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Thread: propeller pitch dimensions question

  1. #1
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    Default propeller pitch dimensions question

    My question is about propeller pitch and how it changes the size of the propeller. If I had a 12 inch diameter propeller with a 12 inch pitch and I changed to a 18 inch pitch, would the horizontal dimension increase and by what amount ?

  2. #2
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    Default Re: propeller pitch dimensions question

    No. Diameter is diameter and pitch is pitch. But more than that, the question's off, too much unstated. Get "Propeller Handbook: The Complete Reference for Choosing, Installing, and Understanding Boat Propellers" by Dave Gerr.

  3. #3
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    Default Re: propeller pitch dimensions question

    Rough and ready an inch of pitch equals about 100 rpm, and an inch of diameter equals about 200 rpm.
    That being said, there are a lot of other factors, like the reduction ratio of the gear, the max rpm of the engine, is it a 2,3, or 4 blade, what is the DAR(a ratio of surface area of all the blades) etc.
    If your existing prop was allowing the engine to reach its rated max rpm’s, then any good prop shop will be able to calculate the change you are thinking about.
    You can reduce sound by reducing Diameter thus increasing tip clearance to the hull, then pitch need to be added to fully load the engine.
    A lot of traditional sailboat prop apertures were woefully small and resulted in quite inefficient use of the inboards power.

  4. #4
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    Default Re: propeller pitch dimensions question

    My question is not about selecting proper pitch but how does pitch effect the size of the propeller. I have a 12 inch prop with a 12inch pitch. If I lay it on a table it
    is 3 !/4 inches high. If I switch to a 18 inch pitch would that 3 1/4 inch dimension change and about how much?

  5. #5
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    Default Re: propeller pitch dimensions question

    A totally new question from the OP. Really, get Gerr's book.

  6. #6
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    Default Re: propeller pitch dimensions question

    Quote Originally Posted by Pete H View Post
    My question is not about selecting proper pitch but how does pitch effect the size of the propeller. I have a 12 inch prop with a 12inch pitch. If I lay it on a table it
    is 3 !/4 inches high. If I switch to a 18 inch pitch would that 3 1/4 inch dimension change and about how much?
    No one will be able to add 6” of pitch. The total depth(fore and aft dimension)of the propeller will change, but you will not be able to turn a 12 x 12 prop into a 12 x 18.

    A visit to an active propeller repair shop will give you the info you seek about cost of a new prop, depth, etc.

    sounds like you are worried about it fitting within your aperture?

  7. #7
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    Pleasant Valley NS Canada
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    Default Re: propeller pitch dimensions question

    Quote Originally Posted by Pete H View Post
    My question is not about selecting proper pitch but how does pitch effect the size of the propeller. I have a 12 inch prop with a 12inch pitch. If I lay it on a table it
    is 3 !/4 inches high. If I switch to a 18 inch pitch would that 3 1/4 inch dimension change and about how much?
    Yes, it will change the "on the table" height a bit, but how much elicits my favorite response to technical questions; "it depends". Basically, changing pitch involves twisting each propeller blade about its root axis, so if you take a round-ish blade that was laying flat on the table and twist it 45 degrees, it will no longer be laying flat and will therefore have greater height off the table. The "it depends" comes from the great variety of blade shapes and where their root axis lies along that blade shape. They will all change 'height' when pitch is increased, but by how much varies depending on the blade shape and where ther root axis lies.

    Does this answer your question, sort-of?
    Hope for the best, but plan for the worst.

  8. #8
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    Aug 2020
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    newburgh,NY
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    Default Re: propeller pitch dimensions question

    yes -I am building a steam launch and have a 12 by 12 prop. I was wondering after I complete it and wanted to change to a larger pitch would it still fit in my aperture. I would buy one but the sites I have been to do not give overall dimensions. Not ready to buy a new prop, just thinking ahead.

  9. #9
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    newburgh,NY
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    Default Re: propeller pitch dimensions question

    Thankyou-that answers my question.

  10. #10
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    Athens, GA
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    Default Re: propeller pitch dimensions question

    These guys do excellent work and there is loads of information on their website:

    https://www.generalpropeller.com/?gc...xoCwHAQAvD_BwE

  11. #11
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    Default Re: propeller pitch dimensions question

    The ever shifting questions here speak to Pete H's need to make friends with Gerr's book. Prop aperture is but one of the issues to be harmonized. We might expect questions gradually introducing engine RPM; shaft RPM; foot-pounds thrust; shaft diameter; diesel v gas v electric v reciprocating steam. . .

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