Size of motor boat propellers

[Tsilatipac]

I notice that most people seem to be using the 50mm Robbe propellers (folks that use them). Has anybody looked at using the 60mm propellers? I'm thinking about ordering 6 for my project but don't want to waste $50 if they are oversized. I'm using the 500GPH bilge motor cartridges. What are your thoughts?

[bossfrog]

I also purchased the 500 gph pump cartridges for my ROV. After reading on Steve's website, I decided to go with the 40mm for forward and reverse thrusters. They had slightly less thrust, but used far less current. As I'm going to be running on board batteries, this was important to me. I did get a 50 mm for the vertical thruster as I'm only going to be running on thruster for up-down.

[Rover]

Yes, you are correct, the bigger the prop, the larger the load on the motor.

[sKooTr]

I have done some recent tests of motor load v propeller size and style that may be of interest.

Using a Rule 500gph bilge pump, a 12V power supply and a selection of propellers:

• - Robbe 3 blade 35mm (R1473) (http://www.cornwallmodelboats.co.uk/aca ... llers.html)
  - Robbe 3 blade 50mm (R1471)
  - Robbe 3 blade 60mm (R1465)
  - Graupner 3 blade 50mm (2308.50) (http://www.cornwallmodelboats.co.uk/aca ... llers.html)
  - Graupner 5 blade 40mm (2298.40)

With just the standard wire length they are supplied with (ie no long tether so there is 12V across the motor):

• - Unmodified pump with impeller = 1.60A
  - Robbe 3 blade 35mm = 2.18A
  - Robbe 3 blade 50mm = 3.0A
  - Robbe 3 blade 60mm = 3.6A
  - Graupner 3 blade 50mm = 2.5A
  - Graupner 5 blade 40mm = 2.22A

With a 30m long tether (2 x 2.5mm^2 CSA):

• - Unmodified pump with impeller = 1.53A @ 10.7V across motor
  - Robbe 3 blade 35mm = 1.84A @ 10.4V
  - Robbe 3 blade 50mm = 2.5A @ 9.85V
  - Robbe 3 blade 60mm = 3.0A @ 9.5V
  - Graupner 3 blade 50mm = 2.1A @ 10.1V
  - Graupner 5 blade 40mm = 1.85A @ 10.4V

Summary: bigger propeller = more current. More current = greater voltage drop across tether.

Now I need to do some thrust comparisons / measurements next.

Hope this helps.

Scott

[sKooTr]

For the record, I have had a terrible experience with Cornwall Model Boats and no longer recommend them.

I ended up buying 40mm 5-blade Graupner propellers from www.gliders.uk.com without any problems.

[Saab340]

Are the bilge pumps left or right turning? Or should it not matter because I can reverse the wires to make one to the left and the other right turning?

[thegadgetguy]

In my experience, they all work exactly the same in either direction. The wires have a positive and negative, but they only matter if you are trying to use the units as a pump, because the impeller will only push water in one direction.

[Saab340]

So I can make one pump a left turning and the other a right turning, put respective turning props on them to eliminate any asymmetric thrust issues?

[sthone]

Yes, you can run counter rotating props, I did this on my Seafox Retrofit but I'm not sure if it made any difference or not (it couldn't hurt though.)

As to the original thread question, I found that going with the larger 60mm props actually put out less thrust then the 50mm ones. (this was on a 750gph pump too)

You can see my test results on this page ....

http://www.homebuiltrovs.com/mayfair750test.html

-Steve

[KR2_Diving]

Just thought I would share this little piece of info...

Sent an e-mail off to the Prop shop of England just to see what their techs had to day. You can see my e-mail and their response below:

Hello,
I am currently building a small submersible remotely operated vehicle.
I am looking for a prop that offers the same power (or similar) spinning in either direction. (FWD & REV)
Would you be able to assist me in selecting the correct prop? I am sure you will probably require more details. At the moment, I have very little firm data, as I am still quite early into the design.

The ROV is likely to be 350mm x 450mm x 150~200mm. I would like to have 4~6 thrusters mounted on the ROV, which will then be propelled using a Vectored thruster arrangement. Motor size will depend partly on the prop selection.

Any assistance you can provide would be appreciated.

Thanks,
Ryan

Their Response:
Dear Ryan,

Many thanks for your enquiry, which I passed onto our technical advisor, Simon Higgins for his comments.

Simon states : The obvious choice for thruster propellers would be something from our range of kort nozzle propellers. Our Kort Nozzle, part number K1315/4 which is 32mm diameter would be best as they will work on 380, 400 or 540 motors and have very similar thrust in forward or reverse. 4 to 6 of these on a sub, the size you have described would maneuver it very quickly and precisely.

Kind regards,
Mrs Lucy Rimmer - Administrator
on behalf of Simon Higgins - Technical Advisor for Prop-Shop
Swan Precision Castings & Engineering Ltd