I read that the record for an electric is 120mph and was done with a prop that was spinning at 48,000rpm. I'm interested in nitro out-riggers and was wondering if a prop is more efficient at high rpm with a small diameter or slower rpm with a bigger diameter prop. The nitro guys say you get to much prop walk with a big prop (surface type).
Prop speed
- Thread starter Mark
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Climate Models
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A larger prop will always be more efficient than a smaller one. It simple physics.
The problem with props on the models is that you need to balance efficiency with reality.
A large prop may not be the most efficient for the motor driving it, or for the design of the hull it is driving. The motor has to maintain a certain RPM to be in its "Power band" Going to a larger motor is one option, but then you encounter other problems such as weight, balance, etc. Prop walk is one of these problems.
We aim for the best balance of all factors to achieve the highest speeds. This often means using a prop that is smaller and therefor less efficient.
Going to more blades on a prop can help to reduce prop walk, but again you loose efficiency in a multi bladed prop.
The overall package is a good balance of all factors and becomes acceptable within the limitations we work with.
The moment you come up with the "Perfect" solution, please be sure to let us all know.
Peter R.
www.climatemodels.com
The problem with props on the models is that you need to balance efficiency with reality.
A large prop may not be the most efficient for the motor driving it, or for the design of the hull it is driving. The motor has to maintain a certain RPM to be in its "Power band" Going to a larger motor is one option, but then you encounter other problems such as weight, balance, etc. Prop walk is one of these problems.
We aim for the best balance of all factors to achieve the highest speeds. This often means using a prop that is smaller and therefor less efficient.
Going to more blades on a prop can help to reduce prop walk, but again you loose efficiency in a multi bladed prop.
The overall package is a good balance of all factors and becomes acceptable within the limitations we work with.
The moment you come up with the "Perfect" solution, please be sure to let us all know.
Peter R.
www.climatemodels.com
Chris Thomas
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- Nov 10, 2002
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- 475
Hello Peter, why do you say that there is an efficiency level loss in a multi-blade prop? How or where did you derive this info. I am interested to see how they got this info or documentation.
G
Guest
Guest
If you use a for example X640-2 blade and your boat runs a 32 mph. using 40 amp's at 16,000 RPM
Now you put on a X640 -3 blade and your boat runs at 33 mph. using 50 amp,s at 14,000 RPM.
Just because you gained 1 MPH does not make it more efficient.
Now if you use a X638 - 3 blade and your boat runs a 32 mph. using 40 amp's at 16,000 RPM
same as your original setup. But now you have less prop walk.
LARRY
Now you put on a X640 -3 blade and your boat runs at 33 mph. using 50 amp,s at 14,000 RPM.
Just because you gained 1 MPH does not make it more efficient.
Now if you use a X638 - 3 blade and your boat runs a 32 mph. using 40 amp's at 16,000 RPM
same as your original setup. But now you have less prop walk.
LARRY
Climate Models
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Hi Chris,
It is well documented that a multi blade prop loosed efficiency with each blade added.
When I used to race pylon planes back 15-20 years ago, the top guys in the fastest classes all used 1 bladed props. Why? because it was much more efficient. This is in part due to the blades interfering with the "bite" of the next blade as it passes through a disturbed medium from the previous blade.
The single blade got a clean bite no matter where it was in the revolution.
The more blades you have stirring up the medium, the less they have to grab. There is also the frontal area of the prop. In theory, the prop is a solid disk when spinning.
The more blades it has, the more "real" this frontal area actually is.
Now I am not suggesting that you go out and rig yourself up a one bladed prop for sprint racing. Heck, the thing would walk like crazy. Each time that blade hit the water the back of the boat would probably move over an inch or more.
Look at the post by Larry. It pretty well explains efficiency. It is a balance of factors, not just one simple part of it. A good racer will have several different props for the same boat.
One might be for rough water where the driver knows that the prop will be getting a lot of air. You can go up in pitch and diameter because the prop will be spending less time in the water. It is more efficient in this configuration.
The others will be for different conditions like smooth water, SAW runs, etc.
Efficiency = the highest RPM + the least heat + the least prop slip + the lowest resistance on the hull from air and water.
Peter R.
www.climatemodels.com
It is well documented that a multi blade prop loosed efficiency with each blade added.
When I used to race pylon planes back 15-20 years ago, the top guys in the fastest classes all used 1 bladed props. Why? because it was much more efficient. This is in part due to the blades interfering with the "bite" of the next blade as it passes through a disturbed medium from the previous blade.
The single blade got a clean bite no matter where it was in the revolution.
The more blades you have stirring up the medium, the less they have to grab. There is also the frontal area of the prop. In theory, the prop is a solid disk when spinning.
The more blades it has, the more "real" this frontal area actually is.
Now I am not suggesting that you go out and rig yourself up a one bladed prop for sprint racing. Heck, the thing would walk like crazy. Each time that blade hit the water the back of the boat would probably move over an inch or more.
Look at the post by Larry. It pretty well explains efficiency. It is a balance of factors, not just one simple part of it. A good racer will have several different props for the same boat.
One might be for rough water where the driver knows that the prop will be getting a lot of air. You can go up in pitch and diameter because the prop will be spending less time in the water. It is more efficient in this configuration.
The others will be for different conditions like smooth water, SAW runs, etc.
Efficiency = the highest RPM + the least heat + the least prop slip + the lowest resistance on the hull from air and water.
Peter R.
www.climatemodels.com
Hey Chris,Peter,and other Interested Boaters,
How about a formula to calculate propeller tip speed? Remember that the ideal
tip speed is between 180 and 200 MPH.
RPM x 3.1416 x prop diameter in inches, divide this by 1056= tip speed in MPH's
Example: 21 SG outrigger:
32,000 RPM x 3.1416 x 1.77"= 177940.224 divide that by 1056 =168.504 MPH
This is an example of one of my boats fellas.
Just A Little FYI For Ya,
Thanks,
Mark Sholund
PROPS-4-U
How about a formula to calculate propeller tip speed? Remember that the ideal
tip speed is between 180 and 200 MPH.
RPM x 3.1416 x prop diameter in inches, divide this by 1056= tip speed in MPH's
Example: 21 SG outrigger:
32,000 RPM x 3.1416 x 1.77"= 177940.224 divide that by 1056 =168.504 MPH
This is an example of one of my boats fellas.
Just A Little FYI For Ya,
Thanks,
Mark Sholund
PROPS-4-U
Last edited by a moderator:
M
mark poole
Guest
Hey Peter. I have never flown planes so i have a stupid question. :huh: What sort of balancing.....or counter balanacing tricks do those guys use to run a one bladed prop??
Climate Models
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- Nov 23, 2003
- Messages
- 140
When we used to custom make our one blade racing props, we would take a stock high pitch 2 blade prop, cut off one blade, leaving about 1/2" -3/4" of the cut prop blade still on the hub.
We would attach steel weights to this stub on both the front and back of the prop with small bolts and lock nuts.
We tried lead but it was way to soft and would fly apart at 30000+ RPM.
The poor plane would beat itself to death with the off balance prop in a fraction of a second. There was usually nothing left of the plane or motor when this happened.
The counterweight was then enclosed inside a spinner. We used the CB associates spinners as they needed to be cut out for the prop blades and you could only cut out one side if you wanted too.
The entire assembly was placed on a static balance shaft and small brass washers were added to the bolts holding the counterweight on to get a perfect balance. It took some time to get it just perfect. You could cheat when you got close and use a drill to make a small hole in the aluminum backplate of the spinner to take off just enough weight to get it perfect.
Man those were the days. Flying zippy little airplanes around at 150 MPH with those .15 engines just on the edge of blowing apart.
Here is a thread I found on the subject over at RC Universe.
http://www.rcuniverse.com/forum/m_396244/mpage_3/key_/tm.htm
Peter R.
www.climatemodels.com
We would attach steel weights to this stub on both the front and back of the prop with small bolts and lock nuts.
We tried lead but it was way to soft and would fly apart at 30000+ RPM.
The poor plane would beat itself to death with the off balance prop in a fraction of a second. There was usually nothing left of the plane or motor when this happened.
The counterweight was then enclosed inside a spinner. We used the CB associates spinners as they needed to be cut out for the prop blades and you could only cut out one side if you wanted too.
The entire assembly was placed on a static balance shaft and small brass washers were added to the bolts holding the counterweight on to get a perfect balance. It took some time to get it just perfect. You could cheat when you got close and use a drill to make a small hole in the aluminum backplate of the spinner to take off just enough weight to get it perfect.
Man those were the days. Flying zippy little airplanes around at 150 MPH with those .15 engines just on the edge of blowing apart.
Here is a thread I found on the subject over at RC Universe.
http://www.rcuniverse.com/forum/m_396244/mpage_3/key_/tm.htm
Peter R.
www.climatemodels.com
T
TomMoorehouse
Guest
simple physics..... is that like jumbo shrimp?Climate Models said:
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