Thrust factors

**kenboyd** · 04-30-2009

In theory, what effect would a one inch decrease in diameter have on thrust, in conjunction with the RPM increase that would be produced, by a propeller moving through the water, if no other factors were changed?
(This is an example to make understanding what I'm asking more understandable. These numbers are for simplicity and clarity only). For instance, if I had a 10 inch prop producing 100% thrust at a given RPM and NO other factors changed (horsepower, pitch, blade shape, style, or number, drag, viscosity, velocity, etc.), and I reduced the diameter 1 inch, the thrust would be 90% at the same RPM, then 80% for 2 inches, 70% for 3 inches....
Now, the question again in its final form. Using the above example: What would the thrust be with the increase in RPM? Would the thrust be less than 10% decrease or more than 10%, for each 1 inch diameter decrease?
I'm not interested in the effects hull design have on the equation. I'm just talking about a prop moving through the water. Blade shape, design, and number, exact figures or Reynolds numbers are irrelevant in this question. I'm aware that these factors all have an effect in reality, but I am not interested in these effects for this theoretical question.
Ken

**Mark75H** · 04-30-2009

Diameter is not as closely related to thrust as pitch is. Your question is beyond science and into the realm of art. No way to know without trying the prop on a boat, the variables are way too many and it will change dramatically from boat to boat, prop to prop and set up heights and angles.

**kenboyd** · 04-30-2009

Originally Posted by Mark75H

Diameter is not as closely related to thrust as pitch is. Your question is beyond science and into the realm of art. No way to know without trying the prop on a boat, the variables are way too many and it will change dramatically from boat to boat, prop to prop and set up heights and angles.

Mark,
I don't think you understand the question. I'm not asking about the relationship between thrust and pitch, or pitch and diameter, or how props react when placed on one or another boat, or differences between props, or setups, or any other variables that might exist in that realm of reality. I am aware of how those things work and relate to each other.
Maybe I shouldn't have said the prop moving through the water. How about a prop in a tank or an enclosed controlled environment, not connected to a boat, but rotating in a fluid of a controlled viscosity and producing a given amount of thrust at a particular RPM, with a set amount of torque. Then reducing only the diameter which will increase the RPM, (I'm not talking about cutting down a prop. Let's say the prop is an exact replica except smaller, no other factors are changed), and then measure and compare the difference in thrust. Of course the resulting figures will be different for different types of props, but the relationships between the thrust differences would be similar. I'm interested in the direction and degree of those differences. I'm sure these factors have been explored by someone.
Ken

**Mark75H** · 04-30-2009

Originally Posted by kenboyd

Mark,
I don't think you understand the question. .... I'm interested in the direction and degree of those differences. I'm sure these factors have been explored by someone.
Ken

I understood exactly what you meant. The only one I can imagine would be Paul Kamen aka "Fishmeal"

**david bryan** · 04-30-2009

i have only ben around boat racing for 50 years so dont know much about thrust but i do know about rpms and that is a propeler officiates increases with the rpm

**kenboyd** · 04-30-2009

Thanks guys.

Ken

**JohnsonM50** · 05-01-2009

Originally Posted by kenboyd

In theory, what effect would a one inch decrease in diameter have on thrust, in conjunction with the RPM increase that would be produced, by a propeller moving through the water, if no other factors were changed?
(This is an example to make understanding what I'm asking more understandable. These numbers are for simplicity and clarity only). For instance, if I had a 10 inch prop producing 100% thrust at a given RPM and NO other factors changed (horsepower, pitch, blade shape, style, or number, drag, viscosity, velocity, etc.), and I reduced the diameter 1 inch, the thrust would be 90% at the same RPM, then 80% for 2 inches, 70% for 3 inches....
Now, the question again in its final form. Using the above example: What would the thrust be with the increase in RPM? Would the thrust be less than 10% decrease or more than 10%, for each 1 inch diameter decrease?
I'm not interested in the effects hull design have on the equation. I'm just talking about a prop moving through the water. Blade shape, design, and number, exact figures or Reynolds numbers are irrelevant in this question. I'm aware that these factors all have an effect in reality, but I am not interested in these effects for this theoretical question.
Ken

Without knowing much science on the matter I have a prop I wanted more Rs out of so I carefully & methodically removed about 10% of the dia. Then with calipers as a measure I evened them & re-edged them [3 blader]. Last but not least I checked for balance. When done it gained about 200 Rs on the exact set up as B4 & about 2 mph. So, in percenages that will be 'about' 10% off = 4-5% RPM and speed. This started out as a 7 1/4 cleaver that is now 6 1/2 and my fastest prop for the boat I run it on.
What I leave to

is by needing to trim back the leading edge some to re-shape, has the pitch increaased? Otherwise Id think Sam has it right that its an art. I got lucky, no way to be sure but to do it at your props risk.

What to disclaim: at the same time I thinned the prop a little more again using calipers to slide in & mark along the blades & connect the dots A pattern of thickness appears from the lines drawn to see where work is needed. [this was definitely a few late nights at the bench]

**kenboyd** · 05-01-2009

JohnsonM50
That answer is in the direction that I was looking for. Thanks Mike
Ken

**JohnsonM50** · 05-02-2009

Originally Posted by kenboyd

JohnsonM50
That answer is in the direction that I was looking for. Thanks Mike
Ken

Glad to help. I find by figuring potential speed [B4 slip/resist loss] & subtracting real speed that cleavers are very efficient. Of course 2 blade props wont be as efficient at all, the few round eared 3 bladers I have dont do as well. This doesnt mean cleavers are faster across the board, just that 2 of them are working for me. The fastest is a low rake cleaver, it airs the flat bottom out just right. The favorite is a high rake cleaver, its nearly as fast, has better traction & acceleration. The boat is...