Prop pitch verses diameter

[bwells]

I'm trying to understand the dynamics of props due to size and pitch. My question is as follows: Octura X642 1.6x1.65=2.64
m545 1.5x1.77=2.65
X448 1.4x1.90=2.66
Would all of these props do the same thing due to the final pitch, same boat and motor Etc.? It appears that they should but I think the diameter is confusing me, just can't get a handle on it. Thanks Edit: would the x4, x5, x6 be called the "nominal" pitch and the 2.64 Etc. be called the "final" pitch

[domwilson]

In Octura props, the first number is the multiplier. For instance, a x442 would be 1.4 X 42mm. A X642 would be 1.6 X42 mm, m545 would be 1.5 X 45 mm with the "m" meaning the tongue removed. As far as the diameter, that depends on a number of things. Too large a diameter will probably draw too many amps and in some cases introduce "torque roll" as too small a diameter will not have a sufficient "thrust cone" to push the hull. Bigger is not always the answer to more speed as it may load the prop too much and not allow the motor to reach it's full rpm potential. The best thing is to try a few props to see which will get you what you are after.

[bwells]

Dom, I understand that but the final pitch on the 3 props is the same. I have 5 props that the final pitch goes from 2.48 to 2.90, differing nominal pitch (x4, x5, x6). Would the above stated props be exactly the same due to the final pitch or does the diameter override any pitch change for the motor being used and the boat it is in? This may throw in an other variable and one I don"t want to consider, too complicated!!!

[domwilson]

Not sure I'm understanding. Although the final pitch is the same, the loading is different. The larger diameter may cause a higher amp draw. The best thing to do is experiment with your particular setup.

[tiqueman]

Diameter wouldnt necessarily over ride it, but it is another factor. The same pitched prop, lets say a 37mm vs a 45 mm w/ the same final pitch (im just throwin those numbers out there, not sure if a 37 and 45 ie "x" series will come out w/ exact same final pitch) anyway, its going to take more "umph" to turn the 45 and as Dom said, more amp draw. And the 37 may work great on boat "a", but horrible on boat "b".. and vice versa. So just because final pitch is the same or close to the same among various props.. characteristics because of diameter will be noticeably different.

I dont know if that was all ramble or kinda what you were looking for... Im tryin!

[bwells]

I got it and I am leaning toward the diameter as being the deciding factor as opposed to the final pitch ratio. I am not sure why but my guess is that the pitch difference is so small compared to the diameter that the size of the prop that the motor can swing is the deciding factor. I think I may be over my head on this one!!!!

[tiqueman]

Its all trial and error. Like I said, one prop that works incredible on one boat, may not do the trick on another, even if very similar hulls. Test, test, test... it never ends.. cause then you can mod props, de-tongue, thin, cut down... If you want some really good answers, ask Egneg, aka the prop god.

[bwells]

Yep, He did 3 of my props and they are perfect, I checked 'em. Thanks for the reply

[Rex R]

I've come to figure that larger dia = more blade area, more blade area = more power needed to 'swing' the prop.
rex

[m4a1usr]

I'm trying to understand the dynamics of props due to size and pitch.

"Pitch is the distance the prop will go through the water in a single turn. A 12" pitch prop would be 1 foot forward for every Revolution. That is the theoretical distance the prop would travel but due to slip it actually does not. Slip is the ratio between the height difference and the real distance."

Diameter is effectively the blade surface effecting the loading. A 38mm prop with a 1.6 pitch and a 48mm prop with a 1.6 pitch will both theoreticaly travel the same distance. But if used on the same size/design/weight/rpm'd hull the larger prop will create an increased load due to its blade larger diameter and surface area. Cupping lessens prop slippage.

John

[bwells]

Thanks for the replies. Alright then, I will go with diameter as being the kicker on this one and do trial and error to make the proper choice. (proper choice, good one). Thanks again.

[FE Wannabe]

Some really good info here.

Not trying to hijack the thread but I have a somewhat related question.
What makes a prop a "High Lift" prop? What are the physical properties of the prop that cause it to lift more than a normal prop? Where on the blade is the lift generated, at the tongue, trailing edge, or the whole surface?

Brad

[egneg]

Some really good info here.

Not trying to hijack the thread but I have a somewhat related question.
What makes a prop a "High Lift" prop? What are the physical properties of the prop that cause it to lift more than a normal prop? Where on the blade is the lift generated, at the tongue, trailing edge, or the whole surface?

Brad

Good question - Lift has to do with the thrust cone. A wide thrust cone produces more lift because the water is being directed more to the side so if you narrow the thrust cone more to the back this will reduce the lift. The shape of the blade versus pitch plays a major role as far as lift is concerned.

Note: This is true for sub-surface props only as submerged props are a whole new ballgame.

[forescott]

What about a two blade vs. three. Doesnt a three blade prop create more of a load because of the extra blade. Isnt a two blade prop more efficient? If so then whats the advantage of three blades? And what type of prop is best suited for each hull type?

[egneg]

What about a two blade vs. three. Doesnt a three blade prop create more of a load because of the extra blade. Isnt a two blade prop more efficient? If so then whats the advantage of three blades? And what type of prop is best suited for each hull type?

That is a loaded question. It all depends on the setup ... and at this point there are to many variables to answer your question.