Prop optimization at 6KW and 4500 rpm

Hi all.

This thread is about the optimization of a propeller for an electric direct drive outboard at 6000W and 4500 rpm.
It originates from the Overvolting af a Minn Kota outboard thread.
http://endless-sphere.com/forums/viewtopic.php?f=39&t=38897&start=15#p594246
I felt it was time to stop parasitizing and start a new separate topic about this prop idea.

Anyway, here goes.
I tried out some more optimization in Java prop to see what it took to get heaviers blades yet keep a decent efficiency performance.
I came up with a 2-blade prop at 22cm dia with 63% efficiency. The velocity 6 m/s and 102mm pitch. Medium blade loading.
See the pic and saved Java Prop attachments.

Please have a look and I will listen to your verdicts. I'm very grateful for all inputs.

Probably as close as you're going to get to good efficiency at that rpm, I think. It may well be that you can get a commercial prop that's pretty close to that, too, probably an outboard prop, as the pitch is a bit finer than many inboard props.

Wow, thanks! That sounds great. 8)
Now I just have to figure out how to visualise the prop dimensions in real life and build it.
The view in Java Prop is not very design friendly and it's hard to get a feeling for the dimensions and physical look of the prop.

How did you render the blade shape into a accurate test model, perhaps in wood, for comparison to the complete prop built in stainless steel?

I make my blades from stainless steel sheet. Start with a rectangular strip that's the maximum thickness of the blade and the maximum chord width.

Use the pitch angles at each radial station in the "geometry" table (r = radius, c = chord width, β = pitch angle) to create a set of triangle templates for each radial station. Fix these upright to a strip of flat wood (I usually print them out and glue them on thin plywood) and you have a blade twist profile gauge. Next, twist the flat stainless bar so that the angle at each station matches the gauge (I use a couple of bits of slotted bar for this, one at the previously twisted station, one at the point where the next twist is needed). Work your way along the strip of stainless, twisting it to the right angles as you go.

Next, make a template for the blade shape from paper, using the chord widths and radii from the same table. Print this out and glue it to the twisted stainless blade, and use it to cut the blades to shape. Next, make up some aerofoil section half templates, one each for the upper and lower surface of each blade at four radial stations. Grind the blades to shape starting with a small angle or die grinder, finishing them with files and a flap disc, checking that you have the right profile at each blade radial station as you go. I sometimes silver solder an extra piece of stainless steel sheet at the root end of each blade to thicken the blade section there and allow it to be shaped more easily.

It sounds long-winded, but the first blade I made like this took me an hour and half, now I can make a blade in about 40 minutes, or a complete prop in around an hour and half.

View attachment prop template.pdf

Thankyou for the informative suggestion to how make a prop.

I have decided to initally design a good efficient prop for 1100 rpm to be used with my homebuilt Magmotor outboard.
I will compare speed and power consumption to a standard 2-blade Motor Guide prop.
Hopefully I can get both 50% more top speed and slash power requirements at lower trolling speed.
Please see the attachment on my homebuilt outboard rig using the Magmotor S28-400.