twin blade design

Hi there,

I am now busy to make a twin blade prop. Where am I going wrong?
I have from "propcalc" the information that the propeller must be 257 mm large with a pitch of 1.06.
For me to calculate the angle for me to weld the blade to the middle part, I did it the following way.:

I took the diameter = 257mm x 1.06 = 272 mm = pitch.
I projected the 3.141 x 257 mm = 807 at the triangle and took tan of 807 mm divided by 272 mm = 2,9669 which tan equals approx 71,4 degrees. (or 18.6)

Is this correct?
Bert

 

Yes, and no.... We normally measure the pitch angle from the propeller disc plane, and the angle you have calculated is the pitch angle for the tip.

i.e.: Pitch angle = arctg(D*(P/D)/(D*pi)); for your tip that is 18.6 deg.

Each diameter must thus have a different pitch angle, the blade becomes twisted.

 

Thank you so much. I thought it was that simple. I took the picture from Dave Gurr's book and it looked like it was my answer. How do I calculate the weld than?
My power is only between 1 and 5 Hp, thus nothing fancy. Maybe I should just gamble and judge an appropriate angle. At that speed I am going, I believe a small error will not make the boat going much slower or faster.
Thanks for the input.
bert

 

Hi baeckmo,
does it mean that if I take the diameter of the center i.e. in my case
45 mm x 3.141 = 141.345 mm
I project that against the pitch of 272 mm tan = 1.92436 = +/- 62.5 degree or 90 - XXXXXXX degrees. >>>> wrong must be 141.345 / 272 = tan 0,51838 = 62.6 degrees

With other words the top of the blade must be 18.6 degree and the weld 62.6 degree. I can then calculate what the angle must be at various points along the blade.

Do I have it now correct?
Bert

 

The formula is atan(p/d/(p*r/R)) so for any nondimensional radius, ie 0.70r/R the proper angle would be atan(1.06/(pi*0.70)). which is 25.7 deg. At the root (0.15r/R) the angle would be 66 deg and at the tip as Mr. Backmo stated would be 18.6 deg. Since most of the work of the prop is done from the midel to the outer radii, you should probably ignore the angles for the inner radii and weld the blade on at something close to the 25 deg angle. you would be overpitched at the tip, which would lead to more cavitation and less efficiency than necessary, so perhaps you could twist the blade by a few degrees to get it closer to optimal. It might also work acceptably to trim the chord length way down at the tip? Not sure what the blade looks like, so this is just a guess.

 

This is just a guess too from a guy that had no money and tried to make a prop. Go buy one !!!!!

 

Hi Frosty,

That is part of the fun to find out what is ticking and how it works. I bought a leath/milling machine and could have bought lots of props for that money. Although I have two welders, of which one can weld stainless steel, that part I rather leave over to somebody who has a steadier hand than me. I will only buy a prop, when I make a mess of it.
Bert

 

Thanks John. However I am now confused. I thought that I grasped how to calculate it. You bring a figure of 0.70 into the calculation. Where does that come from? Bert

 

Its just a convention. Normally, in a properly designed prop, the formula p/d/pi*r/R doesn't work out to be constant. Meaning if you solved for pitch after measuring the angle and the radius, you wouldn't get a constant value across the prop's radius. By convention, you take the pitch calculated at 0.7R as the nominal pitch of the prop.

 

Thanks Phil, John and Beackmo.

I made an error in thread 4, I hope I got the picture (calculations attached) Thanks for all the help.
Bert

 

In the formula r/R is the nondimensional radius. By convention, this is generally how the prop geometry is defined, by pitch, chord, rake and skew provided at a set of nondimensional radii. Historically, the 0.7 r/R is used to provide information about the propeller. So with a constant pitch propeller, the P/D is a constant and you get pitch angles at each radius by inserting the nondimensional radius (r/R) into the formula and calculating the angle. So your diagram is correct.

Many propellers are made constant pitch for ease of manufacture and repair.

 

Thanks John. At least I know now, that it may work out fine.
Bert