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Old Jun 21, 2014, 09:20 PM
FLOOD-CHAMBERS... awesome !!
RC fetish's Avatar
United States, MI, Sterling Heights
Joined Jun 2014
21 Posts
Question
Replace 2-blade with a 3-blade... how much smaller ??

Hey guys,

PLEASE help me

I would like to replace my stock 2-blade prop with a 3-blade prop
(I often read how much better a correct-sized' 3-blade prop is over a 2-blade)
And preferably a CNC Alloy prop, I think?

Stock 2-blade is a 35mm x 1.4" pitch spun by a 2850-3300Kv inrunner & 3S 25C 2700LiPo's in my 22" Offshore Infinity... how much smaller OD should I go with ?
(ESC & motor barely get warm with above combo)

33mm, 32mm, or is that still to large?
(I'd love to get the Red Anodized CNC Atomik 32x1.4 prop IF that is the best size ?)

OR does someone have a cheap multi-pack' of plastic 3-blade props of various sizes below 35mm I can buy & try?

I don't know guys.... you tell me ?

Thank you very much for your time
Peace, Dave
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Last edited by RC fetish; Jun 22, 2014 at 11:54 PM. Reason: new
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Old Jun 23, 2014, 07:03 AM
Steps? What steps?
zozer's Avatar
United States, TX, Humble
Joined Jan 2010
218 Posts
Why do you think that a 3-bladed prop is "better" than a 2-bladed version?

Cons:
- not nearly as many choices in diameter, pitch and style
- can cause more lift, upsetting boat attitude
- can be difficult to find the right pitch/diameter for application
- increases amp draw for same pitch (speed)

Pros:
- might reduce torque roll
- cool looking

If your heart is set on a 3-blade then since the 35mm is working well, go to a 32mm. But the pitch will likely be 10% less so the boat might actually slow down. You can try a 35mm with the same pitch, this will probably increase amp draw but you say you have head room. As long as you check for high temps the first few short runs you should be fine.

BTW, most of the fastest R/C boats in the world ran 2-bladed props (although some ran 3-blades).
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Old Jun 23, 2014, 07:54 AM
Registered User
Joined May 2012
317 Posts
Some boats run better with a 3 blade prop other much worse! All you can do is try a number of props until you find what works the best for your boat.
On some of my boats (mostly the monos) I find a 3 bladed prop has more grunt out of the corners but a bit less top end, probably loosing rpm due to the added load.
Never had much luck with 3 bladers on tunnel hulls, for hydros I usually run a 2 blade prop as well. More choice as posted above. Also much easier to balance and modify a 2 blade prop.
And ALL of then need work out of the package unless you buy from somebody who runs a sharpening and balancing service. No stock mass produced prop is ready to run regardless what the web page selling them claims!
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Old Jun 23, 2014, 06:57 PM
FLOOD-CHAMBERS... awesome !!
RC fetish's Avatar
United States, MI, Sterling Heights
Joined Jun 2014
21 Posts
Sounds like I should just stick with using my finished' 2-blade Octura X435 I already bought
(35mm x 1.4P, same OD & Pitch as stock plastic)

I was thinking the 3-blade Atomik CNC Alloy red anodized 32mm x 1.4P would be much better than the Octura X435... but I guess not

But the darn' Octura is sooo heavy... would a lighter CNC Alloy prop spin-up' faster / run' better than the Octura X435 ??
(although less durable)

Otherwise I'll just use my new Octura X435

Thanks again for your time guys
Dave
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Old Jun 23, 2014, 08:31 PM
RcBoater
Joined Feb 2014
251 Posts
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Last edited by Dennis Manning; Jun 27, 2014 at 07:03 AM.
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Old Jun 25, 2014, 05:05 PM
Studying Engineer
MarcRacer18's Avatar
United States, CA, Santa Barbara
Joined Jun 2009
685 Posts
HobbyKing has a small selection of cnc props at a pretty good price.

For under-the-hull drives less blades is a good thing. There's less drag and higher rpms. Higher rpms usually means faster speeds.

For surface drives this logic doesn't always pan out as black and white. Hydros and other really high rpm models can end up spraying more than pushing. In other words, the rpm is higher but the ratio of how far the boat actually moves per revolution goes down more than the increase of rpm. If you notice any cavitation problems then a 3-blade prop may be what you need.

I'd get 1 or 2 to try out and see what the difference is. As long as your current temps are good, going with the same diameter and pitch shouldn't hurt. Trying out a slightly smaller prop first is always a good thing to do as well. Try the 32mm Red Atomic prop, its surface area is 83% that of your current 35mm prop, the drag increase will not make the difference.

Good luck!
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Old Jun 28, 2014, 10:49 PM
Registered User
brushless55's Avatar
United States, CO, Longmont
Joined Nov 2011
3,573 Posts
Quote:
Originally Posted by RC fetish View Post
Hey guys,

PLEASE help me

I would like to replace my stock 2-blade prop with a 3-blade prop
(I often read how much better a correct-sized' 3-blade prop is over a 2-blade)
And preferably a CNC Alloy prop, I think?

Stock 2-blade is a 35mm x 1.4" pitch spun by a 2850-3300Kv inrunner & 3S 25C 2700LiPo's in my 22" Offshore Infinity... how much smaller OD should I go with ?
(ESC & motor barely get warm with above combo)

33mm, 32mm, or is that still to large?
(I'd love to get the Red Anodized CNC Atomik 32x1.4 prop IF that is the best size ?)

OR does someone have a cheap multi-pack' of plastic 3-blade props of various sizes below 35mm I can buy & try?

I don't know guys.... you tell me ?

Thank you very much for your time
Peace, Dave
if your temps are fine there is no reason you need to go smaller when picking out a 3blade prop
I run 3blade props on a couple of my race boats and love the way they handle in race waters
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Old Jul 01, 2014, 04:18 AM
Redneck Injuneer
bill34207's Avatar
Bradenton, Florida
Joined Aug 2007
6,390 Posts
Quote:
Originally Posted by RC fetish View Post
But the darn' Octura is sooo heavy... would a lighter CNC Alloy prop spin-up' faster / run' better than the Octura X435 ??
(although less durable)

Dave
I have experimented with some of the CNC props and they tend to be slower than a nicely sharpened & balanced Octura of the same size.
IE I bought a pair of counter rotating CNC 445's to prop down a little from Octura 447's. Big loss in speed. Just for giggles tried a pair of Octura 442's and picked up a couple mph over the CNC 445's.
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Old Jul 01, 2014, 07:29 AM
Registered User
brushless55's Avatar
United States, CO, Longmont
Joined Nov 2011
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Quote:
Originally Posted by bill34207 View Post
I have experimented with some of the CNC props and they tend to be slower than a nicely sharpened & balanced Octura of the same size.
IE I bought a pair of counter rotating CNC 445's to prop down a little from Octura 447's. Big loss in speed. Just for giggles tried a pair of Octura 442's and picked up a couple mph over the CNC 445's.
Same here Bill
I have a few of those CNC props and not really a fan of them
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Old Jul 02, 2014, 02:40 PM
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cyberhoops's Avatar
Seattle
Joined Feb 2006
979 Posts
From what I recall from the mathematics of propellors - if all things are equal then going from 2 blades to 3 blades will increase amp draw by 1.5x. All things being equal meaning you have the same blade shape and area, which is more likely with airplane props, but not usually even close with boat props.

Meanwhile, the effect of diameter on amp draw is dia^4. Going from 35mm to 32mm would be 35^4/32^4 or 1.43x.... once again all things being equal which would balance out the 3 blades especially once you factor in the reduced pitch and smaller individual blade area.
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Old Jul 02, 2014, 06:27 PM
Studying Engineer
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United States, CA, Santa Barbara
Joined Jun 2009
685 Posts
Quote:
Originally Posted by cyberhoops View Post
From what I recall from the mathematics of propellors - if all things are equal then going from 2 blades to 3 blades will increase amp draw by 1.5x. All things being equal meaning you have the same blade shape and area, which is more likely with airplane props, but not usually even close with boat props.

Meanwhile, the effect of diameter on amp draw is dia^4. Going from 35mm to 32mm would be 35^4/32^4 or 1.43x.... once again all things being equal which would balance out the 3 blades especially once you factor in the reduced pitch and smaller individual blade area.
1.5x is a huge increase. If you find where you heard this I would be interested in logic testing this theory.

1.5x amp increase means that the rpm of the motor is being reduced to 2/3 the previous speed. That means that the speed of the boat would be reduced to 2/3 the previous (everything else being equal). I cannot conceive this to be true (especially when when my rc hydro runs 5mph faster with a 3-blade vs a similar sized 2-blade). Could your calculation be assuming that; the speed of the fluid relative to the prop is constant (0), and/or the increase of each blade's fluid acceleration from the previous blade is also constant (0)?

Diameter^2 would give you the ratio of surface area between both props.
Specifically:

(Pi*(D1/2)^2)/(Pi*(D2/2)*2)

What does the Diameter^4 reference in your calculations?
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Old Jul 02, 2014, 09:21 PM
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United States, CO, Longmont
Joined Nov 2011
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Quote:
Originally Posted by cyberhoops View Post
From what I recall from the mathematics of propellors - if all things are equal then going from 2 blades to 3 blades will increase amp draw by 1.5x. All things being equal meaning you have the same blade shape and area, which is more likely with airplane props, but not usually even close with boat props.

Meanwhile, the effect of diameter on amp draw is dia^4. Going from 35mm to 32mm would be 35^4/32^4 or 1.43x.... once again all things being equal which would balance out the 3 blades especially once you factor in the reduced pitch and smaller individual blade area.
This is incorrect for real world use
my race boats are faster and run better with 3blade props
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