Discussion in 'Props' started by BertKu, Oct 3, 2012.

1. Joined: May 2009
Posts: 2,521
Likes: 47, Points: 58, Legacy Rep: 223
Location: South Africa Little Brak River

### BertKuSenior Member

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

Last edited: Oct 3, 2012
2. Joined: Jun 2009
Posts: 1,568
Likes: 572, Points: 113, Legacy Rep: 1165
Location: Sweden

### baeckmoHydrodynamics

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.

3. Joined: May 2009
Posts: 2,521
Likes: 47, Points: 58, Legacy Rep: 223
Location: South Africa Little Brak River

### BertKuSenior Member

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

#### Attached Files:

• ###### DGerr.jpg
File size:
38.8 KB
Views:
495
4. Joined: May 2009
Posts: 2,521
Likes: 47, Points: 58, Legacy Rep: 223
Location: South Africa Little Brak River

### BertKuSenior Member

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

Last edited: Oct 4, 2012
5. Joined: Nov 2011
Posts: 252
Likes: 16, Points: 18, Legacy Rep: 117
Location: New England

### johneckSenior Member

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.

6. ### FrostyPrevious Member

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

7. Joined: May 2009
Posts: 2,521
Likes: 47, Points: 58, Legacy Rep: 223
Location: South Africa Little Brak River

### BertKuSenior Member

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

8. Joined: May 2009
Posts: 2,521
Likes: 47, Points: 58, Legacy Rep: 223
Location: South Africa Little Brak River

### BertKuSenior Member

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

9. Joined: May 2008
Posts: 2,599
Likes: 374, Points: 83, Legacy Rep: 1082
Location: Beaufort, SC and H'ville, NC

### philSweetSenior Member

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.

10. Joined: May 2009
Posts: 2,521
Likes: 47, Points: 58, Legacy Rep: 223
Location: South Africa Little Brak River

### BertKuSenior Member

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

#### Attached Files:

• ###### prop1.jpg
File size:
90 KB
Views:
836
11. Joined: Nov 2011
Posts: 252
Likes: 16, Points: 18, Legacy Rep: 117
Location: New England

### johneckSenior Member

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.

12. Joined: May 2009
Posts: 2,521
Likes: 47, Points: 58, Legacy Rep: 223
Location: South Africa Little Brak River

### BertKuSenior Member

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

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.