Unexpected prop testing results

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04-06-2021, 03:42 PM
sciesielka

Unexpected prop testing results
I'm looking for some help understanding data from some prop testing. First, I am running a Leopard 4082 1250KV/2.5Y motor with water jacket. I have the OSE 150A ESC and running 2 identical and quite new [7 charge/dis-charge cycles so far] 3-cell RoaringTop LiPo batteries (55C 6500mAh 3S2P) to get 6S for the boat. Details on the boat and the full prop testing chart can be found at this thread on the forum: Joysway Invincible Razor 37 inch mono assembly

I tested all except one prop on the same day and same battery packs. I did not have a GPS for speed but I was mostly looking at AMP draw and temps for these tests. For all tests, everything stayed cool. Air temp was 52F, water temp was about 45F. Wind speed was steady at 10mph out of the south and the long part of the pond runs north-south so each run was 3 or 4 "round-trips" in order to even out the impact of the wind and waves. The Graupner 51mm prop was tested on a different day with the wind essentially the same but air temp was only 40F. I had tested a few other props but they were smaller diameter and the results I got were as expected. These the the ones I'm not sure about:
- CNC 4816-D (de-tongued) is 1.6x48=77mm pitch. Ran 30 sec. Max Amp Draw=130.1. Max Watts=2,705. Max RPM=23,100
- Graupner K2317.48 is 1.4x48=67mm pitch. Ran 35 sec. Max Amp Draw=108.2. Max Watts=2,229. Max RPM=25,380
- Graupner K2317.51 is 1.4x51=71mm pitch. Ran 2 min. Max Amp Draw=108.5. Max Watts=2,225. Max RPM=25,320
The first surprise to me was that the Graupner plastic 48mm dia. and 51mm dia. props were nearly identical in data output. I expected the larger prop to have a higher Amp draw. Is anyone else surprised?
The other surprise was how high the Amp draw was with a metal de-tongued 48mm dia. prop. Total pitch isn't that much different than the 51mm prop but the Amp draw was, in my opinion, significant. I realize the pitch ratio is higher but being de-tongued, I thought that helped, especially to allow higher RPMs. But in this case, the RPM max was lower. Any ideas?

I have run it a few times now using the Graupner 48mm and did record a top speed of 39mph so far, but that was also in windy, wavy conditions. I want to get a good metal prop (CNC or Octura) for durability, but don't want to keep spending money on props if I don't have to.
Thank for the assistance!
04-07-2021, 01:49 PM
NativePaul

This is no surprise to me, I have said on here before that Graupner carbon props are the most efficient props available.

Not only are they a very good tongueless shape, but they have flex designed in that is just not possible with a metal prop, under high loads like hole shots or accelerating out of a corner they flex and reduce pitch which reduces peak currents, as the boat accelerates the prop unloads and the pitch increases.

If you use Graupner carbons to test for prop size, when you find the right prop for your boat, you need to order a metal prop 1 size smaller.

TP's low blade area CNC props are clones of Graupner Carbons, and while they are the second best aluminium CNC props out there (closely behind Tenshock), they give the same speed and less run time than a Graupner that looks and measures the same.

The 51mm prop is flexing and depitching more than the 48 on acceleration, giving similar peak current, but you will find that the average current during a run will still be higher with the 51mm, and you will see slightly higher speeds.

Graupner carbons are very fragile though, when a decent amount of power is put through one hitting the stem of a leaf can be enough to throw a blade, and if you are running big power, just gunning the throttle can break one, as such they are not really suitable for long term use these days, and I would only recommend them as a cheap way to work out what metal prop you need for newbies that don't yet have a box full of metal ones to try and has no access to a friend/clubmate willing to lend them props.
04-07-2021, 02:54 PM
sciesielka

Quote:

Originally Posted by NativePaul

This is no surprise to me, I have said on here before that Graupner carbon props are the most efficient props available.

Not only are they a very good tongueless shape, but they have flex designed in that is just not possible with a metal prop, under high loads like hole shots or accelerating out of a corner they flex and reduce pitch which reduces peak currents, as the boat accelerates the prop unloads and the pitch increases.

If you use Graupner carbons to test for prop size, when you find the right prop for your boat, you need to order a metal prop 1 size smaller.

This is exactly what I was looking for! Thank you! I had read some older threads in this section looking for answers and there are a LOT of opinions on props! I came out of it with more questions than answers!

Quote:

Originally Posted by NativePaul

Graupner carbons ... I would only recommend them as a cheap way to work out what metal prop you need for newbies that don't yet have a box full of metal ones to try and has no access to a friend/clubmate willing to lend them props.

You just described me! No local club that I know of and just starting to get serious with props on this new boat I put together. All our RTR boats and scale boats, we never felt the need to play with props, except the Stealthwake. So yep, no box of props, and just using the Graupners to figure out what I need for a metal prop.
I'll probably get a few metal props based on your information and then when the weather and water warm up I can retest and make sure that I don't overheat anything. By mid-summer we see air temps in the 80's and 90's Fahrenheit (mid 20's/low 30's Celsius) and water temperature probably around 70F (20C).

Thank you again for the great information!
04-12-2021, 08:24 PM
sciesielka

1 Attachment(s)

Full prop testing results

I finished testing with lots of different props. The de-tongued props gave me the most problems in all sizes I tested. Sure they spun fast, but the boat would take forever to get on plane and then was probably cavitating (?), aerating (?) during the run which I assume is why they had lower speeds. At first I thought it was due more to the pitch ratio versus being de-tongued since the de-tongued props were 1.6 whereas almost all the other props were 1.4. However, when I ran the cnc-4219250, a 42mm dia. prop with a pitch ratio "around" 1.9, the boat ran fine and didn't have any issues getting up on plane.

Based on the test results and the speeds basically all ending up around 40mph (64kph) I ordered a pre-balanced and sharpened Octura x447 prop.

Here are the detailed results. The first set from March and April 1st I did not have a working GPS so I didn't have speeds. But the data from those tests is what prompted my initial post here. The second set of data I retested with the GPS to get speed data to back up my observations. I also ran the Graupner 51mm prop to see if the average Amp draw was higher during the run (it was, as NativePaul said) and it also generated a fair amount of heat. Thus the smaller "final" prop!
Attachment 172354