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Old Nov 08, 2014, 08:37 AM
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Does a 4 Blade Prop give 1.33x Tthrust of a 3 Blade

If a motor is working well within its limits so motor efficiency plays no part does a 4 blade prop give 1.33x the thrust and use 1.33x the amps of a 3 blade where the blades are individually identical with the same pitch etc?

I am thinking of scale model brass props.

Logic says yes but.........................
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Old Nov 08, 2014, 08:57 AM
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I always suspected so, but I've never seen any DATA to prove the point.

In my arena of SCALE boats, it's always been a moot question. If the real boat has 4 blades, I use a 4 blade prop, etc. And with scale boats (as opposed to go-fast racy boats), it's easy to over-power the model regardless, so I don't much worry about it.

I suppose I could expand my CFD propeller study to compare 3 and 4 blades...
http://www.rcgroups.com/forums/showt...2#post28741482

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Old Nov 08, 2014, 09:36 AM
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not exactly, as a few variables come into play. The blade area will no doubt differ, as there is less "room" with the extra blade, amp draw will increase which is a play off with run time as batteries will deplete faster, in theory.

I have a 7kg Veron Huntsman 28, 4 foot long. I like to dabble and tinker, general sailing isn't my thing. I have experimented at great length with props, batteries, motors etc. Assuming you have a single screw boat, have a read of this, you might pick up some useful info, its a build log of my boat, with a lot of testing using eagle tree data logging.

http://www.modelboatmayhem.co.uk/for...c,21584.0.html

I also run a belt driven gearbox, which I can change the ratio with depending on the lake I sail on, small lake, fast acceleration, no need to a big top speed, large lake, the opposite. At the end of all my work, there was very little difference between a plastic cheap 52.5mm "x" prop and the 50mm 3 blade custom made cleaver type prop

Paul
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Old Nov 08, 2014, 09:59 AM
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The difference in performance between a three and four blade propeller with the same area on each single blade of the same pitch and diameter is in the amount of slippage reduction due to smaller bites of equal proportion around the diameter of the prop.

The four blade propeller will also generated less vibration due to the fact that more blades will remain under load thru the rotation of the blade. In full submerged use the four blade propeller will produce more thrust at a given fixed RPM.
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Old Nov 08, 2014, 10:27 AM
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Hmmm... sounds good... though I'm reminded of the fable of Plato's horse:

A number of students were arguing about how many teeth a horse should have. After the expenditure of much oxygen and expelling of similar amounts of hot air, it was suggested that they could walk over to a horse and COUNT its teeth. Duh.


a. Find two props of same diameter and pitch, with 3 and 4 blades.

b. Mount on the boat of your choice.

c. Hook it up to a sensitive fish scale to run a bollard pull test.

d. Monitor motor current with a WattsUp meter or similar.

e. Run them and see if a difference can be found.
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Old Nov 08, 2014, 11:02 AM
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Patmat2350
I was rather hoping that someone had done just that! If you do expand your study please post the results.

pmdevil
Love the build thread. Such a nice boat> I must try & get to Wickstead this year.

I rather hoped that someone had done a practical experiment with props that are identical apart from the number of blades. With model aircraft everybody tests lots of props with each motor but in boats it is rarely spoken of.
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Old Nov 08, 2014, 11:08 AM
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Originally Posted by ChrisE View Post
Patmat2350
I was rather hoping that someone had done just that! If you do expand your study please post the results.

Ah, please don't mistake me for someone who might actually do this! As I said, it's a moot issue for scale boats...
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Old Nov 08, 2014, 01:40 PM
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SO, many variables, I have yet to see a 4 bld, that has the "same" size, and shape blades, as the corresponding 3 bld. As others have said, try it and see. Watt's meter or, digital VOM meter, RPM tester, AND, a "high quality" Digital Fish Scale. The ones that run $80 - $100 +. THEN, you will have a pretty good idea of what you have.
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Old Nov 08, 2014, 01:50 PM
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If you were to do testing of the different props it should be done in a test tank and both props should be running at the same RPM when under static load. You will also be able to check thrust along with the amp load on the motors running at rpm.

The four blade propeller should be pulling a higher amp draw due to the increased load at operating rpm.
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Old Nov 08, 2014, 09:34 PM
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Quote:
Originally Posted by patmat2350 View Post
Ah, please don't mistake me for someone who might actually do this! As I said, it's a moot issue for scale boats...
Exactly!!

If it's a scale boat, you install the scale propeller that looks correct.

If you want performance, the scale prop is of no use. (at all)

For example: I have a 48" Dauntless with some "pretty" brass props that look just gorgeous when the boat is on display.

When I put the boat in the water, nobody can seen the black nylon monsters used to blast the huge boat out of the water and make it race across the pond!!

I hope this gis you some perspective.

Dumas Dauntless Brushless Motor Test (1 min 4 sec)
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Old Nov 08, 2014, 09:45 PM
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There is some reason racing boats nearly all have 2 blades. When I was racing outboards in the 1950's I did see a few 3 blade props but obviously they weren't competitve. 2 vs 3, 3 vs 4, probably the same factors apply, more weight, more friction and a bit of black magic.
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Old Nov 08, 2014, 10:53 PM
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Originally Posted by 420TEE View Post
There is some reason racing boats nearly all have 2 blades. When I was racing outboards in the 1950's I did see a few 3 blade props but obviously they weren't competitve. 2 vs 3, 3 vs 4, probably the same factors apply, more weight, more friction and a bit of black magic.
The 2 blades will have the best top speed, but the three blades will come out of the water faster. More blades give better power for cruising or battling in rough water. Flat out on smooth water, the 2 blade prop wins.

On a small 9.9 horsepower outboard, the difference could be negligible. The 2 blade prop would probably have the overall race advantage on a small hydroplane. The optional three blade props were for pushing the heavier weights of a larger fishing boat through choppy waters.

So why so many blades? The trick is to have power and top speed efficiency. This can get really pricey if you want all things to perfection. You can add more blades with a slight reduction in top speed, but your gonna need a slide ruler and a good engineer.

Lastly, when you have a ton of power, you can only make a 2 blade prop so big and then you have to resort to 3 blades and so on....

This is why a turbine powered catamaran will use a 5 blade prop to harness 1000's of horsepower. The boat will still be able to keep top speeds in calm water near shore but can transition to rough water on the outmost part of the course.

I'm using a three blade nylon prop for better pushing power to get on plane and there's only so much clearance under the hull.
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Old Nov 09, 2014, 10:45 AM
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Blades.

Or---- you could go this route. lol
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Old Nov 09, 2014, 01:39 PM
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Yo dawg, I heard you like blades...

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Old Nov 09, 2014, 03:28 PM
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some really good points here, also, a heavy boat will need more torque, hence racing lightweight boats generally prefer the alloy 2 blade props. I recently did a twin screw project and again achieved great performance (25mph with eagle tree data logging, yes pedestrian for a racer, but quick for a 6.5KG ply 4 foot scale fireboat!)

The nylon two bladers where as good in performance, but these custom made prop shop props gave an extra 5 mins run time as they dragged less amps with only a 5mph top speed reduction. (and they are pretty!)

We are really getting deep here, perhaps you could advise what boat, what motor etc you are using, as stated before, there are so many variables
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