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[Darren007]

Would 2 30mm props give as much pulling power as 1 60mm prop on a tug?

[derekwarner]

Darren... just a few basic points


edit.........Area diameter of a circle/disc = Pi x r^2......[and am using diameters as a size of the propeller, and blade shape & surface areas are assumed as the same relative to the outer diameter]

one 30 diameter circle/disc = 7.069 cm²

so, two 30 diameter discs = 14.138 cm²

however, one 60 diameter disc = 28.74 cm²

From this you can see the one single larger [60 diameter] disk still has approximately twice the surface area to the of sum of the two smaller [30 diameter] discs

It is fair to assume, that 3 blades surface area on the 60 diameter propeller will have still approximately   twice the total surface area of 3 blades on the two 30 diameter propellers

It is also fair to assume that the one 60 diameter propeller would require and consume approximately twice the total input power as compared to the two 30 daimeter propellers

Hope this helps

Derek

[Fastfaz]

  Hi Darren,
         A lot depends on the size of your tug and whether you want to tow bigger vessels, with a twin screw tug it's has far superior manouverability over a single screw. I have always used either 60mm or 70mm 4 blade props in Kort nozzles when doing towing (I used to do competitions) depending on the size of the tug, the final choice is down to your preference and what you intend to use the boat for. Hope this helps.
                          Regards, Pete 

[Bunkerbarge]

Diameter of a circle/disc = Pi x r^2......[and am using diameters as a size of the propeller, and blade shape & surface areas are assumed as the same relative to the outer diameter]

Surely:

Pi x r² 

is the area of a circle, not the diameter?

[Darren007]

So In theory a single 60mm prop would be better than x2 30mm props for towing?

[derekwarner]

Absolutely correct BB....my error and incorrect terminology    , however the calcs remain correct...[posting noted as edited]

[Geoff]

What a very interesting question and response, I have often wondered this and had always assumed a single large prop would be more efficient than two smaller ones. I do have an observation though, would I be correct in assuming this would all be correct assuming the props are rotating at the same speed.


Not wishing to confuse what I think is an excellent explanation but if because the smaller props are smaller they would rotate faster than the larger props given the same power input so the practical difference may not be as much as the base calculations would suggest.


So for example if the smaller props rotate at twice the speed of the larger prop they would move much the same water and create the same level of thrust? I still think the larger prop would be more efficient.


Cheers


Geoff

[GG]

Unfortunately no one has mentioned the effect of "PITCH" on changing sizes of propellers.


What follows is a typical Engineers "back of the envelope" style of calculation.  No claims for absolute accuracy but it's often good enough to avoid serious problems and show if you are moving in the right direction.


If the the propellers are geometrically similar, then doubling the the diameter will increase the cross sectional area of the cylinder of water that the propeller tries to push rearwards in one revolution by four times. But, doubling the diameter also doubles the pitch, so this cylinder is twice as long.  Hence, doubling a propellers diameter would in this situation actually increase the volume (and so mass) of this cylinder by no less than eight times.....


In this situation doubling the diameter (and pitch) would result in astronomic increase in the power demand to maintain the same shaft speed.  Using this idea, doubling the rate at which water is pushed rearwards only requires the propeller diameter to be increased by around a quarter.


I remember writing something along these lines in "Model Boats" a few years ago.  I wanted to show that it was worthwhile to take a few propellers along when carrying out sailing trials on a new model.  A slight change can have a drastic effect on a models speed and power consumption.  Testing is the only way we can be sure our models are working well and it is never "obvious" which way to go.  I remember changing down to a slightly smaller propeller, which allowed the electric motor to spin faster with less current drawn, yet still the model still ran at the same speed.


Glynn Guest

[Bunkerbarge]

Can you clarify please, I'm afraid I can't quite see how doubling a propeller's diameter necessarily doubles the pitch. 


If you double the diameter of the propeller and all other aspects of the propeller are simply similary doubled to create an exact scale replica then the pitch, the theoretical distance moved through a solid in one revolution, which is directly proportional to the angle of the blade to the perpendicular at a point 75% of the radius must surely remain the same? 


If the angles the blades make to the perpendicular remain the same then the pitch must be the same surely?

[GG]

Bunkerbarge,
              Taking two propellers which are part of a geometrically similar range, i.e. they have the same Pitch/Diameter ratios.  Lets rotate two in a single complete revolution, but one being twice the diameter of the other.


It's easier to consider the tips of the blade rather than a point 75% of of the radius.


The smaller propellers tips will cover a distance = Pi x D


The bigger propellers tips will cover a distance  = Pi x 2D


You could take any position on the blades and it would give you the same result.


Being geometrically similar the large propeller would, without any slippage of course, have advanced twice the distance of the smaller one.


Drawing right angled triangles will show this, the hypotenuse is the distance traveled by the tip, the opposite side is the pitch or the advance.  The angles are the same for both propellers but the hypotenuse of the larger one is twice the length of of the smaller one.




Glynn Guest


   

[Bunkerbarge]

I'm afraid I still cannot see the relationship beween radial angular movement of the tip, which I agree would obviously be greater in a larger propeller, and how this relates to axial movement, which is determined by nothing more than the angle of the blade, which is the same for both size propellers.

[chas]

I found it helped me to think of this like drill bits, one small one large, both with the same pitch. Ignoring slipage, the big one travels further in than the small one.

[GG]

Chas,
       Drill Bits are a useful analogy.  Much easier to see than attempting to describe in words.


Glynn Guest

[Bunkerbarge]

I'm afraid the drill bit isn't the best analogy as it is always effectively slipping. 


The best analogy's usually quoted are either a wood screw through wood, or, by far the best, a bolt being screwed into a nut.  With a bolt its much easier to envisage, you proportionally increase the size of the bolt, you increase the distance the bolt travels for a single revolution.

[rhavrane]

Bonjour Darren,
I would not be able to give an objective answer to your question but would give you another point of view.
I would also like to point out that my suggestion only makes sense if the two propeller shafts have independent functioning.
My answer is “maneuverability”. I have three tugs (including one side paddle) in this configuration and it is a real pleasure to make short turns with them.

I don't know if they would have the same behaviour in real life but if you wish having a glance on the models I evoke :
Resolve : https://youtu.be/gT0vajnl5ps

Aggrip  : https://youtu.be/UngCLWfR7Es

Pilat     : https://youtu.be/qLmOZyA-yKQ

[Andyn]

Can you clarify please, I'm afraid I can't quite see how doubling a propeller's diameter necessarily doubles the pitch.

That one is the simplest thing to work out, seeing as on model propellers the first two measurements we use are the pitch ratio and diameter. If we look at the Octura surface drive props, the most popular are the 1.4 pitch series - whereby all the pitches are 1.4 times the diameter of the prop - the 430 has 42mm of pitch, the 460 has 84mm 


The thing is, we’ve only considered pitch in this thread, which is by far the least important factor when considering the effect a prop will have in moving the boat along. Pitch progression, bar and rake play a far more pivotal role. ABC propellers give their bar and rake numbers in the chart, so a 2614-10-50 and a 2614-15-50 will behave completely differently. Then we get into the shape of the blade itself…


For those seriously interested, there’s an interesting paper at the link below, with a part on dimensional analysis of propellers. For those uninterested, I agree with the second post in this thread that states a bigger prop is better than two small ones 


http://1nitrorc.com/527_3.pdf

[Subculture]

Aside from maneuverability, the main advantage of twin screws is cancellation of torque roll, assuming the props contra rotate, natch.

[dodes]

I can only say that for towing for real vessels, a single screw vessel was more balanced steering than twin screws on a astern tow where as twin screws have to be balanced usually with the  MoDs R class by the engineers when in passage.