Custom propeller - Do you want to test it?

[Christian Holmstedt]

I haven't been on here in years but I went through my old RC stuff the other day and found my half-built stash of stuff and tons of files with ideas and concepts.
Big plans back then to get into FE tunnel racing. Got a PX-300 (?), nice kit, but it was never finished. Got all the cool telemetry stuff too. I had plans for a custom hull that also never materialized. The paper design got quite far but nothing got done. One thing that actually got done was a custom 4 blade propeller. This was 5 or maybe closer to 6 years ago and it's been sitting in a box ever since.

It's designed for 3.50 cc Nitro and P Ltd/P Spec/ P Class Fast Electric Outboards. I read up on what basic size and characteristics racers used used at the time but since no boats were finished I never got to actually try it.
I have no idea how it performs. It might be way too big and hard to drive...

It's designed from scratch with all parameters calculated in a spreadsheet and had it made by a 3D printing service out of bronze. It's a 4-blade cleaver with the following specs:

Diameter: 42mm
Pitch: 55mm
Pitch/Diameter ratio: 1.6
Rake: 20mm
Pitch progression: 5mm (leading edge pitch is 50mm, final pitch is 55mm)
Cup: 23mm (pitch at the very trailing edge is 55+23mm)
Since it is a lost wax casting the final prop is slightly smaller than specified. I think the diameter is closer to 41mm.
It fits on a regular 3/16" fitting but due to shrinking it took some work to drill it out to fit.

I'm still itching to know how it performs so I'm thinking about lending it out to someone to have a go at it.
The guy/gal trying it would have to have their tunnel stuff together and running fast already. I am interested in performance numbers such as speed, acceleration (data logs) and over all drivability/feel of it compared to propellers available today.

It was quite expensive to have it made (~$60) so the main thing I am asking is to get it back undamaged and in a reasonable time. It's all about the honor system, if you know what I mean...
I will ship it to you at my expense. You do the test runs and post results here and ship it back to me (you pay shipping).
Are you interested?
Post some pics & stats of what you run now and we'll see what can be arranged.

Attached are a few pictures of it mounted on an OS lower unit in its raw unfinished state and pictures of it after sharpening and balancing was done. It's not perfectly balanced but I think it will be good enough.

 

[shoboat]

Christian,
I have my doubts that any OS-21 OB is going to pull that pitch on a four blade. Maybe an electric motor?
The motor mount will need to be able to come up really high on the transom to have any chance of pulling it with good RPM. I have done quite a few four blades in the past few years and the only luck we had was on a sport 40. It looks pretty good in the pictures.

Have Fun Testing Though,

Mark Sholund

 

[Christian Holmstedt]

Thank you for the feedback!
I have no illusions that a stock OS will pull it for sure. The goal was FE P-Spec/P-Ltd class but maybe it'll take a full P-Spec electric to run it?

 

[shoboat]

Christian,

It is going to add a lot of lift to the transom of the boat. An electric motor
will be a good test for it.

Thanks For Sharing,

Mark Sholund

 

[John Beardslee]

I always had trouble balancing and polishing the smaller 3 blade props, there’s just not much room between blades, a 4 blade has even less room. The custom 4 blades that I built for Jim Babbie we’re 57mm dia. and 70mm dia. and we’re still difficult to work on. These were also counter rotating which brings other challenges for working them.
Thanks John