the "M" props might help to solve your problem, BUT, please do not quote me on it.

You stated that your problems are the strict result of a prop change, so that thread would be the best place to start.

Good luck, I hope that gets you going in the right direction.

[paultbg]

Most probably the Octura is lifting the boat more out of water vs the plastic one therefore the boat is instable and the prop i snot efficient.
You need to either adjust the tabs for this new prop, or better, start by moving the COG a bit to the prop.
Regards

[J.W. Pepper]

Most probably the Octura is lifting the boat more out of water vs the plastic one therefore the boat is instable and the prop i snot efficient.
You need to either adjust the tabs for this new prop, or better, start by moving the COG a bit to the prop.
Regards

The reason the Octura is lifting the boat is because that's what the "X's" do, precisely why I suggested he read the "X" vs "M" prop thread...
BTW: Octura "M's" don't lift because they've been de-tongued, had you read that thread you would have learned that.

Fine tuning an FE boat requires research & reading, seems like alot of the posts I see on OSE are... Just gimme a solution / answer / quick fix, I don't want to put forth any effort into trying to figure out why my boat is doing... this or that. Sorry, but every boat is different, there are no easy answers.

The thread I referenced will answer his questions, and ultimately lead to a solution, he just has to put forth the time and effort.

[TheShaughnessy]

Pepper, I know you are trying to help but the tounge of a prop isn't the area of the blade responsible for lift. It's at the opposite end of the blade, the trailing edge, where the lift happens. Sometimes a boater will modify a prop with what is called a Barr cut or a Texas cut to reduce the amount of lift from a given prop. Another trick is to spoon the tips which will condense the thrust cone, cupping also has this effect but cupping adds pitch so you need to watch temps when doing this.

One way I think of it is : look at the rooster tail, the prop is throwing a considerable amount of water into the air right, well then it must also, to some degree be throwing water down below the water line as well, that is where the lift is coming from, thrust that is directed down rather then straight back and that is why condensing the thrust cone reduces lift.

Disclaimer: I myself am a novice FE boater, while my description is to the best of my understanding it may contain information that is not 100% accurate.

To the OP. you got that x442 like razor sharp right, the the point where you are scared to touch it cause it will cut you?

[paultbg]

@Pepper- please read my words again; I am saying that the Octura range is lifting the boat more than the plastic prop; Now, what is wrong here, can you explain?
Also why you guys recommended cutting the prop to a beginner as long as he did not tried all the other possible remedies, like trim tabs, COG,etc?
Not all the boats with all the setups are running well with Octura M or X range, some will need either less lift or they will need to have the prop deeper in the water.
There is no other way then trial error.Still, telling someone to go and read is not quite helpful.My 2 cents...

[J.W. Pepper]

Pepper, I know you are trying to help but the tounge of a prop isn't the area of the blade responsible for lift. It's at the opposite end of the blade, the trailing edge, where the lift happens. Sometimes a boater will modify a prop with what is called a Barr cut or a Texas cut to reduce the amount of lift from a given prop. Another trick is to spoon the tips which will condense the thrust cone, cupping also has this effect but cupping adds pitch so you need to watch temps when doing this.

One way I think of it is : look at the rooster tail, the prop is throwing a considerable amount of water into the air right, well then it must also, to some degree be throwing water down below the water line as well, that is where the lift is coming from, thrust that is directed down rather then straight back and that is why condensing the thrust cone reduces lift.

Disclaimer: I myself am a novice FE boater, while my description is to the best of my understanding it may contain information that is not 100% accurate.

To the OP. you got that x442 like razor sharp right, the the point where you are scared to touch it cause it will cut you?

Thanks for the clarification, I do appreciate it. You were a big help and contributor to the thread I referenced in both my replies.

I checked Tower, they're saying the stock Spartan prop is a 42mm x 59mm - 4mm bore.

[Heaving Earth]

X442 is almost exactly the same size as 42x59.
The 442 is 42x58.something.
I noticed the spartan prop has rounder tips, which reduces lift a bit. You might try rounding the 442 tips off.
1.4 series props have less lift the 1.6 which are higher lift.

[Heaving Earth]

Pepper, I don't know where you're getting your calculations but 42mm is equal to 1.65
And 59mm is equal to 2.32
You might want to get your facts str8 before you comment anymore

[martin]

The Prather 230, x548 & x648 are way way to big on this boat.

[jetnfast]

Good discussion here guys. I totally understand its all trial and error! I had the boat dialed in nicely with the stock prop and switched back to the x442 (I have gone back and forth between stock and x442 about 2 or 3 times). I will continue and fine tune the settings, next I am going to move the CG forward a good deal to help keep it in the water. I don't want to mess with the strut angle too much since I feel its best at neutral, so my only other options are trim tabs and batt shift, hopefully the combo of the two and lots of testing will provide good results. BTW, I have no problem reading threads, in fact I have been doing that for the last year or so since I got involved with FE, a lot can be learned, and has been learned! That's always the first thing I do. I don't expect someone to answer a very vague and general request to tell me why my boat is doing this or that....so no offense taken either way. I have been involved with RC for 25 years (over 2/3 of my life!).

[J.W. Pepper]

Pepper, I don't know where you're getting your calculations but 42mm is equal to 1.65
And 59mm is equal to 2.32
You might want to get your facts str8 before you comment anymore

I went back to the drawing board and did more calculations, yes I made some mistakes, sorry I'm human, happens to the best of us.

For the record Tower Hobbies states that boat comes with a 42mm (1.65354") x 59mm(2.32283") prop. I used the this website... http://www.towerhobbies.com/help/convcalcs.html - to convert to inches then tried coming up with the correct prop size.

Question: Why does propping a boat have to be so bloody hard? About 95+% of all R/C cars & trucks use 48-pitch gears, some 4wd & exoctic race setups use different pitch gears, but that's not mainstream. I can go to my LHS and buy a Robinson Racing pinion gear and put it on pretty much any standard R/C car / truck, best of all it only costs $6.95 for a pinion & spur set.

With boats there's at least 1/2 to a dozen different measurement systems: Octura, Prather, ABC, Kyosho, Grim Racer, this system, that system and none are compatible or interchangable. Why? Where is the logic in that??

Example: chart @ AQ's website says that Prather S215 (aka PB 1.6" x 2.5") is same as Octura x440. BULL****!! I have the PB, also have Kintec CNC'ed aluminum x440 & x442 props, the 440 is a little smaller & the 442 is a little bigger (talking diameters). So basically, the Prather S215 is the same as a x441. Gee that's nice, there's no such thing as x441 - unless you make it yourself.

Some of us are new to the FE boat thing and don't have the ability to cup, cut down, bend this, change that to get the exact precise prop our boat may need for what-ever the reasons may be. I can't speak for other people, but I personally find that very frustrating.

Heaving Earth - I removed the erroneous info that I accidentally put in post #10 and cleaned this one up as well, I was frustrated and cranky at the time that I originally did this post. Lotta stuff on my plate lately.

I promise I won't make anymore posts to this thread, obviously I'm way outside my league here, sorry for any mis-information or confusion I created, my intentions were to try & help out. Sometimes the best intentions don't always work out the way we hope.

Peace