What R/C Props to Try?

Discussion in 'Props' started by BlueBell, Nov 19, 2017.

  1. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Wondering if you may be able to help some Boy Scouts with some prop size options for the following set-up.
    -500 watt electric motor 75% efficiency rating, 465RPM loaded.
    -6 knot advance
    -200 watt input from solar panels only
    -400 watt input from battery only

    In the battery only endurance heat we must last 25 minutes to go the furthest distance.
    We've calculated a 400 watt throttle setting should last 28 minutes.
    If we approach the LVC of 30V, we can throttle back to be sure we last the 25 minutes.
    Boat weight ~60kg (135 lbs)

    For the two sprint heats, we want to go as fast as possible for ~45 seconds on the panels alone.
    Boat weight 85kg ( 185 lbs)

    We are interested in testing half immersion props as well as full immersion.
    The motor is mounted on a swivel with a five foot prop shaft like a Thia long-boat.

    Thank you
     
  2. alan craig
    Joined: Jul 2012
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    alan craig Senior Member

    Bluebell, I entered this fun competition three times, the power levels are similar to your competition:

    Almost everybody used R/C 'plane props. I used Graupner Super Nylon (3 blade 10" dia.) and APC Sport (2 blade 12" dia.) both are made for IC engines and will cope with the bending loads. You can put two props on a shaft if you want. Experiment with a few different pitch versions.
     
  3. BlueBell
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    BlueBell Ahhhhh...

    Thanks Alan,
    Yes, R/C airplane props are great for this application.
    What pitch were you using?
     
  4. alan craig
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    alan craig Senior Member

    Bluebell, 10 x 7 and 10 x 8.3 three blade (that boat is in 2011 CCC video) and I tried 12 x 6 to 12 x 12 two blade in 2013. But I've just realised that your motor is slower and probably more powerful than my drills so my guess would be to start with about 14 dia. x 12 pitch two blade. Good luck, and I'd love to know how you get on. It looks like the Boy Scouts have a slightly more scientific approach than the average "cordless" canoeist.
     
  5. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Interesting.
    Thank you.
    Yes, we have a 6:1 reduction gear built into the motor.
    It doesn't create any more power as 500 watts is 500 watts, a measure of power, but it does create more torque which is likely what you meant. So, yes the diameter and pitch are going to be bigger and, in theory, more efficient.
    We're hoping to do a drag test and create a speed/drag graph which could be helpful in choosing props to test.
    What did you do with your race boat?
    Cheers!
     
  6. alan craig
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    alan craig Senior Member

    I thought my drills were about 250W but I've just checked my records and I measured 430W static with a big air propeller, so nearly as much as your motor. Be aware that if you do a static test, the power goes down when the boat is moving probably because the prop is stalled when static.
    After three competitions I sawed up my "race" boat into little pieces!
    Dennis Adcock in the 2017 CCC has been officially measured at over 10kts and will go into the Guiness Book of Records.
     
  7. Barry
    Joined: Mar 2002
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    Barry Senior Member

    You need to choose appropriate props before considering two of the same pitch prop, one in front of the other. If the exit velocity of the water stream is above the theoretical pitch velocity of the second prop, the second prop will just produce drag. ( the following numbers are here to show the concept, they will not be accurate as there are too many variables
    at this point to get close) So say the canoes speed is 10 feet per second so that is the inlet speed to the first prop, and picking a number out of the air, and say that the exit speed of the water off the first prop is 40 feet per second. Assume that if the second props theoretical pitch velocity is 30 feet per second, then this prop will drag and slow the boat down. So the second prop might need a theoretical pitch of 40 plus to put some load on it.
    upload_2017-11-23_19-54-55.jpeg
    upload_2017-11-23_19-53-45.jpeg
    Unless you can get more blades in the same plane, or counter rotate the second set but even then the pitch must be larger than the first
     

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  8. alan craig
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    alan craig Senior Member

    What I meant was, use two two bladed props to avoid bending the blades of an overloaded prop in water (there is not enough centrifugal force to keep them straight). Just rotate the rear prop 90 degrees plus an angle equivalent to the hub length, then they will both effectively be in the same plane.
    edited: hub length not dia!
     
    Last edited: Nov 25, 2017
  9. baeckmo
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    baeckmo Hydrodynamics

    Hello BlueBell, there is some confusion about your motor data that must be straightened before diving into calculations. If I understand you correctly, you have an electric power of 400 W going into the motor, with a 75 % total efficiency, resulting in a shaft power of 300 W. First question: at what shaft speed?
    The same question goes for the 200 W input case.

    Then: I'm a bit curious about the hull shape you are going to use. I understand it is a slender monohull? Main "wet" dimensions neeeded to identify possible "obstacles" in terms of humps in resistance characteristics. Important to the propeller performance during acceleration.
     
  10. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Hello Baeckmo,
    First answer: 467 shaft RPM loaded, according to the motor spec sheet.
    Second answer: Same RPM for 200W and 400W inputs.

    The hull shape:
    6.25meters LOA & LWL
    0.35m BWL
    0.15m draft
    Cp ~.45, very fine, vertical stem bow; fuller buttock lines starting aft of midships, with vertical canoe stern.
    Hard chined, flare is about 20 degrees off vertical.
    Rocker: 0.11m
    Two, low buoyancy, "V" cross-section pontoons ( 1.25m BOA ) only just touching the surface, mounted aft. 1.0m LWL each, 0.1m max BWL
    (They may be reduced or even removed during sea-trials and training.)

    The motor sits low on a swivel with a 1.5 m shaft-drive, like a Thai long-boat.
    We'd like to try half submerged props too if you think it worthwhile.

    Thanks
     
    Last edited: Nov 26, 2017
  11. baeckmo
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    baeckmo Hydrodynamics

    That was what I suspected, and I'm afraid you are not quite correct. The performance map of a PM motor (which I guess you are using) is three-dimensional. At reduced power settings, you can find a specific power in a number of operating points, but there is only one unique combination of voltage and shaft speed that meets the maximum efficiency for the requested power. The maximum power is never occurring along the max efficiency.

    Since it seems that this is causing some confusion now and then in our forum, I made up a typical chart to demonstrate my point. What you read from the motor specifications is often the max power (top of the yellow surface) at maximum Volts. Note that this operating point occurs at half the zero-load ("runout") speed. Now, the best efficiency is not found at the rpm giving max Power, but towards lighter load, i.e. at some higher rpms. In order to gain your medals, you have to find the "sweet spot" for the two cases you will be running; input 200 W and input 400 W. The sweet spots are found along the cutting line between the power (yellow) and the max efficiency (red) surfaces.

    So, since I will be the one to blame for bad performance if I recommend a propeller according to your statements, I am a bit concerned about accuracy of the underlying information. As you can see on the performance map, you can not select shaft speed at will if you are to extract maximum work from your rig.
     

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  12. baeckmo
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    baeckmo Hydrodynamics

    ......and when you have contemplated the above, we add two propeller characteristika to show the impact of propeller selection upon Power and efficiency. Please note that the props don't care about voltage, only about available Power and shaft speed.

    In this graph I have erased the efficiency surface for better clarity; the optimal loading point still along the red/yellow intersection. Note that the two props have maximum efficiency at different voltage, power and rpm (thick black vertical line in the red surface). So, where is now your operating point again??

    PMmotorMap2.jpg
     
  13. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Hello Baeckmo,
    Hmmm, I was under the impression voltage would be constant ( 48V ) and amperage would vary to throttle power.
    Looks like I was mistaken.
    I've asked for clarification on the electrics forum thread here:
    Battery Advice Needed For Middle School Competition - Page 4 - Endless Sphere http://endless-sphere.com/forums/viewtopic.php?f=14&t=65894&p=1336667#p1336667
    There you can see all the components recommended for our project.
    I have voiced your concern there and hope "DrkAngel" may provide some insight.
    (S)he has been most helpful thus far.
     
  14. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Okay, as above, I heard back and it looks like ~300 RPM for 200W input and ~400RPM for 400W input.
     

  15. baeckmo
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    baeckmo Hydrodynamics

    Ok, that means you have shaft powers of 150 and 300 W respectively with a 75% total efficiency (including controller). You should come fairly close to optimum with a 2 blade, BAR 0,38 prop with 13" dia and 20" pitch. For the 200 W run, you may be better off with a slight increase in pitch. But there are too many unknowns in your project to come closer by calculation. (BTW, I checked into the forum you referred to and find that the graphs you pick your motor rpms from is not appropriate for a propeller load. the "thrust cuve" shown there has an exponent of about 2.3, while the theoretical propeller curve has the exponent 3.0). When it comes to winning you must understand the underlying mechanisms; the devil hides in the details!

    The fact that the same prop will (close enough) suit both operating conditions depends on the non-linear behaviour of hull resistance, which, of course in turn depends on my guessing of hull characteristics (of which I have no hard facts due to lack of information).

    You have estimated the maximum speed to 6 knots and I take it for the 400 W case. Recalculating speed for the 200 W - 85 kg case, I find you should reach something like 3,8 to 4 knots. This means that you will be close enough with the same basic prop for both runs. BUT: that depends on how good your drag estimates have been. The propeller thrust will be ~83 N and ~62 N for the two cases, all with a wake factor of 0,9. Since I have no idea of the shape of the hull, and thus no figure for the drag values, this is a rude guess, and the selected pitch is most probably slightly on the low side, but it is better to increase pitch (by cupping f.i.) than to decrease.

    There is nothing speaking in favour for surface piercing propellers for this application as far as your information goes at this stage. One thing to keep in mind though; with a very slender hull the propeller torque may cause a heeling that should be compensated for. Asymmetrical loading will reduce drag on the amas if you use those.

    So, good luck, and keep us posted!
     
    Last edited: Nov 27, 2017
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