In search of the killer 20 mono propeller

[David Homer]

I've been inactive for a few years now, but in the past few weeks I've been looking through stuff, going through some old listbot threads and tech articles, and the lightbulb of inspiration lit again...lol... I probably won't be racing anytime soon, but I'm going to find the time to play a little locally...

Before I put the boats on the shelf, I began experimenting with cut down larger props (X445, X447/3, X646/3, etc.) for my CMB .21 LS powered "Wild Thing". This was before a very informative thread on propellers showed up back in 2003 on Marty's listbot. It appears according to that discussion, I'm on the right track, but I also found out what I WASN'T doing, either.... (the results of my earlier experiments became a lot easier to understand...lol)

I realize the X440/3 is the prop du jour for 20 mono. It just becomes very apparent to me this prop is very limited in what it can do... Assuming 30,000 RPM, theres not a lot one can do with this prop to make your mono exceed 42 MPH.. (which may be the design limit for a lot of the hulls we have in heat race conditions, but thats another thread...lol)

I have some very specific ideas I want to try, but before I start grinding beryllium in earnest, I thought I'd start a discussion here. Has anyone else tried the same thing? Successes? Failures? Theories? Always figured a few minutes of education is worth the money saved on junk equipment...lol

 

[izitbrokeyet?]

I've been inactive for a few years now, but in the past few weeks I've been looking through stuff, going through some old listbot threads and tech articles, and the lightbulb of inspiration lit again...lol...  I probably won't be racing anytime soon, but I'm going to find the time to play a little locally...
Before I put the boats on the shelf, I began experimenting with cut down larger props (X445, X447/3, X646/3, etc.) for my CMB .21 LS powered "Wild Thing".  This was before a very informative thread on propellers showed up back in 2003 on Marty's listbot.  It appears according to that discussion, I'm on the right track, but I also found out what I WASN'T doing, either.... (the results of my earlier experiments became a lot easier to understand...lol)

I realize the X440/3 is the prop du jour for 20 mono.  It just becomes very apparent to me this prop is very limited in what it can do...  Assuming 30,000 RPM, theres not a lot one can do with this prop to make your mono exceed 42 MPH.. (which may be the design limit for a lot of the hulls we have in heat race conditions, but thats another thread...lol)

I have some very specific ideas I want to try, but before I start grinding beryllium in earnest, I thought I'd start a discussion here.  Has anyone else tried the same thing? Successes? Failures? Theories?   Always figured a few minutes of education is worth the money saved on junk equipment...lol

90952[/snapback]

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I like the 40x 53 /3 blade with the tongue cut just a tad. It slips a little more on the bottom but seems to carry better speed on the top with little difference in handling

 

[MarkScott]

I spied this prop at last years IMPBA Internats.. You'll have to inquire with Erine Lafleur on where to get one..

Mark.....

 

[Ray Sametz]

Dave I run a N/R powered Wild Thing. My best prop for Heat Racing is a X645 two blade cupped 0.10 Runs 46 in practice off Garmin GPS. MY 2 cents Ray

 

[Ron Olson]

Dave, I've got a few that you can try out including the M-445 before you spend the bucks. I've got a spare new in the package Andy Brown Prather 220 that you can try. You've got to stop by sometime and see what all I've got in the basement now.

 

[shoboat]

David,

I can tell you have been away awhile. Most of the HOT 21 Monos here in

District # 2 can easily hit the 50's now. As far as propellers their are many

to choose from: ABC 40 x 52/3,40 x 53,M-440,X-442,H-4,X-440/3,PS4213/3,

X-640,and the X-642 are just a few that come to mind right now. You may

have to cup them a little bit or remove the tongues a little,but I'm sure your

boat will get into the 50's also. I doubt if your motor is turning 30,000 RPM,

but that is a number that a lot of people talk about. :unsure: :unsure: :unsure:

If I was you, I would start with a ABC 40 x 52/3 and remove the tongues a little

bit like Kevin has stated above. This propeller with a N/R 21 will go into the 50's

and beyond.

Make sure you use a toxic mask and some gloves.

Good Luck,

Mark Sholund

 

[David Homer]

Thanks guys, I really appreciate the input.. Ron, we're gonna have to hook up.. I'm very interested in setting that Andy Brown prop on my pitch gauge...

I'd like to get a closer look at the 40X53/3. I'm just a tad curious why you guys remove a bit of the tongue. I know why its done, but as it was mentioned, the boat "slips" a little on the low end..

Ernie's prop... a picture is worth a lot...lol... Two things... the brazing/silver soldering on the hub means this is a one-off... the second thing I notice is that the prop has its full tongue area... again I believe this helps confirm what I'm thinking here..

I have a 40X62/3 of which I tried on my old K&B powered AC mono once (the ol' Saginaw world beater...lol).. Prop wouldn't launch worth a bleep, but lo and behold, that thing hooked up.. boat was easily going 10 MPH faster than it ever did. Just put that on my pitch gauge.. leading edge 2.5"...Hmmmmmmm...

Has anyone taken a pitch gauge and measured the progression from leading edge to trailing edge on their favorite prop? Would be interesting to see if the theory I've read holds true.. Interesting to read some of you guys adding cup...hmmmmm

 

[shoboat]

Dave,

I can tell by reading your post you are interested in going faster. The leading

edge pitch and trailing edge cup all will do what you are looking for. Cupping

is the way to go for quick performance. Believe me you have never even

extracted all the performance out of your propellers yet. There are a lot more

things that you can do to make that mono go faster. You need a 40 x 52/3?

Good Luck,

Mark Sholund

 

[AndyBrown]

David,

I introduced the importance of leading edge pitch in relation to the optimun speed potential of the prop to the model boating community.

I measure leading edge pitch (first 25%) and use that measurement at 100% in the speed formula.

The amount of progresssion required is dependent on hull drag and blade area.

If the LE is 2.5" and the rpm is measured at 30,000 while the boat is doing X mph, if X mph does not match the speed formula using the LE at 100% then the prop does not have the correct progression.

If the boat is going faster than the LE at 100% dictates, then the trailing edge pitch is "pushing" the LE into a wall of water. This means you can reduce TE pitch and increase LE pitch and go faster with the same amount of power.

If the boat speed is less than the LE pitch/rpm ditates then the TE pitch or blade area is too low.

Most .21's are turning low to mid 30K these days and competitive speed in heat racing is mid to upper 40's.

Our 21 MAC powered microbusrt runs 51-53 mph with a stock X440/3 when it's running loose, but in normal heat race ride 47-48 wins races.

Welcome back to r/cboat racing,

Andy Brown

www.cmdracing.com

 

[Watercadet]

Had the radar gun out at the club race today... my N/R (short stroke) 31' Microburst and a stock x440/3 was 40mph on the nose... it was loose and lean. It was the fastest of the 20 monos there. 50's must be just silly fast :blink: Need the correct stinger dia though...

Side note: my FF.12 with a OS .12 TR was @ 37.5 with a P215 B)

Adam

 

[Scott Schneider]

I have a couple .21 monos that run a surface piercing drive ( Stuffing tube exiting the bottom but set right on the bottom of kiel ) and run X437/3 blades on them. Tougue cut back and rolled outer edges with some trailing edge pitch up.

Running Picco P-5 EXR engines with a pretty short pipes at @ 29K to 31K and seeing @ 52 MPH on the gun.

Because of such minimal prop area, throttleablity is great, and get my speed by getting engine to turn very high RPM.

Smaller diameter props have less torque effect on hull and prop walk less.

They will in most cases handle better than bigger props too.

The biggest trouble I see with the little .21 monos set up with surface drive is how sensitive the setup is the exit height on transom and distance prop is set from transom.

These two points when played with can and will make or break your success in building a truely fast .21 mono that handles well and has throttle controll.

In most cases guys tune for top end speed and in doing so don't finish all their heats.

They either come off the pipe and putt around or just die and park along with the rest of the boeys giving the other racers something else to avoid while racing

JMHO. Scott

 

[David Homer]

Thanks again guys... I believe Andy answered the question I was mulling here.... A few years back we saw Marty's tech article on the X-blades... and we started cutting the tongues as shown on the illustrations... Worked pretty good on the Prather 215's I put on my sons' tunnels... but when I experimented on my mono, I was less than pleased...

I'm learning new stuff every day...lol Spent most of the day yesterday with a new X447 and an X452/3 on my Hughey pitch gauge mapping out the leading edges .064 at a time... What I found kinda confirms what I see on Ernie's prop there. The leading edge of the tongue on these X-blades vertically from the hub to the outer leading edge is the lowest pitch area. The cut that Marty recommends takes off an area with acceptable pitch, and I believe has a lot to do with launch and acceleration for the heavier higher drag monos.

I figured a LE pitch of 1.88 would get me at my target speed. Measuring the X447, I could map the target pitch from the hub to the outer leading edge.. darned near vertically.. connecting the dots and looking at the prop from the side, the LE appeared practically perpendicular to the hub (again take a closer look at Ernie's prop...)

I am assuming then just as long as the leading edge pitch, blade area, and pitch progression are right, blade shape would have more to do with launch, acceleration and handling than speed.. I have a starting point.....

Mark, if you could email me on a price for a X447/3, an X646/3, and that ABC prop you mentioned, I'd appreciate it. Thanks again everyone!!