Large prop low pitch or small high pitch?

[adamb]

What is the differance between running say a large prop (1.4 pitch) versus a smaller prop (1.6 pitch) if they have the same overall pitch? Hopefully I'm making myself clear

[BakedMopar]

It depends on the hull and setup. Hopefully someone with more knowledge will chime in.

[adamb]

[crabstick]

Im no expert, heres my take on it: A bigger prop can provide a bigger thrust cone. Even though a small prop may have near the same overall pitch, it will work a lot harder to get the boat on plane and keep it there.

The bigger the hull, the bigger the thrust cone it needs to get moving.

Does that make sense?

[forescott]

also, would'nt a smaller prop have more slip compared to a larger prop with less pitch. hmmm......

[crabstick]

I think it would, yes.

[MarkF]

I found monos like the bigger low pitch props and hydros usually benefit from the smaller high pitch props.

Mark

[forescott]

Yup, the mono hull has more wetted surface area and pushes through the water rather than riring on top of it like a hydro. It would benefit from a larger thrust cone.

[bwells]

From what I understand, a 1.4 pitch moves through the water 4" for every turn were as a 1.6 moves 6 "(kind of). Pitch is speed and diameter is thrust. More thrust for a larger hull. If the motor does not heat up with an X6xx, step up in diameter. It seems there are a lot of variables in choosing props so maybe borrow or try the Graupner plastic ones. My 2 cents

[forescott]

From what I understand, a 1.4 pitch moves through the water 4" for every turn were as a 1.6 moves 6 "(kind of). Pitch is speed and diameter is thrust. More thrust for a larger hull. If the motor does not heat up with an X6xx, step up in diameter. It seems there are a lot of variables in choosing props so maybe borrow or try the Graupner plastic ones. My 2 cents

Actually pitch is 1.4 inches for each revolution. Total pitch is actually the diameter multiplied by the pitch. "2.0 x 1.4= 2.8 total pitch" Less any slip which could be anywhere from 15-30%.

[bwells]

Thank you for the correction. 1.4 inches makes more sense than 4". I will change my "cheat sheet". Do not want to steer anyone wrong!

[graill]

What is the differance between running say a large prop (1.4 pitch) versus a smaller prop (1.6 pitch) if they have the same overall pitch? Hopefully I'm making myself clear

Hi Adam, in seeing this question i take it your semi new to boating? There are a huge amount of factors hidden in your question that need to be aswered first, i can try to simplify things a bit.

In both cases, all things being equal in a particular boat for both props, the larger diameter 1.4 pitch will draw more amps than the smaller diameter prop even though it has slightly more pitch. Cross section and the area of work the prop is using will affect how hard the motor works and how many more amps it will try to pull to reach a requested rpm (your throttle request). The resulting work/wait is what burns boats to the water line if that request takes to long, usually milliseconds for an esc, among other things. Thats why as a fast electric person you always need to do your math.

Many, many other factors apply but the above is a simplified explanation, hope it helps.

[Simon.O.]

This discussion will be interesting.
I generally like lower dia props to get the rpm up and then feed in pitch to get the speed.
Yes there is a see-saw effect there as the picth will increase load and amps etc.

For my riggers, I go with rpm and smaller dia prop. To increase speed there are two options.
1. Increase rpm with more Volts or more Kv(motor) There are limits here as always
2. Increase pitch with a prop change

example.
My mid size rigger with a Feigao 7S loves a 632 prop.
A 432 is tame until it is fed with 3cell lipo, I have done that too.

My smaller cats like a small prop but I am having some issues with my big (29") Cat as it appears that even on twins the K40 props may not be enough blade in the water.

Overall I find there is a size range of props for a given hull size and type and from there it is up to the user to decide which route they take.
My roads to speed are.
1. Small dia, low pitch and insane rpm
2. Small dia, high pitch and high rpm
3. Bigger dia, lower pitch and lower rpm.

I favour the first, but there is a lower dia limit for most hulls. Especially cats in my experience.

I hope I have not muddied the water more for you.

[raptor347]

To answer the question, it really depends on the application. Higher drag boats need more blade area to push then through/over the water. Very low drag boats don't need the same amount of blade to push them. My spec hydro runs a prop that started out as a 1650, it's now cut to 44mm and has over 5inches of cup at the trailing edge. This prop on an equivalent size/weight mono would not come up on plane. At the same time an m445 which is fantastic on the same mono will only push the outrigger 53mph instead of the 68-70mph its running with a more suitable prop.

You really have to pick the prop for the application.

Remember that the 1.4 and 1.6 pitch designations are actually pitch ratios. The actual pitch it 1.4 or 1.6 times the prop diameter. So a 40mm 1.4 pitch prop has a pitch of 56mm per revolution, a 50mm 1.4 prop has 70mm of pitch.

There are quite a few other factors that come into play. Blade area and how it's distributed. For example look at the blades on an X440, 1440 and 40x52. All have very similar pitch. The 1440 is a lifting prop that's suitable for hydros and tunnels(with a bit of modding), the other two are low lift props that are often considered interchangable on any given application. However the 40x52 has more blade area out toward the tips witch give more options for fine tuning the blades by cupping etc, it also tends to bite a little harder with less cavitation. If you have a boat that is very sensitive to torque, you actually are better off with the X440 even though you may give up a little potential performance to have a better handling boat.

After that, there's rake, pitch progression, blade thickness. There are quite a few variables outside of pitch and diameter.

[adamb]

Thanks guys. The reason I asked is I'm running an m445 on my cat. It ran fast but warmer then I wanted it to. Since the motor is running at 29xxxx rpm's I'd like to try a smaller prop so It can use all those rpms.