# Determine pitch along the length

Discussion in 'Props' started by Solarboat, Jun 22, 2020.

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### SolarboatJunior Member

I've found a document on the internet where they show how to determine the pitch along the lenght.
But is this the right way to do it?

D = radius of the diameter
P/D = pitch ratio

Ps. I attached the document

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### gonzoSenior Member

Are you trying to design a propeller? Otherwise, the nominal pitch is at about the middle of the blade.

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### jehardimanSenior Member

Get a copy of the development of B series. It can be found in several works, especially the 1967/1977 errata/1980 edition of Principles of Naval Architecture...it takes you through the whole B series development.

edit: A principle is not a pal.

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### SolarboatJunior Member

Yes, I'm trying to desing a propeller. And I'm trying to figure out how to calculate the pitch over the lenght of a blade. Because I coundn't find it in Gerr's book.

Can I find here how to calculate the twist of a blade.
If so, where can I get a copy of the books?

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### gonzoSenior Member

Designing propellers is a very specialized branch of engineering. A book is not going to be enough. From your post, it seems like you are at a basic level. I am not trying to offend you. However, even though I have an engineering degree, designing a propeller is a daunting task. Are you prepared to spend a few years studying? In general, propellers get specified, not designed for an application. There are cases, like military submarines, where they have huge budgets and teams of engineers to design a specific propeller.

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### SolarboatJunior Member

We have a project for school where we have to build a solarboat. I have been given the task of designing a propeller. But if I have to believe you, you cannot design a propeller with Gerr's book.
I have a few pictures of propeller as an example here in the appendix. These are all custom made.
I also added an Excel file. It contains all the calculations I have done so far.

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• ###### Example.png
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### gonzoSenior Member

Gerr's book is for specifying propellers. They are simple formulas that work remarkably well. He adds enough theory for readers to have a basic understanding. At this point in time, propeller design has reached a plateau where only small improvements can be made. In general, those are for specialized applications. Are they requiring you to start from nothing? In fact, nobody does that. Even large research facilities will start with a series that approximates their requirements and refines it for the application.

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### jehardimanSenior Member

If you are a student, go talk to your librarian, they can get Principles of Naval Architecture for you. Here is a scan of the 1988 edition, Volume II, which covers propellers. Principles of Naval Architecture Second Revision Volume II @BULLET Resistance, Propulsion and Vibration https://www.academia.edu/37034241/Principles_of_Naval_Architecture_Second_Revision_Volume_II_at_BULLET_Resistance_Propulsion_and_Vibration

Realistically, if this is for a low power solar vessel, a standard B series is not what you want. They are made to fit conventional cargo ships and all the things that come with a cargo ship. You need blade element theory to get maximum performance from a non-standard situation. Blade element theory - Wikipedia https://en.wikipedia.org/wiki/Blade_element_theory
Additionally, you really need to get your head around the propeller blade helix. While the root and the tip may have the same pitch, the path they take and their geometry on the blade is very different. The idea is to make the thrust generated by each discrete element of the blade equal by changing the width of the blade element while accounting for the speed and angle of attack of the blade element along its specific helix line.

While not as bad a picture as Gonzo paints, he is correct that propeller design is fairly specialized. From a practical point of view, a propeller made from twisted sheet metal can be made to be 75-80% efficient...as long as the pitch and blade shape are properly selected. The propellers you showed are getting there, but have too much drag at the tips. It should look more like this...

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### gonzoSenior Member

Do you men 75-80% of the efficiency of a commercially bought propeller? I would think the blades wouldn't be able to be loaded as much.

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### jehardimanSenior Member

No, not commercial, but for small low power or HPV's. We are only talking 100 lb (50kg) of thrust at full power bollard. The SubHuman III set above produced 175 lbs bollard for the 3rd ISR. The SubHuman II set below produced 220 lbs bollard for the 2nd ISR and the Guinness record. 33 inch diameter, ~0.1 in thick, calculated 88% open water efficiency, just over 100% behind the hull.

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### HeimfriedSenior Member

Regarding your question "... is it the right way to do it?", my answer is no. In my view the authors of the linked paper didn't understand, what they were writing, it is contradictory and plain wrong. They confused radius and diameter and also things about pitch. Variable pitch propellers vary the pitch only slightly along the radius of the prop. The range of the shown numbers (from 1.65 to 6.61) are very much out of it. May be they tried to calculate a kind of pitch angle and did it wrong.

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### BlueBell. . . _ _ _ . . . _ _ _

Solarboat, are you still around?
How's the prop making going?
You'll find a radio controlled model airplane propeller to be a great design for a low powered solar boat.
It would simply be a matter of matching diameter and pitch to your power and drag.
But, you have to make your own.
It can be done with flat steel, a vice, and a grinder.
Do a search here on this forum for an explanation.

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