Lifting Or Non-Lifting Prop - How Can You Tell?

[Tim van Schyndel]

I understand that the Octura X series props are non-lifting, and the 14 and 16 series are medium lifting props. Why??........



There is obviously a blade shape difference but what's in the shape that effects the lift characteristics?



An experienced boater looked at a prop on the weekend (ABC 2218) and said that it would be a very high lift prop purely by the shape (I didn't mention what prop it was). Was this correct and again, if so, then what is it about the 2218 that makes it high lift compared to a 16 series for example!



If this has already been discussed, or if the answer is too complicated to write on this forum, then can I please be pointed in the right direction for reference material etc or older threads that would be great.



Thanks again Int Waters community!



Tim

 

[Glenn Gates]

I cannot tell what is different about the shapes of the props. The X series seem to work well on pickle fork hydros while the 1400 series seem to work better on the round nose boats.

 

[Andy Brown]

Low pitch to diameter ratio (1200-1400) is normally, but not always, low lift. Higher pitch to diameter ratio (1600 - 1900) is normally, but not always, higher lift.

Rake angle however can change that general rule. Low rake angle lifts more than high rake angle.

A high pitch to diameter ratio prop can be designed to lift less by designing with higher rake angle.

There are other factors, but those are the major points to look at.

Pitch to diameter ratio is fairly easy to guesstimate with the naked eye. Contrary to popular belief, rake angle can not be.

It really does take a lot of experiance to just look at a prop and have a good idea what it will do. I don't think there is enough written that can replace experience, years of it.

 

[Tim van Schyndel]

Low pitch to diameter ratio (1200-1400) is normally, but not always, low lift. Higher pitch to diameter ratio (1600 - 1900) is normally, but not always, higher lift.

Rake angle however can change that general rule. Low rake angle lifts more than high rake angle.

A high pitch to diameter ratio prop can be designed to lift less by designing with higher rake angle.

There are other factors, but those are the major points to look at.

Pitch to diameter ratio is fairly easy to guesstimate with the naked eye. Contrary to popular belief, rake angle can not be.

It really does take a lot of experiance to just look at a prop and have a good idea what it will do. I don't think there is enough written that can replace experience, years of it.

Thanks Andy for your insight here.



If I can draw a bit more from your experience and use the above prop ABC 2218, 2.2" diameter, 3.96" pitch, ratio 1.8. I am assuming you may have one in your vast collection to inspect or already know off the top of your head - what would be your general thoughts on this prop?? high, low, medium and if possible a short brief on why!



Cheers and thanks



Anyone else please chime in....... Mr Sholund - I know you like banging the beryillum.....

Tim

 

[shoboat]

Tim,

At a 10 degree rake angle, it is going to be a pretty high lift propeller.

What are you going to run this on? The blade shape can be changed

however, and it will change the way that propeller runs. Jim did make

Some 2216 & 2218's I believe that had a much higher rake angle, but

they pulled the front end of a hydro up off the water. If you let us know

what you are running we may be able to help you more?

Thanks,

Mark Sholund

 

[Tim van Schyndel]

Tim,

At a 10 degree rake angle, it is going to be a pretty high lift propeller.

What are you going to run this on? The blade shape can be changed

however, and it will change the way that propeller runs. Jim did make

Some 2216 & 2218's I believe that had a much higher rake angle, but

they pulled the front end of a hydro up off the water. If you let us know

what you are running we may be able to help you more?

Thanks,

Mark Sholund

Thanks for your input here Mark

I have a 67 Eagle SGX that I have completely rebuilt and I am not running the centre sponson/ski.

Andy has already suggested the H32 as the prop for this boat and if it’s coming from Andy, then I assume it’s a given.

Please note that I am not trying to reinvent the wheel for this boat as I am sure every variation has probably been tried, but like many, I am attempting to learn through experience and experiment and to gain a better understanding of props in general. I have read many articles on this and other forums and am starting to get a grip on the principles, but have yet to find anything that addresses a props lift other than a brief mention of rake angle and that cupping can reduce lift (excluding Andy’s comments above).

I am aware that the 2218 will be too small for this boat but it was purchased on the basis that it would be a good prop to run in the new CMB and that they are good props for the 45 JAE which is my next project.

In reading Andy’s post above he mentions:

Low rake = high lift

High rake = low lift

Low ratio = low lift (1200 – 1400)

High ratio = high lift (1600 – 1900)

Andy has graciously provided the values to be considered in relation to ratio, but what are the benchmarks to be considered with respects to rake? You have mentioned 10 degrees to be a high lift prop and therefore I can assume that 10 degrees is a low rake angle. What therefore would be considered a medium or high rake angle?

Also, the 2218 has a “cleaver” style blade shape which from appearance provides a very constant LE curve and pointed tip, compared to a 16 or H series prop which has a rounded TE tip and a proportion of the LE is far straighter. How do these very different blade shapes differ in their lifting characteristics? Does the rounded TE tip have an impact here?

Marty Davis’ tech notes is great information but it only relates to the 14 and 16 series Octura props which have a distinctively different blade shape to the cleaver style blades. Would most of the modification principles described in his article convert to a cleaver style blade?

Is annealing a must do when modifying props or is this only required if they have been heat treated?

Thanks again Mark for your advice and information!

 

[Christian Lucas]

Hi,

belong to the position of the blade on the hub you can force or delet lift . This do not eleminate the basic that high pitch prop lift moore than low pitch prop but it will give you the option . This designs i gotvfrom tethered line propellerfreaks from Bulgaria and Ruschia . You easy see it and can virtualy feel it witch position create higher lift and what grap moore down .

 

[Jeffmaturo]

Good info on this thread. Would anyone advocate the use of a pitch gauge?

 

[shoboat]

Jeff,

A good pitch gauge is a must if you want to learn about propellers.

I have six of them now and they all give me different information,

which can really help getting the pitch and cup right on the money.

Recently, I have been measuring leading edge pitch a lot more than

I did in the past. Propellers are a great challenge to learn what they

do, and why they do what they do? What we are finding recently with

some new blade shapes and designs is that the overall blade

diameter is still very important along with how much blade area ratio that

the propeller has in it. Currently testing some new choppers and cleavers

from ABC propeller in Columbus, Ohio. Jim is starting to change the rake

angles and blade area ratios a lot more. Testing at the lake is still the best

way to learn more about propellers.

We are liking the more rake in the propeller, I think it is a big step in the right

direction. A lot less lift.

Thanks For Reading,

Mark Sholund

 

[lohring]

I tried to cover some of the basic theory and propeller geometry topics in a series of Propwash articles. They are in the October 2013 (Theory), April 2014 (Geometry), and April 2015 (Practical) issues. Lift is a more advanced topic and I was mostly trying to cover the basics. I hope they will inspire a few people to roll their own and gain the experience to become the future prop experts.

Lohring Miller

 

[Hydro Junkie]

All this information is great but, for a layman like me, some if it is just technobabble. Pitch and diameter are easy to understand, pitch changing along the blade is as well. To me, however, a rake is a garden tool. Could someone explain what rake is and how it relates to pitch and diameter?

 

[John Beardslee]

Hi Mark, aka Hydro Junkie,

Rake is probably the most difficult thing about a propeller to explain in writing. In person or with a video and prop its fairly quick to understand. Hummm maybe I could do a quick video with my new smart phone. I'll see. Lohring may have one already or illustrations that could help. In the jet boat world and racing jet skis positive rake was a huge performance booster and can be very helpful in and open propeller application as well. I've have more success with using cup from the nose of a prop up to the tip to increase rake rather than bending the entire blade all the way to the hub. Sorry, didn't mean to add to the "tech - no - crap

Thanks, John

 

[Hydro Junkie]

You didn't add anything I would call crap, John. What we all need to remember is that prop science is based on some theory and a lot of testing. The problem is, as I see it anyway, that the selection of a prop isn't much more than a wild guess unless you understand the theory behind why one prop may work and another doesn't on a given boat. I'll be the first to say that my prop knowledge, especially in theory, isn't anywhere it needs to be to be useful in knowing what prop to try other than an X457 is a good starting point on a scale

 

[lohring]

Rake:

Rake.bmp Zero Rake.bmp

Lohring Miller

 

[John Finch]

I knew the trailing edge raked back was a good way to reduce the thrust cone width but i have recently been told the whole blade needs to be tilted back on the hub to get the proper effect. Any experience there?

John

 

[lohring]

From Propellers, Part 2 (see link above):



[SIZE=11pt]Another propeller parameter is the blade rake as shown in the blade cross section picture above. This can be difficult to measure, especially if the propeller doesn’t have a straight rake that goes through the propeller axis. Place a small straight edge along the blade to find a position where it touches from hub to tip. The angle this makes with the axis is the rake. Most stock model propellers have the same rake from leading edge to trailing edge and the rake line goes through the shaft axis. That situation is what the blade cross section picture above shows. [/SIZE]

[SIZE=11pt]The above is for computer people. For the more mechanically inclined, also from the same article:[/SIZE]



Rake is set by angling the fixture in the milling machine. In this case we matched the ABC 2018’s 10 degree rake.

Lohring Miller

 

[Terry Keeley]

I remember Tom Prezentka years ago was asking us how his 21 series props were working on our SAW boats, he was interested because this profile has some rake angle (as well as nice progression). Not sure if any of his other props are "raked".

Funny how that works...

 

[John Beardslee]

Hi John,

If you look at a prop shaft with a prop on it (left hand rotation) directly from the back like looking down a rifle barrel - positive rake is when the blade is tilted to the left and negative rake is when the blade is tilted to the right. You end up with negative rake when you thin a blade to much and it opens up when you run it on a boat and that causes the tail end of the boat to lift or hop out of the water. Very hard to repair - most times I threw them away. I like to use smaller rake by cupping the blade from the tip down toward the nose or lead rather than the whole blade. My reasoning is due to the high RPM's we operate in compared to other applications.

Thanks, John

 

[John Beardslee]

On one of my first visits to Mercury their propulsion engineer Joe McElhaney (sp) showed me what rake was and explained some of the basics on their theory at the time of how it was used or not used.

Thanks, John

 

[John Finch]

From Propellers, Part 2 (see link above):

Hub + Blade Section.jpg

Another propeller parameter is the blade rake as shown in the blade cross section picture above. This can be difficult to measure, especially if the propeller doesnt have a straight rake that goes through the propeller axis. Place a small straight edge along the blade to find a position where it touches from hub to tip. The angle this makes with the axis is the rake. Most stock model propellers have the same rake from leading edge to trailing edge and the rake line goes through the shaft axis. That situation is what the blade cross section picture above shows.

The above is for computer people. For the more mechanically inclined, also from the same article:

P1010260.JPG

Rake is set by angling the fixture in the milling machine. In this case we matched the ABC 2018s 10 degree rake.

Lohring Miller

I see what you are saying now. Nice equipment you have there.

John