Changing pitch vs changing diameter - what effect gear reduction?

[Finnerty]

If a 2 inch change in pitch with a 2:1 gear reduction caused a 150 rpm change in WOT, would a 1 inch change in diameter give roughly the same result (assuming a 1 inch change in diameter is similar to a 2-3 inch change in pitch) or in this case because of the 2:1 reduction would it double the result (ie. 150 rpm X 2 = 300 rpm) ???

 

[Barnacle_Bill]

Re: Changing pitch vs changing diameter - what effect gear reduction?

No. Changing diameter would have very little affect.

 

[mpsyamaha]

Re: Changing pitch vs changing diameter - what effect gear reduction?

What he said ^

a 1" change in diameter has very little effect on wot rpms.

 

[tpenfield]

Re: Changing pitch vs changing diameter - what effect gear reduction?

If a 2 inch change in pitch with a 2:1 gear reduction caused a 150 rpm change in WOT, would a 1 inch change in diameter give roughly the same result (assuming a 1 inch change in diameter is similar to a 2-3 inch change in pitch) or in this case because of the 2:1 reduction would it double the result (ie. 150 rpm X 2 = 300 rpm) ???

You can not really equate changes in diameter to changes in pitch that simply. A diameter change of that magnitude will have quite an effect and could be more significant than the change in pitch. There is no absolute way to tell for sure., except to try it. Since the gear ratio is consistent, it would have nothing to do with the comparison, only the absolute value of the RPM change.

Increasing the diameter has a similar effect as increasing the pitch. It is somewhat hard to say exactly how much though.

A lot depends on what propeller diameter you are starting with. For example: if you start with a 14 inch diameter prop and go up to a 15 diameter prop, keeping the same pitch, the effective prop 'area' (discounting the hub) increases by over 15%. You could expect a 15% decrease in RPM.

Not sure that answers your question though . . . :noidea:

 

[Finnerty]

Re: Changing pitch vs changing diameter - what effect gear reduction?

You can not really equate changes in diameter to changes in pitch that simply. A diameter change of that magnitude will have quite an effect and could be more significant than the change in pitch. There is no absolute way to tell for sure., except to try it. Since the gear ratio is consistent, it would have nothing to do with the comparison, only the absolute value of the RPM change.

Increasing the diameter has a similar effect as increasing the pitch. It is somewhat hard to say exactly how much though.

A lot depends on what propeller diameter you are starting with. For example: if you start with a 14 inch diameter prop and go up to a 15 diameter prop, keeping the same pitch, the effective prop 'area' (discounting the hub) increases by over 15%. You could expect a 15% decrease in RPM.

Not sure that answers your question though . . . :noidea:

That does help thank you. The percentage change in effective prop area is something I can calculate directly, thanks again.

 

[steelespike]

Re: Changing pitch vs changing diameter - what effect gear reduction?

A 15" pitch should behave like a 15" pitch regardless of the diameter however a 1" diameter change is significant and will effect rpm.
I think finding props of the same pitch and design with a 1" inch different diameter is unlikely.
Generally a heavy boat will do better with a larger diameter prop and will likely have a greater ratio reduction.
If you look up props you will find that generally as pitch goes up diameter goes down ,as number of blades increases diameter goes down.

 

[smokeonthewater]

Re: Changing pitch vs changing diameter - what effect gear reduction?

you CAN'T just calculate prop area... the tips of the prop, moving at the highest speed, have much more efect while near the hub has very little..... the needed diameter is determined by the hp, shaft rpm (gear ratio) the size of the boat and the geometry of the prop.... since we common folk are just buying generic props for a given engine family then they aren't specifically designed for each boat because that would cost thousands of dollars.

When selecting a prop look at pitch and geometry and honestly most people are better off describing their current boat, engine prop and performance to a professional prop man and letting him suggest a prop.

The specifics on individual props are not very transparent and the experience gained by years of trial and error is all but impossible to replicate by surfing the internet..... I put months into the diameter question and basically, if your boat is fairly average then you don't need to worry about it.... If you have a huge boat w a small engine then you are in a small group not well served by the average prop. If you have a small boat with a really big engine then you are again out of the average but since that group is constantly throwing lots of money at their boats looking for another 1/2 mph, there is lots of info around for selecting props

 

[Frank Acampora]

Re: Changing pitch vs changing diameter - what effect gear reduction?

Having done that on a smaller boat with rated horsepower-- 13 X 19 prop, 15 footer with 90HP-- I can say that in that particular case, reducing diameter had almost no effect. On a high horsepower lightly loaded hull, pitch changes have the most effect.

Necessary blade area is a function of load and blade slip.

I can not provide specific details because I did not write them and it was a long time ago. However I do seem to remember that on my 14 foot go-fast with 125 HP, the stainless cleaver 12 X 21 with less blade area gave the same top speed at a higher RPM than the Ballistic 13 X 21. So there you see the effect of slip.

The Bronze two blade was about 24 pitch and loaded the engine down so both RPM and top speed suffered losses.
Depending upon prop design, the tips may or may not be delivering more thrust due to higher linear speed. MANY props are designed as a segment of a helix with constant pitch throughout. think of a wood screw or an inclined screw conveyor. The outer edges do not advance more than the root. On the other hand, progressive pitch props would develop more thrust at the blade tips.

 

[Texasmark]

Re: Changing pitch vs changing diameter - what effect gear reduction?

View attachment 219979View attachment 219980View attachment 219981View attachment 219982View attachment 219983
The outer edges do not advance more than the root.

Interesting comment. Thinking of tip velocity w/respect to noise on air moving props and translating noise in air to possibly cavitation bubbles in water. I noticed the newer props on a C130 Hercules are built like the props on our Nuke Subs with the tips rolled back to prevent excessive tip speed, hence noise, as are some of the portable house fans that are really quiet.

Remembering a '71 Chrysler 85 hp I had with the stock alum prop. It had a reversal on the leading edge just out from the root (you know what I am talking about) and mine kept the paint burned off in that area (about the size of a quarter dollar) from cavitation.

Mark

 

[limitout]

Re: Changing pitch vs changing diameter - what effect gear reduction?

a change in diameter is just like changing the size of the load you are pulling up a hill with your car so it is a much bigger change in performance then pitch unless you offset that with a much different pitch at the same time.

if you reduce the diameter size its the same as putting smaller wheels on your car and the engine will turn higher rpms for the speed to stay the same. to account for this it is common when changing diameter to also change pitch to make up for the difference.

if you go with a smaller diameter prop the engines load is decreased so you often go with higher pitched prop to help increase the load to offset the reduction of load on the engine trying to turn the now smaller diameter wheel.

if you go with a bigger diameter the engines load is increased so you often go with lower pitched prop to help reduce the now increased load so that helps offset the engine working harder to turn the bigger wheel.

but there are a lot of other things that can make a big impact on the results you see so you cant overlook things like the "rake" and "cupping" and also the 4 blade props may or may not be what fits your needs best.

 

[steelespike]

Re: Changing pitch vs changing diameter - what effect gear reduction?

All props are sold by pitch; diameter is only mentioned to match the existing prop or to be sure it is meant for the motor.
Diameter may be discussed for certain particular applications like heavy or hard to push hulls.
Pitch is measured in inches and pitch blocks come in fractions of an inch.
Some props are sold in fractions. Never seen degrees mentioned. I suppose some very high tech props may mention degrees.
Certain very particular applications might use a diameter change to fine tune rpm.
General rule of thumb 1" pitch change equals about 150 to 200 rpm. I don't think a 1" diameter change could result in up to 600 rpm.