What R/C Props to Try?

Hi Baeckmo,

First of all, thank you.

Sorry I couldn't give you better info.
I thought I did a pretty good job describing the hull(s).
I'd post a picture, but unable.
I could email you one if you PM me an E'ddress.
I can tell you, this design did 4.5-knots WOT under two-man, kayak-style power, ( three on board, 240kg ) and won the race.

Our challenge now is to find 13 x 2o, 21, & 22 props to test.
They may not be available.
I can't find any on-line.

What happens when we go to a 3-blade?
There may be more selection.
And if the diameter decreases, all the better for our shaft angle.

The race isn't until May but will keep you posted of our progress.

 

Sadly, nobody makes a production prop in this range, not even close, as they are basically useless on R/C airplanes.
So, back to the drawing board.
Any ideas on alternatives that may be more available?

 

You might have a look at the vari porps...

varioPROP http://www.ramoser.de/home_e/varioprop_e/varioprop_e.html

Serveral sizes, blade forms and hubs for number of blades...all that are pitch tunable so you can optimize the advance to match your speed/power...

The 12d series looks like it would be approximately correctly sized.

 

Well, doesn't that look promising, thank you WTC.

I can't find max pitch anywhere in their literature.
Does anyone know? Someone who's used them, or owns a set?
I'd like to test all the way up to 22 or even 23 inch pitch on a 13" diameter.
Also nice to have the different blade profiles to choose from for testing.
I'm checking with HQ to see if they are within budget...

 

The data sheet here says from 4 " to 15" for the 12C

http://www.ramoser.de/files/info_prices_200710.pdf

so past "square"....I'm not sure you would want to go further...I believe you would find aboulute best efficiency with advance ratio of ~ .7....but you can get very good with much lower advance ...Dr. Larabee (gosamer albatros, condor prop designer) set those up with advance ~ .2 for minimum induced loss...

There is a small plot in this paper here that shows the effects of advance on efficiency versus attack angle...section 11.7.4.5
11.7 Performance of Propellers http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node86.html

There is not much more in it past advance of ~ 1 (so "square")...but your static trust, acceleration performance is going to be really bad to get to speed. You can see the power curves go through the roof at advance of 0...ie static start...

Also good data on the APC props here...

http://aerotrash.over-blog.com/2015/01/apc-propeller-data-what-s-new.html

 

Hmmmm...

I guess that means we're making a propeller.
I think this beyond the scope of our Boy Scout pack.

Any suggestions...?

13" x 20" BAR 0.36, 2-blade, RH
Maybe we should drop the 6:1 gearing and go with a direct drive in the 3000 rpm range.
That should drop the pitch considerably.
Maybe also the diameter...?
The whole idea was to use production R/C airplane props...

 

Bluebell,

I do not understand the thought that you need such a high pitch propeller??

Using the numbers from Beackmo of about 300 RPM (200W) and 4 knts, and the high power case of 400 RPM (400W) @ 6 knts...

for a 13 inch prop gives an advance ratio in the area of .4 (.388 and .44 for the two cases) even reducinig to 10 inches diameter only raises this to .5 and .57.

As you can see from the link with the APC efficiency curves a square prop (ie pitch = diam) only reaches peak efficiency with advance near 1 ( or ~.85 -.9 depending on whos data you believe)...see curve for APC 8x8E...

for good efficiency at the much lower advance you will need lower pitch see for instance APC 8x4 that has best efficency near .5-.6 advance (again depending on whos data you believe...

if you want the large 13 in diam, the pitch will be pretty low to match your operating points of ~ .4...certainly the large diameter slow moving 2 blade will be the most efficient (if properly set up and holds its form with stiffness)...or you could run a smaller diam ~10 inch prop, pitch a little higher to match the ~.55 advance ratio...if the two blade 10 did not have enough cord/ disk solidity to absorb the needed power you could go to a 3 blade probably with little loss ( slightly higher frictional loss than you would have with the large two blade)...or higher blade count

Maybe I am incorrect in my understandings?

 

Hi WTC,

The idea came from Baeckmo below.

 

Ok maybe he can jump back in and correct me?

Here is a link that may be in the range of scout building props? The Recumbent Bicycle and Human Powered Vehicle Information Center http://www.recumbents.com/wisil/hpb/prop/default.htm

Also have a look at the pedal power threads...their operating points are very similar to what you are after

Pedal Powered Boats https://www.boatdesign.net/threads/pedal-powered-boats.23345/page-27

maybe read some of Jeremy Harris posts in endless sphere

Overvolting a minn kota trolling motor - Endless Sphere https://endless-sphere.com/forums/viewtopic.php?f=39&t=38897

from the looks of it these guys are using APC props, look well less than square...but they are also on low speed hulls...

APC site has lots of options up to just barely past square (13.5x14)...if my understanding is correct you will want to be closer to 13x7, 13x9?

Internal Combustion Engines Archives | Page 2 of 20 | APC Propellers https://www.apcprop.com/product-category/internal-combustion-engines/page/2/

 

sorry forgot the link where I said "looks like these guys are using APC..."

ment to have the EPcarry link here Electric Paddle https://www.electricpaddle.com/about-ep-carry.html

 

Yes, you are incorrect in many respects! First the advance ratio: it is defined as: Velocity/(Diameter*shaft speed) in consistent dimensions. In the low-Power case, the vessel speed is 4 knots, i.e. 2.06 m/s; the rotational speed is 300/60=5 revs per second and the diameter is 13*0.0254=0.33 m. In addition you should consider the wake after the hull, which has the effect of decreasing the inflow velocity into the propeller disc. Roughly, the propeller ADVANCE SPEED is 2.06*0.95=1.96 m/s in this case, leading to an advance ratio of 1.96/(0.33*5)=1.19!

Next, you take the SUPPLY POWER (200W and 400W) for shaft Power, which is totally wrong; you must correct for the efficiency in every working point of the three-dimensional operating envelope of the motor. To understand the optimizing process here, you must understand the nature of the specific motor. Unfortunately, neither BlueBell or his advisor on the other forum got my point (I thought I was clear enough.....); they refer to a situation with a road-vehicle loading with a fixed gear ratio, leading to wrong conlusion about the optimum shaft speed at 400 W power input.

Instead you have two differing situations here, both with a restricted power supply. In those cases, you have to operate as close to the MAXIMUM EFFICIENCY of the motor as possible! In the three-dimensional diagram showing the motor characteristics, the position of this optimum is found along the break line between the red and the yellow fields. The rest of the power envelope (including maximum power) is of secondary interest. Since there is little in terms of hard facts available, we may assume a maximum motor efficiency of ~75% at about 75% of zero-load shaft speed at each voltage. At this point, the available shaft power is roughly 70% of the max power at the respective voltage.

So, roll up your sleeves again; time for round two!
With the assumptions above, the shaft power at max voltage AND max efficiency is 500*0.75=375W, and it occurs at 465*2*0.75=698 rpm. At this point it requires 375/0,75=500W input power! But there is only 400W, respectively 200W available; the voltage has to be reduced correspondingly. After the math is done, the optimum shaft speed for the 400W input case is closer to 600 rpm (BlueBell states 400 rpm.), which leads to a different propeller. By coincidence, the speed originally given by BB for the 200W case is correct.

 

Baeckmo,

Thank you for taking your time to correct my error (s)!

My unit conversion is ok, I made a worse conceptual error and took the Advance ratio to be the ratio of tip tangential speed to forward velocity! Hence I convert the diameter to perimeter distance and end up with an extra factor of Pi in my conversion!

You are correct I did not include any correction for wake factor. Was only looking for rough approximation, and I do not have enough knowledge to know proper wake factor. (Would you have some reference to point to where I could understand effects of hull shape and distance behind etc)

I did not actually use any power numbers (I had just noted them from the previous posts)...I did not calculate loading/power...I have programmed Larrabee's minimum induced loss equations long ago...but the only files I can find are in Fortran...and probably run on a Vax (I am getting too old...) I will have to learn to use JavaProp.

I do (I believe) understand your explanation of the motor efficiency as you are having to project max efficiency point from the peak power point, should also be near approximately 80% of the free speed (Kv * V) for the respective loadings/voltage for BLDC motor?

So looking at your 400 W input case (~300W shaft) and ~ 600 RPM 13 in propeller with a wake factor of 0.95 gives advance of ~.85...points to approximatley square to just over square prop...depending whos efficiency curve is correct... if the pitch is not enought the boat falls off the prop efficiency curve approching top speed ( and the competitor that has it correct rides off in the distance!)

For boy scout approach could start with slightly larger diameter available prop (APC 14.5x14.5 or 16x16)...maybe too much power? (again I did not calculate it)...the oversized prop with high pitch should hold motor down below best efficiency speed (higher power)...then could begin removing tips in step wise fashion (measuring motor speed/boat speed) to reduce power needs? End up with an over square prop (not perfect spanwise chord or attack...but pretty good)...

Assuming you do a good job of finishing/smoothing tips..maybe a practicle approach from avaialable parts and capabilities?

Again thank you for correcting my original errors (and any others you see in this post also!)

 

The 13" propeller for the 400W case was based on the (faulty) statement of shaft speed to be 400 rpm, so that is where the job starts all over again; new calculation (and wasted time due to false info)! My message here is that when you ask for assistance you must be very careful with the information you supply as background.

 

Baeckmo and WTC, thank you.

I do get the inappropriateness of the car tire graph applied to a boat propeller.

I'm concerned about these RPM numbers rising up above even the unloaded values from the manufacturer.
500RPM will be the upper limit given our max voltage.
Lightly loaded, the manufacturer claims 467RPM maximum.

Baeckmo, It appears 13 x 15 is the boxiest we're going to find in a manufactured propeller.
From what little information I have provided ( ~80 N drag, ~60/85kg displacement, ~4 - 5knots advance, 6.5m LWL, 0.35m BWL, Cp~.45, ~150/300w shaft, ~467/513 un/loaded RPM, ~75% motor efficiency, race/heat descriptions and metrics ) would there be, perhaps, a larger diameter option, at a pitch number more accessible, like 15"?
Or, shall we take a 13" x 3" rectangle of 1/8" plate steel, weld it to a 5' rod of 7/16 steel and get out the oversized crescent wrenches to twist up 20" of pitch.
Then grind some shape into it for a few hours... possible.
It would be nice to have adjustable pitch for sea-trials.
What diameter hub would you recommend we purchase?

At the risk of opening up another can of worms.
Larger diameter props mean steeper shaft angles into the water.
Are there consequences to this?
How steep is too steep?
I wonder how a flex shaft, like 1/4" spring steel, would respond to a swivel motor mount...
I suspect slower steering inputs would be prudent.
But if the prop self-positioned/stabilized to vertical, well, wouldn't that be slick.

Kindest regards.

 

Disregard my last.
I can see you're pretty pissed Baeckmo and I'm sorry.
I provided the information I had.
We'll go with a 14" hub, 4 - 15" pitch .
Thanks again everyone.