Let's Talk Camber

[Meniscus]

So here's a question I've wrestled with in the past with regards to camber of a prop on the leading edge. When you introduce more camber, let's say for a 3-point hydro and a lifting prop, does it increase or decrease the prop walk, with all other things considered equal?

I understand that the slippage is affected which may expound on either an increase or decrease, but I don't have my head fully wrapped around what is going on with the introduction of more camber.

2nd Question: Does the progression of the camber contribute to the change in prop walk as well or is it a moot point?

Last Question: Are these effects more pronounced at lower RPM or a higher RPM setup?

All input is welcome and greatly appreciated.

[Meniscus]

Anyone have any insight?

[egneg]

Not many folks know anything about this and those that do probably aren't willing to say much here as it is a very involved mathematical process. Here is a link to a PDF file developed for obtaining partial fulfillment of a Masters of Science in Ocean Engineering at MIT.

http://dspace.mit.edu/bitstream/handle/1721.1/42632/1/37520848.pdf

Camber is a design characteristic for a given RPM. This is why ships and tugs have variable pitch props to compensate for load while keeping the RPM constant.

[Meniscus]

Great info Chuck! I've found similar, but not to this level of detail. What I am missing is the practical application based on experience. Until you provided this paper, much of what I was finding were conclusions that didn't necessarily translate to the smaller scale and higher RPM.

Perhaps I should clarify my question to include practical application based on experience and subjective conclusions as well as opinions on the subject. Again, any input is welcome and graciously accepted.

[egneg]

Ben I doubt very much if you will. As you can see there is a great deal involved and in our small world of rc boats this info will be closely guarded as the return is very little for the time required to develop successful props. Remember a couple of years ago when Andy Brown of CMDi came out with CNC props? The venture was short lived, even though the props were very nice looking they didn't work.

[Meniscus]

Understood Chuck. I was hoping that some of the seasoned folks with countless years of experience would weigh in so that others can gain a little insight. For my purposes, I was hoping I might be able to incorporate into prop modifications pondside for this and upcoming SAW events next year. Of course there's always things to learn at any level, but I was hoping to have some of this information so that I could discuss with the local folks to see what their opinions and approach are.

[lohring]

Model boat propellers are surface piercing propellers. At speed, only the back surface of the blade matters. The front surface is in an air pocket, and the pocket's shape has nothing to do with the shape of the front face. Because of this, people usually talk about progressive pitch and cup, rather than camber. These are variations in the pitch of the blade's back surface. With progressive pitch the pitch increases gradually from the leading edge to the trailing edge. With cup the trailing edge is bent, increasing the pitch in a short distance. Combinations are often used. As an example, Octura X series props have mostly helical pitch with trailing edge cup. Prop Shop props have progressive pitch. A much more relevant discussion of supercaviting hydrofoils and their bottom shapes can be found here.

Lohring Miller