PDA

View Full Version : Bending pitch into props



mm123521
01-01-2012, 07:54 PM
All,
I built a twin electric outrigger this summer.
As most of you know, there is a limited number of props available in right and left rotation.

I have been cutting down 457's to various diameters between 50mm and 56mm, then cupping the trailing edge, with good success.

Now, I want more center-of-blade pitch, and have been experimenting with bending the entire blade, not just a trailing edge cup.

Check out the pictures.
I built an aluminum block with a cavity cut into it.
I filled the cavity with a low temperature (175 degrees F) fixturing alloy.
Then cast the whole blade into the fixturing alloy.
Then bend the whole blade close in to the hub, using an adjustable wrench.
Then melt the prop out of the fixturing alloy.
This procedure preserves the basic shape of the blade, while it gets bent.

The pictured 457 was 3.3 inches pitch, now its 3.87 inch.
I wonder if it will work OK?? Gotta wait till the spring, the water is frozen right now.

If you look at the middle picture closely, you can see a slight bend near the hub.

So far, I Have a pair of 457's cut down to 51MM with 4.0 inches pitch and a pair 55MM and 3.87 pitch.
What say, all you prop guy's, will this work?
Looking for 100MPH with a 16 pound, twin outrigger, 6S, 1800KVA, running near 40,000 RPM.

Mark

lohring
01-20-2012, 09:13 AM
It works to a point. Where you can't increase rpm and are limited in prop selection, it's the only choice. We did this with Octura 2170 props on our record setting gas boats for years. However, you can't increase the hub pitch, and eventually the inner area of the prop will have a negative angle of attack. In this situation we cut the blade at the hub and rebrazed it on at a higher pitch. This was good for a 6 to 7 mph increase at around 100 mph. Our SAW engines turned 20,000 to 23,000 rpm.

With electrics you have a much better solution. By increasing the kv you can increase the rpm to go faster.

Lohring Miller