Prop questions.

[Samuel Hagan JR]

Does more pitch equal more lift?

 

[shoboat]

Sam,

Good question. I have been working with some higher pitched ABC propellers

recently on my 21 hydro with a lot more pitch and higher rake angle without any

significant lift at all. It will depend what propeller you are working with? If you can

give us an example we may be able to help you more.

Thanks,

Mark Sholund

 

[WillieMcCauley]

No.

Lift happens at the transom.

>Zero degrees rake will lift the transom out of the water and you will have maximum prop walk.

>10 degree rake which is standard for most props gives you less lift on the transom and settles the boat down and reduces prop walk.

>15 degree rake will cause the prop to dig into the water deeper forcing the transom down more. THIS CAUSES THE BOW TO BEGING TO RAISE!

>20 degrees prop is excessive for mot boats as the bow will ride high do to transom digging in deeper.

Pitch is only the distance traveled in one revolution of the prop. More pitch equals a further distance traveled.

 

[John Beardslee]

By running a prop with less rake to start with allows you to maybe tailor it to your boat. It can really lock the tail in and get the best of the performance from the hull and the prop. I've taken a prop that hopped and was almost unrunable and by adding a small amount of rake locked tail and the boat ran really well.

Thanks, John

 

[shoboat]

Willie and John,

I can share some early testing results with you both.

20 degrees of rake on an ABC 1615/3 chopper is not too much rake.

It runs nice and smooth on the water with a 3 to 5 MPH increase in speed

and it throws only a 12" roostertail behind a 21 Mono. Now this propeller

only has a 10% increase in pitch and 5 degrees more rake angle than the 1614/3

chopper that we tested it against. Pretty nice handling and speed improvement

with just a simple propeller change. It does require more horsepower to pull

effectively, but quite a few newer engines have a little more power to start with.

Now on the 21 Hydro we have tested a ABC 1815 and a 1817. The 1817 is 30%

More pitch than are old 1814 that ran well, but I think it may be too much propeller

for a decent heat racing boat. Now the 1815 is another story. It seems to be a much

better choice with only 10% more pitch than our old 1814 propeller. Both of these

propellers have 17 degrees of rake angle designed into them. We have noticed a

Big increase in closing speed versus our older 1814 propeller. We have documented

a 3 to 7 MPH speed increase on three different set ups with the 1815 propeller. The

roostertail is less than three feet high behind the boat with one boat only about a foot

Off the water. It seems like the higher rake angle is lowering the thrust cone quite a bit

more than the older 1814 did. We are still working on more progression numbers thru

the blade face, and we are starting to grind out some of the trailing edge cup and we have

had the RPM's coming up nicely on our test propellers. I think these higher raked propellers

are going to be a Great Addition to our heat racing programs. I have two more prototypes

coming in next week for two other sizes of Hydros, it should be interesting to see how they

work on our 12 and 45 Hydros. I can keep you posted on some of are most recent testing

in the future. In fact, John will see the new propellers in 9 days at our first race of the season in Burton, Michigan.

Thanks For Reading,

Mark Sholund

Props-4-U

 

[John Beardslee]

Hi Mark,

I was going to check to see if you were going to the Mini Gold Cup, glad to hear it's on your calendar. See you there.

John

 

[WillieMcCauley]

Hello Mark,

I am glad you shared the information with us about the propellers. I think it is fantastic how methodical you are doing it as it is the correct way to test props. I am new to RC boating, but raced rc cars for 20 plus years nation wide. I grew up in a boat racing family. My father was a Mercury dealer, my bother was a graduate of Mercury school at age 14 and I was six when they started racing boats. My brothers son is racing a tunnel hull in the sport class and my brother builds the motors, sets up the tunnel and modifies the props for his son as well as two other racers... including one who beats his son. Props are not a black art, they just require and understanding of the basic principles of operation and a very methodical testing program.

I am using 15 degree rake props on my 30cc CP Mono and 29.5cc CP Kat. ABC Props that I cut, shape, thin, sharpen and balance. I test one, log how it ran in all performance areas and may cut/shape/cup more based off of how it ran. If it runs as I hoped, I place in the prop box and start working on another one. (I don't like being caught without a back up or options)

I see you are the owner of Props-4-U and hope to meet you at a race.

Willie

 

[Andy Brown]

Does more pitch equal more lift?

Simply put. Yes, if the pitch is increased and the rake is the same the lift will increase. However most prop designers adjust rake according to the P/D (pitch/diameter ratio) Added pitch requires added rake to achieve the same lift.

Some Octura props have 25 and 30 degrees of rake. Some props have 40 to 50 degrees of rake. Check out the props on the Taiwan Long Tail boats!

 

[WillieMcCauley]

Hello Andy, Just so I am clear on your post... if I am running a 2814 15 degree rake prop and go to a 2815 15 degree rake prop the 2815 will lift the transom more than the 2814?

 

[Chris Wood]

Mark

2 questions

1. Are you cutting off the blades and re brazing them or is Jim making them for you

2. You keep referring to WE IN ALL YOUR POSTS. Who is WE?

Chris

 

[Samuel Hagan JR]

The reason I asked is I'm looking for some more push with props.

450/2 blade has 70 mm pitch

2050/2 blade has 100mm pitch.

 

[WillieMcCauley]

Samuel,

There is not enough information or experience on my part for me to assist you with this answer. In a gas boat (which is what I run) that would be a 30mm increase in pitch ratio or in inches you would be going from a 2.75 pitch ratio to a 3.94 pitch ratio. Too much of and increase for a gas boat unless badly underproped. Your response above does not sound as if you are under proped, so I assume you are running nitro or FE. I do not know or understand what is possible with pitch ratio in those classes so I cannot answer.

The others who have posted to your original question do have that experience to help you.

 

[Samuel Hagan JR]

No problem Willie. Yes I'm running Nitro. Specifically Sport 40.

 

[Andy Brown]

Hello Andy, Just so I am clear on your post... if I am running a 2814 15 degree rake prop and go to a 2815 15 degree rake prop the 2815 will lift the transom more than the 2814?

Yes, that is correct Willie.

The reason I asked is I'm looking for some more push with props.

450/2 blade has 70 mm pitch

This prop has 10* of rake. L.E. pitch of about 55mm and a T.E. pitch of 100 mm, giving a progression of 80%. The Pitch/Diameter ratio is about 1.4/1. This prop will push a heavy, high drag boat, because of the high progression and the low P/D ratio.

2050/2 blade has 100mm pitch.

This prop has 25* of rake. L.E. pitch of 95mm and a T.E. pitch of about 105mm giving a progression of about 10%. The Pitch/Diameter ratio is 2.0/1. This prop will not push a heavy high drag boat. Not because it has 30mm more pitch, but because it does not have enough progression for a high drag boat. This will cause slippage. This prop also has more lift than the X450 because the 25* rake is not enough to compensate for the high 2.0/1 P/D ratio compared to the X450.

My recommendation would be to take a larger X prop, say X455, and cut it to 50mm to get the extra pitch you are looking for. The P/D will increase to about 1.6/1, but you will still have the all important high progression.

One of my favorite props for a sport 45 is a 1450 with the T.E. cupped up to about 4.0" (101mm) The L.E. on the 1450 is 2.7" (69mm) so the average pitch becomes 85mm. I have run 64 mph with that prop on a Phil Thomas Super Sport 45. It handles well and mills easy. A stock 1450 will not work at all. The 1450 must be properly cupped.

 

[WillieMcCauley]

Good morning Andy.

You have and interestng perspective and maybe some day we can meet at a race and discuss it further.

I have not experienced increased lift of the transom when increasing the pitch. Why? If the prop selected does not reach the upper end of the recommended RPM range of the motor, the boat will "always" sacrafice speed and lift. This I agree with and therfore accept your perspective about it being pitch increasing lift. (I generally start with a lower pitch prop and increase pitch when testing so I could come to this conclusion)

But this is not a function of propeller pitch, but a function of nailing the upper end of the rpm range of the motor with the correct diameter and pitch propeller to begin with. If I am at the the upper end of the recommended rpm of the motor with the corret prop diameter and pitch and increase the pitch only to try and get additional top end, then the transom will drop back down and performance is lost due to the increased pitch. The same would be true if I lowered the pitch as the boat would fall out of the optimum rpm operating range of the motor. So my understanding is once you hit the optimum operating range of the motor, any change will cause a less efficent boat with less performance. Therefore it is not a function of propeller pitch.

So, I stand by my understanding of performance and assume you will stand by yours. We could go on for hours, but I would rather do it over a beer and in person. As part of Team Futaba I am sure you would be easy to find at a race.

Willie

 

[Andy Brown]

Good morning Andy.

You have and interestng perspective and maybe some day we can meet at a race and discuss it further.

I have not experienced increased lift of the transom when increasing the pitch. Why? If the prop selected does not reach the upper end of the recommended RPM range of the motor, the boat will "always" sacrafice speed and lift. This I agree with and therfore accept your perspective about it being pitch increasing lift. (I generally start with a lower pitch prop and increase pitch when testing so I could come to this conclusion)

But this is not a function of propeller pitch, but a function of nailing the upper end of the rpm range of the motor with the correct diameter and pitch propeller to begin with. If I am at the the upper end of the recommended rpm of the motor with the corret prop diameter and pitch and increase the pitch only to try and get additional top end, then the transom will drop back down and performance is lost due to the increased pitch. The same would be true if I lowered the pitch as the boat would fall out of the optimum rpm operating range of the motor. So my understanding is once you hit the optimum operating range of the motor, any change will cause a less efficient boat with less performance. Therefore it is not a function of propeller pitch.

So, I stand by my understanding of performance and assume you will stand by yours. We could go on for hours, but I would rather do it over a beer and in person. As part of Team Futaba I am sure you would be easy to find at a race.

Willie

Willie,

I'm not sure about my perspective. So far I have just listed bare facts about some props to answer Samuel's question.

With all other propeller parameters being equal, increased pitch will cause the prop to lift more. I am talking about what the prop itself will do, not about how it effects the engines power curve or hull lift characteristics at different speeds.

When you say you start with a low pitch. Are you changing props to larger pitch props? If so, do you know that all other parameters, diameter, rake, progression, of the prop are the same? If any one or more of them is different the prop will have a different lift character that will mask the true effect of the increased pitch.

Or are you are adding Trailing Edge pitch?, That increases pitch a small percentage and also adds a small amount of lift, but it is not easily detected on some boats.

The added Trailing Edge pitch adds a large amount of load to the engine compared to the small average pitch increase. Adding T.E. pitch is not the most efficient way to increase pitch. The main use of T.E. pitch changes should be in the effort of reducing prop slippage which in turn will increase speed.

Just a few things that I have learned while playing with my r/c boat.

 

[WillieMcCauley]

Thanks Andy,

Hope to meet you some day.

 

[Terry Keeley]

Good morning Andy.

You have and interestng perspective and maybe some day we can meet at a race and discuss it further.

I have not experienced increased lift of the transom when increasing the pitch. Why? If the prop selected does not reach the upper end of the recommended RPM range of the motor, the boat will "always" sacrafice speed and lift. This I agree with and therfore accept your perspective about it being pitch increasing lift. (I generally start with a lower pitch prop and increase pitch when testing so I could come to this conclusion)

But this is not a function of propeller pitch, but a function of nailing the upper end of the rpm range of the motor with the correct diameter and pitch propeller to begin with. If I am at the the upper end of the recommended rpm of the motor with the corret prop diameter and pitch and increase the pitch only to try and get additional top end, then the transom will drop back down and performance is lost due to the increased pitch. The same would be true if I lowered the pitch as the boat would fall out of the optimum rpm operating range of the motor. So my understanding is once you hit the optimum operating range of the motor, any change will cause a less efficent boat with less performance. Therefore it is not a function of propeller pitch.

So, I stand by my understanding of performance and assume you will stand by yours. We could go on for hours, but I would rather do it over a beer and in person. As part of Team Futaba I am sure you would be easy to find at a race.

Willie

Correcting Andy Brown on basic prop characteristics!

Andy's forgot more than most will ever know bud.