A 3-blade prop is faster then a 2- blade ?

[dasboata]

2 blades are faster then 3 blade,, which give you more drag , 3 blades leave the beach better and come out of the turns better they make better heat racing props , when a prop says 70mm pitch no matter how many blades each blade has 70 mm on it ,,most SAW records are set with 2 blades,,, oval records are better set with 3 blades because of there acceleration and better handling characteristics ,, I am pretty much a 2 blade cat guy and 3 blade on a mono guy,,,, although the nitro mono records I set were all done with 2 blades

[Nevs]

(Off topic).Wonder why all records were set with single motor except for cats. Here is the speed record for cats, ( I think):
https://www.youtube.com/watch?v=BeOkOox1ii8
A lot slower then the rigger. But then again also no chance in the weight to power ratio.
Rigger: 3,4 kg7 17kw = 5kw/kg.
Cat:
Hull if it is the same weight as the MHZ. 7300 G
ESC 2400 G
motors 3300 G
lipo 3000- ? G
"Other things"/the rest 1000 G
Total would at least be 17 kg.
That is 5x times the weight of the rigger. So it would take at least 85 kw/43 kw pr. motor to have the same ratio. On 12s. that would be roughly 2000 A or 1000 A each motor. So you would probably have to go 2p. each motor, (19 kilo).

[Speed3]

One thing I was specifically looking for when data logging is propeller efficiency.

I would use a GPS to measure speed then I would downloaded the data from the data logging ESC.

Once the propeller pitch is known.

I would calculate the theoretical maximum speed for the given prop. (Load rpm/60* propeller pitch/12/1.466)

60 is used to convert revolutions per minute to revolutions per second.

12 is used to convert inches per second to feet per second ( remember propeller pitch for an rc boat is in millimeters or inches, I convert pitch to inches.)

1.466 is used to convert feet per second to miles per hour.

So once my theoretical maximum speed is know. I now calculated propeller efficient by using ( actually speed recorded with gps/ theoretical maximum speed.)

Having said that I have gathered that smaller lighter boat of a similar hull type have a higher propeller efficiency.

Adding weight to a hull decreases efficiency.

Just some basics.

I have seen propeller efficiency can be very high on some hulls. Example on a proboat mystic I found the octura m545 to be 94% efficient.

It was almost 78% efficient on the aquacraft revolt.

I did the efficiency on almost all my props.

On the matter of which is faster 3 blade versus 2 blade I don't think it have a 2 blade that is faster than my 3 blade at least on lighter boats. My 3 blade at 46mm loadeds around the same as the m545 as far as ampdraw goes.

It allows the motor to rev the same as the m545 but it has more pitch and is more efficient.

The reason it is faster than even larger 2 blades at least than the x450.

While the x450 have more pitch. It have a lower propeller efficiency than this prop.

And because the x450 load the motor so much more than my 3 blade the decreased rpm of the 450 has it being slower than my 3 blade.

Imaging my 3 blade pulling 50% percent less amps than a x450. So for reference if my prop is pulling let us say 60 amps the x450 is pull 90 plus amps. Actually it more like 65 amps versus 100.

I have a ABC 1814 17 45 2 blade and it is faster than that too by far. And loaded less than that too.

When it was stock it loaded very close to the x450 (95 amps versus 100amps) and the rpm was slightly less than the x450. I am referring to the 3 blade.

The point I am trying to make is this I am not necessarily trying to Bragg about a prop but here is the point. A properly prep 3 blade is faster than a 2 blade.

[Nevs]

94% is insane, (the WR rigger was 88,5%). I think the highest efficient I ever had in a mono was about 79%, (twin). But calculatet with teoretic 14,8V and no logs on RPM or anything.
https://www.youtube.com/watch?v=OXqVuErFflo

[Fluid]

One thing I was specifically looking for when datalogging is propeller efficiency...

The problem with the math is that it is missing an accurate pitch value. Most props used today have some progressive pitch, and all have some cupping on the trailing edge. So what is the effective pitch, that which ‘supplies the thrust’? If you bother to actually measure the blade pitch at several points with a pitch gauge you’d realize this problem - you cannot always trust the nominal pitch values listed by the prop maker.

Too, props are not screws, they can build thrust by the difference in pressure between the face and back of the blade, making blade shape and cross section as important as the pitch value. These facts make all those pretty calculated “efficiency” values pretty meaningless.

In reality who cares about the true pitch (whatever that means), it is the performance of the boat which matters. I spend my time maximizing overall boat performance, not worrying about some unknowable value for an equation. But it’s your time, spend it how you want....


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[Speed3]

I don't go by the manufacture rate pitch. I was experimenting trying to calculate pitch and I came up with a method. Which when used I think is very reliable. Why I think it is reliable because I get the same pitch that the manufacture state for most props.

But the props I didn't get the rated pitch for is the m series octura and the 1814 17 45 abc.

Well the detunging of a prop removes material from the prop so it cannot be the same as the original. I measure the to have less pitch.

The ABC have a higher pitch ratio than 1.4 based on my method. But I measured the grim racer 4255 at 55 mm pitch.

The graupner k42 at 59mm pitch. The octura x442 is around 59mm pitch too. It have other props measured and got the same rated pitch for also.

I just give 3 examples to show how my method stake up against the manufacture rating.

I take a ruler and measure from the center of the torgue to the tip on the side that push water and multiply it by 2 for a 2 blade prop.

I have a prop that is a 6717 3 blade which works out to 114mm when you multiply 1.7 * 67. My measurement gives me 38mm pitch for each blade.

If you multiply 38mm by 3 you get 114mm.

The octura 3 blades are different though.

For octura at least for the x series a 3 blade prop has 3 blades that is similar in size to a 2 blade similar in diameter.

Example a x440 2blade may have each blade measures 28mm per blade using my method. The 3 blade version would have each blade measuring 28mm.

So octura 3 blade props I don't know what to say about them. I will say this though if the method used for the 6717 3 blade prop is used.

Then an octura 3 blade have 1.5* more pitch than it is known for. Example a x646 3 blade has each blade at 34mm. So that would be 102mm total when 34mm is multiplied by 3.

[Nevs]

Think octura is more "real". In a twin setup with x440 2-blade you will have the same pitch and prop area as with a single x440 4-blade, (with same blade size and shape). In the twin each blade of the 2 props would cover 28mm (parralel) with the x440 4-blade each blade would have to cover 14mm. But that is the same as saying there are 2 blades covering a distance of 28 mm