What causes a prop to ?lift??

[jingalls007]

I understand the purpose of lifting props, but what part of the prop does the lifting?

Ex: ABC 2115*17*45 vs ABC 2115*15*63
Which set of numbers dictate lift and how does it work?

Thanks doodes!

[dasboata]

I understand the purpose of lifting props, but what part of the prop does the lifting?

Ex: ABC 2115*17*45 vs ABC 2115*15*63
Which set of numbers dictate lift and how does it work?

Thanks doodes!

It's how the water comes off the blade tips , example a 10* prop is almost 90* to the water,,,,the spray is pretty much going down,,, this will cause transom lift, by the force of the water coming off the tip ,,,, a 17* will push the water more behind the boat , , With gas I prefer 20* rakes and trim the boat accordingly !!!

[jingalls007]

It's how the water comes off the blade tips , example a 10* prop is almost 90* to the water,,,,the spray is pretty much going down,,, this will cause transom lift, by the force of the water coming off the tip ,,,, a 17* will push the water more behind the boat , , With gas I prefer 20* rakes and trim the boat accordingly !!!

Thanks Chris! So the higher the rake the lower the lift?

[dasboata]

Thanks Chris! So the higher the rake the lower the lift?

Yes ,,,,don?t forget it transom lift model boater should refer to


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[NativePaul]

Yes ,,,,don?t forget it transom lift model boater should refer to

Yup, All boaters should refer to transom lift IMO. It is a pet peeve of mine that many full scale boaters refer to bow lift when talking about the amount of lift a prop does or doesn't have.

As far as I am aware no prop can physically have zero lift, never mind the negative lift that would be required to lift the bow when aligned parallel to the water surface. You can just have more or less prop/transom lift, which pivots on the CoG and pushes the bow down more or less.

[raptor347]

Chris is right.

The rake number is the first indicator of lift. Higher rake=less lift on otherwise equal prop designs. Increased rake tightens the thrust cone coming off the prop.

The xxxx-17-45 and xxxx-15-60 designs are very different, not only in rake but also pitch progression and leading edge sweep.

Number of blades and area has a lesser but not insignificant effect on lift. From there it gets more subtle. The leading edge sweep and pitch progression play a part. Even the geometry of how you sharpen the leading edge can change lift.

[jingalls007]

Hydrodynamics is a fascinating topic. It?ll be easy for me to remember that the higher the rake the tighter the cone, in essence.

Are lifting props therefore more sensitive to strut angle adjustments?

[raptor347]

Strut angle is more a function of hull/prop combination. Some hulls require more prop lift than others for proper ride attitude. Say you have a boat that likes a higher lift prop, but with that prop can run a flat strut (zero angle). That same boat can be made to run with a low lift with several degrees of negative angle in the strut. Any given boat/prop combo will have a fairly narrow strut angle range where it runs efficiently. Example: my raptor rigger, the strut angle range is from dead flat to a maximum of .015" under the nose of the strut (speedmaster sport 20 flat bottom strut). I use a feeler gauge on the setup board for the adjustment.

Major generalization warning: Low lift props are usually more efficient than high lift. If you have two props that with the same energy input, the high lift prop uses a higher percentage of that energy in lift.

[srislash]

This thread should be sticky’s. Very good explanation Brian, Chris, and Paul.

It is so hard to find Easily understandable Descriptions. One has to wade thru pages of dynamics in order to decipher a theory,

Shawn

[RossCo216]

I totally agree with Shawn, this thread has really good common sense info for basic concepts of props characteristics and I would like to learn more about the other aspects of props.

I do have a follow up question: Would a round ear 2 blade prop have more lift than a standard 2 blade prop given the same prop diameter and pitch?

I am trying to learn about the basic prop differences so I don?t blindly buy a bunch of props that are totally wrong for my boat. I want start doing some test and tuning sessions on my boat so I can start racing at the local club.

Thanks in advance!

RossCo

[larryrose11]

I am trying to learn about the basic prop differences so I don?t blindly buy a bunch of props that are totally wrong for my boat. I want start doing some test and tuning sessions on my boat so I can start racing at the local club.

Rosco, That is what the CNC props are for, cheaply trying things out.
Caution with the CNC props: They are soft in comparison to other metal props available. DON'T GUN IT with these CNC props on a powerful setup, you can fold the blades.

[srislash]

I totally agree with Shawn, this thread has really good common sense info for basic concepts of props characteristics and I would like to learn more about the other aspects of props.

I do have a follow up question: Would a round ear 2 blade prop have more lift than a standard 2 blade prop given the same prop diameter and pitch?

I am trying to learn about the basic prop differences so I don?t blindly buy a bunch of props that are totally wrong for my boat. I want start doing some test and tuning sessions on my boat so I can start racing at the local club.

Thanks in advance!

RossCo

So a round ear will spread the wash from the propeller whereas a standard pointed tip prop directs the wash more rearward. Round ears are traditionally better for turning/cornering.
I personally have not dabbled with the two.

[dasboata]

I totally agree with Shawn, this thread has really good common sense info for basic concepts of props characteristics and I would like to learn more about the other aspects of props.

I do have a follow up question: Would a round ear 2 blade prop have more lift than a standard 2 blade prop given the same prop diameter and pitch?

I am trying to learn about the basic prop differences so I don?t blindly buy a bunch of props that are totally wrong for my boat. I want start doing some test and tuning sessions on my boat so I can start racing at the local club.

Thanks in advance!

RossCo

a round ear lessen the lift,,by boarding the spray coming off the tips ' I am a big fan of round ears

[Fluid]

Whether a blade is round or pointed, the prop rake and cup are the factors which most affect the shape of the thrust cone. A wide cone will generate more lift, a narrow cone produces less lift. More rake equals a more narrow cone. And more cup usually equals a narrower cone. Blade shape can affect how effective the above two factors are, but by itself it means little. The world’s fastest propellers are cleavers.




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[RossCo216]

Thanks for all the feed back everyone. I have been experimenting with some of the CNC props of various pitch (1.4-1.9, 38-40mm) which do make it nice to try a few ideas out before buying a higher quality prop. If the round ear prop broadcast the spray doesnt that mean it would have a larger diameter thrust cone? And wouldn?t that cause the transom to lift? I am running a x640 right now on my 32?mono hull and it seems to be getting the hull up and chine walking a bit to much. I have the stinger set flat/parallel with the keel. So I was thinking a tighter thrust cone could help reduce this or a set of trim tabs.

Either way I want to try different props and see how they change the boats characteristics. One step at a time or else I will never figure out the puzzle!

I recognize both your names Dasboata and Fluid from other posts I have been skimming and realize you both have a much better grasp of props than me! Thanks for trying to explain it to me.