# What R/C Props to Try?

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

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### WTCJunior Member

Baeckmo,

I'm sorry if I have wasted your time also...and again thank you for the correction.

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### BlueBellAhhhhh...

Now that we've cleared that up.
If a 6', 1/4 spring steel prop shaft would self stabilize, as has been suggested,
I wonder how it would behave on a Thai-style long-boat with the motor swiveling in the horizontal plane only.

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### baeckmoHydrodynamics

Hi again, glad you didn't give up! This project has an educational twist that is interesting, so I think it is worth taking it one step further. But first, there is more to contemplate about the motor selection; we have arrived at the "what if"-point!

Looking at the available info it says: 500W, at rated speed 465 rpm, 13.4 A, and efficiency 75%. The no-load speed is 517 rpm, ie just 1.11 times rated speed. This indicates that the figures show the best efficiency point at full voltage (48 V). Note that since there is no info available on the shape of the curves, generic characteristics must be used.

This operating point will draw 500/0.75 = 667 W from your supply source. With 400 W available, the new optimum working point will be: 300 W shaft power at 400/667*465 = 279 rpm. This would be far from both previous estimates and probably below what is practically reasonable. In short: selecting motor speed can not be done at random, the simplified design chain goes: Hull thrust at design speed>hull propulsion power>propeller efficiency goal>propeller disc area>propeller shaft speed>compare available power>redo if necessary!

I'll take you through the process in steps with the aim at using standard marine propeller elements, but I also wish that some of our hull drag wizards joined us!

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### BlueBellAhhhhh...

This will give adjustable pitch (4" - 15").
What diameter is now the question.
We're hoping to implement a flex drive, 1/4" x 6' spring steel from a 15 degree angled motor mount resulting in a vertical propeller posture.
For 150/300w at the prop, 60/80N drag at 4/5knots advance, would 14" diameter be reasonable?
We'd like to wrap this up by Christmas.

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### WTCJunior Member

Bluebell,

I ordered some of the VarioProp units...they look pretty nice...i got the ~10 inch diameter with 2, 3 and 5 blade hubs. easily adjusted for pitch and variable blade count so I can have a range of total cord area to match to power delivery capablity ( I have a ~ 1kW capable BLDC drive with speed control up to ~ 1000 RPM and cycleanalyst for power measurements). Just a fun boat, not racing anyone...

I hope Baeckmo can give you a better calculation of target size...if not maybe there is a clue in the second post of the thread where Alan Craig was running a 3 blade 10 in or two blade 12 inch APC with power levels similar to yours?

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### BlueBellAhhhhh...

WTC,

Thank you.

Agreed, there are a lot of clues, all over the internet including this website, and, as you say, even right here in this thread.
I have a pretty good idea on sizing but numerical analysis is always a nice source to draw on.
Like any good mariner, I utilize all the navigation aids available to me... Baeckmo is not very available.

This too is a fun boat, but competitive.
Ah, it's a fine line isn't it!
Good luck with your props, you've made quite the investment to get all that variety.
Let us know how they work out, please.

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### WTCJunior Member

Bluebell,

Please keep us up to date on what you and the boy scouts find and I will do the same...I have spent many weekends over the last years on Eagle scout projects!

After the first of the year I will rework my old prop code and also begin some real testing...conveiently, from a profession sense, I have full dynamometer testing and data acquisition...so should be able to properly measure motor/drive and controller performance and efficiencies...I have a motored dyno to measure small/medium propeller performance, but for air vehicles... but no way to put them in the water with proper inflow....so that will have to go to the boat... Hopefully I can develop a good data set and correlation to the analytic models as I go! I will let you know...don't know that it will be in time for your races!

For boats, bikes and cars etc. I find the fine line between fun/ playing around and competition/racing is usually crossed when the second one shows up!

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### BlueBellAhhhhh...

Sounds good.

I wanna know how this guy, Dennis, is doing 10+ MPH ( 8.8 knots ) on two 8 foot pontoons, with two 18 volt drills?!
www.bealeparkboatandoutdoorshow.co.uk/news/
And right beside a stabilized mono-hull!!
I can't find and video of his run...

Edit You have to scroll down a page, click on the picture of the two yellow hulls.

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### baeckmoHydrodynamics

Well, BlueBell, there is a real life to be lived as well.....even if it means you have to wait for a day or two for free advice.

Have you really read and understood the #32 note? In pure numbers it means that you need something like 17 to 18 inch diameter and some 28 to 30 inch pitch to hit the optimum operating point, I don't care to do the detail calc, since it is pretty far from realistic values. You won't get anywhere with the 15" (0.381 m) pitch you are shopping for. Look, if your shaft is rotating at 280 rpm (i.e. 4.7 rps for maximum shaft power output with input 400 W), it will cover the distance 4.7 x 0.381 = 1.8 m/s (=3.48 knots) at zero angle of attack; don't you even try to do the basic math? The necessary Pitch/Diameter ratio required for the proposed motor is simply not suitable!!!! Put your brain at work and let the fingers assist, not the other way around!

Now if you could change the gear ratio to double (or slightly more) the shaft speed (might do with a 1:2 step-up additional transmission), then there are a number of good production propellers available. Get back when you have checked the options for shaft speed and I'll help with the prop decision.

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### BlueBellAhhhhh...

Any RPM you like Baeckmo.
or 467 x 2 = 934 loaded
Okay?

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### baeckmoHydrodynamics

Ok, here are two examples. The diameter and pitch in these cases come from an algorithm developed in the SSPA and depend on the input parameters Power, shaft speed and ship speed. Whether your speed estimate is suitable, I leave at your responsibility.

For the case 300W shaft Power, 280 rpm, speed 6 knot you need:
• 2-blade 18" diameter x 26" pitch; or:
If you can arrange 600 rpm at that load (i.e. a step-up gearing 1:2.15) you end up with:
• 2-blade 12.2" x 13"; or:
I intentionally give the decimal values, not that they are all that important, but so that you may find closest alternatives. If you look at a selection of props for older outboards, there are quite a number of props for solid hubs (i.e. non-through-hub-exhaust). For the limited forces involved here, a simple alternative to fix a 2-blade prop is to take a 3-blade alu screw, cut off two blades at their hub junction (best done by a band saw) and weld one of them back to the hub. The reason the 2-blade is preferred is that it will come closer to the optimum blade area ratio (BAR).

In fact, with some reasonably skilled woodworking, you might even teach your adepts to make a set of 2-blade props from Wood; it is extremely well suited for propellers. If you check for instance with forum member PAR, you will probably get expertise advice on what species to use and how to preserve it. Here at my place, every kid by the age of 10 or 11 has made a propeller in the woodworking lessons.

Now, I'll leave you at that, and wish you good luck!

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### dinoaSenior Member

In the old days aero modellers also made their own wood props. Modern full scale areo props are often laminated maple and sometimes ash.

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