Prop Technicalities

[GTR]

Hi Guys

I know there is a lot of expert knowledge on this forum about props - as there are also a lot of newbies who don't know much about them this looks like some form of black art. I'll start with some of the more obvious questions and we can then digress.

1. What is the difference between a lifting and non-lifting prop (now I don't want an answer that says a 14XX series is lifting & an X series is not - what I want to see explained is WHY one is classified as having lifting characteristics and the other doesn't)

That gives you the idea of what I mean - this is the info that people need to understand not just try this or that.

2. What effect does a Barr cut have on the characteristics of a prop?

3. What effect does taking the ears of a prop have & why?

4. What does back-cutting do & why?

5. How does reducing the diameter of a prop change its characteristics?

6. What effect does a Texas cut have & why?

7. What effect does rounding the prop tips have & why?

I know there are a lot of people who know this stuff but this will help people make informed changes to their props and help understand what/why these changed characteristics effect their boat a particular way. I'm sure there is plenty of info out there in cyberland but what about some simple english descriptions on here.

Lets see what you've got guys.

 

[Propjockey]

Awesome topic GTR!!

Wish I had the answers!!!

 

[TimD]

1. What is the difference between a lifting and non-lifting prop (now I don't want an answer that says a 14XX series is lifting & an X series is not - what I want to see explained is WHY one is classified as having lifting characteristics and the other doesn't)

The X series are progressive pitch props. look at where the blade meets the hub - that will give you some idea. basically the pitch starts off low down on the drive dog end and increases the closer it gets to the trailing edge.

2. What effect does a Barr cut have on the characteristics of a prop?

It relieves the prop of a high pitch part of the blade. Let's the motor unload

3. What effect does taking the ears of a prop have & why?

It relieves the low pitch area of the blade. The low pitch part limits the ability to spin the prop fast

4. What does back-cutting do & why?

see Barr cut.

5. How does reducing the diameter of a prop change its characteristics?

It changes the pitch progression. The larger diameter prop of the same progression ratio has more pitch than the prop of the smaller diameter with the same progression ratio.

6. What effect does a Texas cut have & why?

Similar to reducing diameter.

7. What effect does rounding the prop tips have & why?

no idea.

My feeble attempt and I'm bound to be wrong.

 

[Slideblues]

I'll try to help out on Tim's no. 6 & 7,

From Paul G. I have quoted him many times,

Texas cut and pointed vs. rounded tips:

 

[Dan_Cousin]

Heres a littel fuel for thought. These are good tech notes from Marty Davis. That touch the subject on prop modifications.

http://rcboat.com/prop.htm

http://rcboat.com/jantech.htm

http://rcboat.com/febtech.htm

Our engines are basicly feeble on the low end and monsters on the high end. What we want is as much prop as we can pull up on the low end that will allow the high end to "hook up" an deliver the HP to the water.

back cutting, reducing diameter, texas cutting are all aimed at letting the prop slip under high loads (launch and acceleration) and letting the pitch catch when the motor is at high RPMs.

BTW X props do lift and can hold up more weight than 1400 series props. They just don't lift as high.

 

[nitrotoys]

I think the IMPORTANT INFO HAS BEEN COVERED Already.

Paul Gs info on props are some of the best reading, and learning you will ever come across.

He has played and moded props for YEARS now and really makes the props work with his boats, his fav prop i would say is the H-50.

All these mods will help, but you first need to understand what mod does what, then do what you need to do at the prop, I been back cutting, barr cutting and taking the tongues off most. But you need to first start and see what each MOD does for your boat. Somestimes a stock prop, with tongue trimmed down works very good. if you need more Rs out of it, barr cut it.

To much lift, back cut it from the hub to the tip. Normally i cut it and end up at 0 degrees at tip of prop.

then cup and spoon for performance.

 

[ClayGlover]

I think the various cuts are useful, especially the Texas-cut. But the truly productive way to learn what does what is to take all the information you get, then get a pitch gauge, pliers, anvil, small hammer, prop molds to correct bad bends, and lots of props. Then just put in the test time.

Oh yeah Gary, another potential performance gain (or potential trouble)- blade thinning

Andy Brown was in a very generous mood a while back on the listbot. I was able to save the tips to my long list of various shared boat info that I've picked up over a few years surfing time. Here are some tidbits I have courtesy of the Master and rcboat.com. If you missed these, here's a brief example of the depth of prop design. Hulls, motors, and props? The amount of knowledge that must be the man's head is really amazing. Thanks Andy-

A Brown on prop pitch progression and optimization:

 

Leading edge pitch is everything. The first 1/3 of the blade will determine the potential speed of the prop. The last 2/3 must have a certain amount of progression to maintain water/blade contact. If the boat is high drag the first 1/3 of the blade will accelerate water backward faster than the pitch that just made contact. This is because that small piece of water on the surface of the pond yields to force much easier than a 13 pound Scale boat will yield to force. So now the last 2/3 must have more pitch to stay in contact with the water that is traveling backward at a faster rate of speed than the boat is going forward. If there is not enough progression it will be as if you were missing the last 2/3 of the prop. The last 2/3 of the prop will just be pushing air. That is why you can start with a stock 1400 and begin adding trailing edge cup and see and increase in speed (remember! The stock 1400 prop only has 10% progression). Not because of more pitch, but because slippage is reduced. Without the extra cup it would be as if only 1/3 of the blade area was available.

Yes Ian, some props can stand to have a little leading edge pitch added.

Most of the time X props have to much progression. Reducing cup and/or cutting the leading edge will reduce load on the engine and gain speed at the same time. Often the pipe can then be shortened and even more speed realized. 60-70% progression seems to be enough for most "high drag" boats. 30-40% for most "low drag" boats. Most X props are in the 90 to 100% range.

Overall blade area comes into play here also. More blade area will reduce the amount of progression needed. Less blade area will increase the need for progression. Like everything else, it becomes a trade off between blade area and progression.

In my book, "True pitch" is the leading edge pitch only. The leading edge is the only part of the prop that is making contact with still (non-moving) water.

And remember that small piece of "still" water only remains that way for a micro second. Less than .00015 seconds @ 30,000 rpm to give you a close guesstimate. LOL!

By the time it leaves the trailing edge it can potentially be traveling backward as fast as the boat is traveling forward.

 

Andy Brown

 

 

Andy Brown on prop cup measurement location:

 

About 70% out from the hub and starting at the trailing edge.

 

"So is there an optimal pitch progression? I have noticed since using the pitch gauge that in general terms the better mono props seem to have quite a bit of pitch progression while rigger props seem to have less."

 

Ian, There is an optimal progression and this will be different for every different hull/drag condition.

 

High drag needs high progression. Low drag can use low progression as you are beginning to learn from your prop measuring experience.

The key is to use as little progression as possible.

 

A stock 1400 or 1600 series prop has about 10% progression. Meaning the trailing edge has 10% more pitch than the leading edge.

The Octura X series props have nearly 100% progression. Again, meaning the trailing edge has 100% more pitch then the leading edge. You may find 2.5" on the leading edge and 5.0" on the trailing edge of an X460. Good for acceleration and good to pull a high drag boat out of the water but not good for top end speed because you can not go faster than the (2.5" pitch X rpm).

In the mean time the 5" trailing edge is working the motor to death and not contributing to forward speed. Actually it does somewhat, but at a high cost in power usage.

 

Andy

 

[Ron Olson]

WOW! Prop School 101 is now in session!

Gary, I'm glad to see that you brought this up. Some people including myself are kind of afraid to ask questions like this or are afraid to try to mod our own props, ya know, sharpen, balance, toss it on the boat and hope for the best.

Andy explained it very well but to the amateur prop modder it's still hard to comprehend. Does something like the Dy-kem method of painting props and removing what doesn't come off in the water help to find out how to mod a prop for those of us that don't have the fancy equipment?

 

[ClayGlover]

HAHA, I can see it now- no prop's ears are safe!

I use the red dykem. It works, so long as you don't let fuel (or alcohol, etc.) touch the prop at all. The nitro fuel strips the dykem right off and you have to start over again. It's not a quick process, it takes some runs to see the dykem wear.

 

[david cox]

Hi Guys, great topic!

Which modification(s) can be made so that a prop is more "driveable" or race friendly. Here is an example:

My Sport 40 will run the fastest with an X452 prop. Often when I throttle down with this prop, my boat will not get back "on the pipe" unless the needle is perfectly set. I have tried lengthening the pipe slightly but it didn't seem to make any difference. Therefore this is not a great racing setup as it will not allow for milling speed. As long as I can keep the boat at full throttle then it will run like stink, but once I let up on the throttle there seems no way of achieving the initial speed.

Is there a mod that will help driveability, but not sacrifice speed?

Thanks, David

 

[shoboat]

David,

You need to reduce the blade diameter 1 mm at a time until it mills.

1 mm off of each blade should do it. You could try a Barr-cut at the

hub also,a 1/4" notch aprox.

Thanks,

Mark Sholund

Props-4-U

 

[Jerry Wyss]

Great Topic Men,,Mr. Dan Cousin, I think that first rcboat.com segment is gonna be good medicine for my sport 40-II.

I have several 1455 props, 2 and 3 blade, but never knew how to mod them much. At sea level here on the Oregon

Coast they run great just the way they come , (with thinning,sharpening and balancing of course) But take them inland

to a race and the ass is dragging low , stearing left all the time is the order of the day. My remedy has usually been going

to a Prather 240 thats now at the bottom of the pond.

Bravo! On the Timely Topic. Jerry

 

[Brian MAhoney]

Good discussion here.

For some of us beginners it would be usefull if we had some simple pictures of the various cuts.

I think but am not certain I know what they all are.

Eg

Barr-cut I think this is a semi circular notch in the trailing edge of the blade near the hub.

Also do we ahave a FAQ section, it would be a shame for this discussion to drift away into the past.

Brian

 

[nitrotoys]

Well Thanks Andy for the great info. Hes got to be the best around for props.

Unreal how he gets them soooooooooooooo thin.

This is a great topic, I try to learn what i can on props, and the only way your going to learn is do one thing and test it, write it down, test a stock prop. WHat did it do for you? good-bad?

 

[Slideblues]

Brian here is a few pics for examples,

Texas Cut:

 

[Slideblues]

Prop on left is stock, Prop in the middle has been Backcut and slight Tongue Relief cut:

 

[Slideblues]

Prop on left has extreme Barrcut and large Tongue relief, Prop on right is stock:

 

[Slideblues]

Std example of Barrcut here:

 

[Slideblues]

Extreme thin and Way sharp,

Dat all I got........

AB 1440 as recieved:

 

[lohring]

An interesting website is http://www.mercurymarine.com/chapter_4_-_p...ller_technology . Be sure to read all the way to the bottom.

Lohring Miller