Modelling a Propeller

[zul8tr]

It's not going to be that complex, we don't have too much time for this project. If I want to go into further detail then it will be in the future and in my own time. All I want to do for this project is change the hub so I can use a removable hub kit, and change the shape of the bits which connect the inner hub to the outer hub, so they are effectively a continuation of the blade (as if you took an over-hub cleaver and changed it to through-hub). This should allow me to modify the same design in the future to create an over-hub propeller from the same model.
I do want to learn more about propellers, but for this project I've been told to keep it relatively simple. I don't have enough experience to be able to modify the design of the propeller itself, so I'm trying to keep it realistic.

That is a good approach since you were told to keep it simple. Keep us informed.

[Fastjeff57]

...(On my homemade props) "...do you balance them yourself?"

I spin balance them in the fashion of a ceiling fan! (Crude but effective.) Each blade is carefully weighed before welding with a digital scale and 'adjusted' until they are equal.

Jeff

[Roflhat]

update on this, sent the propeller away, 11x18p Mercury Cleaver. Got the IGES file back a few days ago, here's a couple screenshots from Rhino




I've bought a 13 spline RUBEX hub which I'm going to use as a reference to model around, I'll start modifying the design hopefully towards the end of January

[zul8tr]

That looks like a progressive pitch prop based on the curveed blade section with high rake. Now that you have the 3D digital file you can (I suppose) take a radial cut thru the blades at a given radius to see the 3 blade foil x-sections. Then calculate the pitch on the thrust side of the blade from lead to trail edge along that radial cut. Do that for numerous radii and plot the results and get a contour map of pitch at all points measured. It will show you the accuracy of blade manufacturer (generally not the same). Total average blade pitch can then be determined with contour area averaging of all blades. Then you have a record of what you started with.

In case you do not know it, at any point on blade thrust surface: Pitch = 2 x PI x R x Tan(Angle)
PI = 3.14159....
R = radius from hub shaft center (inches for pitch in inches)
Angle = measured at 90 degrees to pitch point interested in on the blade thrust side cut of the foil x-section rearward to a line parallel to prop shaft centerline. The angle is measured in the same plane as the radial cut was made (geometrically, the plane is at 90 degrees to the foil x-section cut and tangent at the pitch point interested in).

[zul8tr]

OK but at least you have the digital file for the future.

[Roflhat]

Thanks zul8tr, that's all good information. I'm looking for literature on this, do you know of any good books regarding propeller design? I'm hoping to get Marine Propellers and Propulsion by John Carlton from the University library.

[zul8tr]

Here are some links I have collected discussing props. Note the pitch gage and the angle measured on the blade relative to the prop shaft centerline at a given radius. That is what the formula above represents.

http://www.boatracingfacts.com/forum...gressive-Pitch

http://www.boatracingfacts.com/forum...ght=pitch+gage

http://www.vicprop.com/propeller101.htm#7

http://www.screamandfly.com/showthre...sh-prop-or-not

I have this book by Dave Geer and is a good reference on propellor basics

http://www.amazon.ca/The-Propeller-H.../dp/0071381767

Jim Russel has a book on propeller design. He has a lot of great info on this site

http://tunnelboats.com/secretsofpropellerdesign.html

[Roflhat]

I'm just starting on this now, anyone have any advice regarding extrapolating the blade shape further into the hub? I want to remove the fillet where the blade joins the outside of the hub and move it inside the exhaust tube, with the blade continuing through.
Here's a photo of a propeller which looks like what I'm aiming for.

[Fastjeff57]

I did that to one my props, whose hub is smaller in diameter than the 'bullet'. Thought it would reduce some nasty cavitation I was getting on plane off. (My boat was used to take grand kiddies for rides--you don't want to hang over the bow with them aboard to get on plane!) The 'barrel' really didn't help, so I removed in and lived with the cavitation.

Jeff

PS: Lots of folks, with 'jet prop' exhausts, use a prop with non-jet prop type hub. The motor will over-rev a bit on plane off, due to exhaust-caused cavitation, but that goes away soon as you get on top. (I'm sure you already knew this!) Your prop could dispense with the outer hub completely.

[Roflhat]

Yeh, I'm thinking with this design the exhaust hub will be structurally redundant, so to turn it into and over-hub exhaust prop you could simply remove the exhaust hub.
But for this project I want to keep the exhaust hub, but just make the blade shape like the photo posted above. I figure this will increase the effective blade area, as the fillet between the blade and the hub is moved to inside the exhaust hub.