Propeller Design

propulsion

#1

Hello,

I m doing a project on development of thrusters for ROVs. Can anyone please tell me the procedure to design a propeller for certain specifications?? As to how to select the blade profile(foils) and model it.

I have been searching about it, but cannot understand as to how should i select the foil for the blade.

Thanks in advance.


#2

Hi Anupam:

Your question hasn't an easy fast answer. A serious propeller design from scratch is a thing that can only be performed by hydrodynamics labs.

The Goal is arriving to a propeller that at the desired speed, requires the minimum torque.

Propeller expulsion current at the desired vehicle speed, must be almost the same than the relative free flow speed. At the same time, fluid at the wake astern of the propeller must have the less possible induced rotation.

This way, the correct thrust with minimum energy consumption is achieved.

The general procedure is:

1-] Calculate total vehicle resistance at the desired service speed.

2-] Set maximum propeller diameter. Take into account clearence and induced wake field.

3-] Better propeller will have:

Best performance is achieved with maximum thrust provided by the minimum torque. Hence .....

Maximum diameter and propeller surface area ratio, with the minimum pitch and RPM would be the best.

Performance decreases with the blades number, hence the less blades the better, but, balance-vibrations and cavitation risk increases.

A balance between vibrations, cavitation, area ratio, pitch, diamater and RPM must be found.

Hydrodynamic pitch and thrust can be improved, at the same time than cavitation is held under control and torque is reduced, by means of selecting a correct blade profile.

But, fluid speed and resultant flow angle change along the blade, and hence the profile and the geometrical pitch must also change.

Behind each blade, a modified pressure-speed field develops due to the distortion created by the blade. Hence, next blade will not work on a "clean" field, but into a modified one.

Fluid adquires three main induced speeds; Radial, axial, and tangential, that change the hydrodynamic angles, speeds and the way they are distributed through the propeller.

At the tips, where the fluid moves faster, and the fluid hydrodynamic angle is minimum, high turbulence areas develop. The plan shape of the propeller (not ducted), must try to approach an ellipse in order to minimize this effect.

In short words:

The propeller is designed in order to work inside a fluid that has been disturbed by the propeller itself plus the hull.

Not an easy thing .........

Best way to go, is starting from the systematic propeller series, making the choice among them, and applying some modifications in order to addapt the standard to your vehicle.

The series to choose will depend on the navigation features of your design.

Regards


#3

The only propeller I designed was for a class assignment. It was for a container ship driven by a slow speed diesel, direct drive. So not quite what an ROV needs, but the basics and discussion given by Ion above is a great explanation of what you are getting into.

All I had to work with when specifying a diameter, pitch, area ratio, blade profile for clients that were doing a new construction or a repowering came from the Wageningen b series. Then I would talk to propeller manufacturers. They always promised better performance than the Wag b series and also delivered. But they would never disclose their Kt, Kq, etc.numbers as they were proprietary. So to find the best prop for our use most likely will be a lot of trial and error experimenting with what is already on the market.

The effect of the hull shape has a large influence on the flow that enters the propeller disk. I just started building my first OpenRov, so I have a lot to learn, but I can see the possibility that the propeller for the vertical thruster may be, or should be, different than the horizontal thruster props. Different purpose, different inflow.

For now, it appears trying different props from the hobby community is a smart way to identify trends. Blade profile, area ratio, etc. I'm looking forward to experimenting with the OpenRov.


#4

Hello "Master of disaster":

Fine to have a Naval Architecture buddy on the forum. That's nice to find people speaking the "same lenguage" :-)

Some comments:

You've surely observed that the inflow stream to the main propellers of the ROV is quite inefficient. The wake factor results quite high and the assymetry of the flow field really big.

I posted some months ago a CFD cutplot of the ROV for an estimate speed of 1.08m/s, the eddies and turbulence ahead of the propellers is so big, that the props do not get broken only due to the tiny scale. (Relative strenght is huge when compared with the diameter).

As I've also commented sometimes on here, I've been trying to get some technical information of the props from Graupner, but they did not answer me. So, I made some research for other model propeller builders, looking for something similar to the Graupner2308 family.

I finally found them at PropShop.co.uk. Once contacted, those guys answered me. They explained that their model props are based on the Gawn Burril, Wageningen, KCA-ducted, and Scimitar systematic series. Hence, the theoretical approach can be performed from the standard polynomial series.

Re correction.....

For a free flow speed of 1.08m/s, and 1500RPM for the prop, once axial and tangencial induced flows calculated -------> Averaged Re=46770

The Graupner 2308 can be approached from a Gawn-Burril propeller, with no rake, and a skew distribution given by:

r/R º
0.13 33
0.2 31.98
0.3 32.21
0.4 30.67
0.5 27.48
0.6 23.18
0.7 18.59
0.8 13.5
0.9 7.84
0.95 4.2
1 0

Where:

First column: Radius fraction

Second column: Angle of the radius-vector for each section, measured from the reference line(Vertical axis HUB-TIP) to the maximum camber point of the corresponding blade section.

Angle is negative for L rotation and possitive for R. Resulting blade from those data:


About the choice of a different prop for the vertical thruster, it has been already done by the OpenRov team. A two blades prop. has been choosen, it improves the diving performance, and balances the UP and Down thrusts a lot better than the 2308.

Once again, pleased to meet you. Hope to get some feedback from you.Kind regards ........-Ion-


#5

Hello Ion,

It's been a couple of days to reply, but that's is what my day to day life is like. I try to get everything done before bedtime, but doesn't always work out that way.

So your mention of a CFD cutplot has me confused. Is that a Computerize Fluid Dynamic display of what is it front of, going through, or behind the propeller? If so, how do you get such information? Also, I am not sure I would know what to do with such information.

Then you say:

"Second column: Angle of the radius-vector for each section, measured from the reference line(Vertical axis HUB-TIP) to the maximum camber point of the corresponding blade section."

So I need some help here. I was of the understanding, via education, that a 'section' of a blade profile is at a constant distance from the center of rotation, but if the propeller blades have some rake, where is that (Vertical axis HUB-TIP) line? At right angles to the center of rotation, or from somewhere on the hub to the blade tip?

The first plot you presented is the blade profile. No? The second plot is the section's chord looking along the (Vertical axis HUB-TIP) line? Same prop, same scale? The axises are not labeled and the units on the scales are not identified. As I suggested, I'm years behind you in propeller designs, these plots may be understood by yourself and peers, but I'm a little behind the curve.

I assuming propeller designs have come a long way since my school days, so I need to do some catching up.

Please stay in touch. Our OpenROV is coming together slowly. Hope to have it running for our next dive trip to the Carribean.


#6

Hi Master:

That's my fault, and ........ may be a little bit to blame on Excel too.

About the CFD.

Yes it's related to a Computerized Fluid Dynamics analysis.

At our Dpt. we own some licences for some comercial CAD&CAM and CFD tools, as well as some own developed software.

The cut-plot is obtained from:

- 3D model of the OpenRov from plans provided at GitHub.

- Building materials from the same source.

- Init conditions:

Fluid relative speed = 1.08 m/s

Computational Domain (dimensions relative to OpenROV lenght):

Lenght ahead: 2.5 LOA

Lenght astern: 4.5 LOA

Height: 3 ROV heights

Breath: 3 ROV beams.

Analysis: 3D- type EXTERNAL

Rotating regions: 2 at 1500 RPM counter rotating.(Still trying to get real RPM data)

Mesh: Tetrahedron. Fluid cells 77820, Total cells 96130

Evaluation of minimum gap size: Automatic

Evaluation of minimum wall thickness: Automatic

Static Pressure: 199395.57 Pa

Temperature: 293.20 K

Turbulence intensity and length

Intensity: 0.10 %

Length: 0.002 m

Machine: 2x8=16 cores (Intel and AMD)

CUT PLOT: Midship plane. Pressure distribution & Stream Lines.

About the propeller:

Yes, my description of the geometry may be confusing. The graphs I posted come from Excel, that is not the best software for such things.

We've developed a joint of spreadsheets and macros that help for optimizing the propeller for finally exporting the result to a propeller 3D file for further analysis. It works fine, but excel graphs are only an user simple guide and not a plan.

Blade profile= Dome type.

Skew: PIC -1-

Measured from the reference line to max camber point of each blade section. As the sections are dome type, max camber is located at the middle of each section chord.

RAKE: No rake for this propeller. Anyway: Rake is designed in most propellers in order to help for improving flow at the blade tips. In propellers working close to the surface, where ventilation may happen, RAKE helps for avoiding it.

Rake also increases pressure gradient astern of the disk.

See pic-2-

Check next pic, I've added some lines for the skew measurement clarification.

NOTE about pics: Each one belongs to a different propeller.

-1- Graupner (HUB in black)


-2- 15ºR Wagenningen


If you are interested, I can send you the offsets 3D table of the propeller Im using as the Graupner model.(pic-1-)

Second pic belongs to a Real ship Raked Wagenningen-B, Diameter= 6m. Ship under construction.

Regards and pleased to share with you.


#7

Hey guys,
Pulling this topic out of the grave…
So, i had developed a thruster…with a three bladed propeller from Wageningen Ka3-65 series along with a 19A nozzle. I took the blade profile data from a 1970 published paper on Wake adapted ducted propellers.
Such a thruster had already been developed by a team from ITB for their SHRIMP ROV and has published in the Indian Journal of Marine Sciences, Sep, 2009. I refereed to their work for my project.
But, the result has been a total failure. I designed the propeller with three 10mm dia blades. It was designed for give a thrust of 2.5kgf at an RPM of 1750. But on testing it gives only 0.15kgf.


Any clues where could i have gone wrong??


#8

Hey Anupam- One thing that stands out right away is that the tips of your propeller are flat. A Kort nozzle blade should be curved to fit snugly along the inner diameter of the nozzle.

We’ve been doing a bunch of testing with four and three-bladed Kaplan propellers in Kort nozzles and they’ve been giving us some pretty good performance. Our 50mm three-bladed prop can produce around 1.2kg of thrust at an efficiency of around 0.02kg/W.

You may also want to examine the effects of the grill plate over your nozzle. You may find that having that in the way is responsible for a large portion of your reduced efficiency.

Best of luck!

~Eric


#9

Hey Eric,
Thanks for the input. I had also noticed the gap(alas, only after the tests failed) and am now working on the same.
The grill plate was designed seeing that most commercial thrusters do have them, in order to protect the propeller from collision damage or just to filter out big junk pieces that might be floating around in the water. Anyways, I will conduct simulations to analyze its effect on efficiency.

How do you guys manufacture the propeller? I got mine 3D printed with Polycarbonate material using SLA process to get the smooth surface finish, but it was very costly. Is there cheaper manufacturing method available?

Also, can you share the source or blade profile details(spreadsheet) for kaplan propeller? I searched for it but couldn’t find.

Again, thanks for all the help.

Anupam


#10

We’ve got a Formlabs Form 1+ printer in house here at OpenROV that we’ve been using for prototype props, but there is also an “Extreme Detail” option for printing parts using Shapeways that we’ve been pretty happy with. We’ve learned that a tremendous amount of efficiency loss (like on the order of 40%) can come from flexure of the blade so finding something that is very rigid is worthwhile.

I’m away from the lab until after New Years and I don’t think I have the prop profile data with me, but I’ll try to dig that up and post it once I get back.

I’m excited to help with this!

Eric


#11

Dear friends:
My intention was answering to Chandra, but I see Erik already did it.
I agree with Erik’s answer.

Kind regards and happy New Year