ORIGINAL: terryking
IsThere anything similar for that class of boat? I understand much of what is here is applicable.
I have sized props for fullsize boats with 200HP etc. Diameter / type from prop mfg guidelines, PITCH from calculating RPM, speed, and slippage.
I don't see anything specifically about PITCH here. Mostly just diameter. What am I missing??
I am not sure of what you mean by your first question. Similar to what? motors? hulls?
As far as props here's the lengthy explanation of why?
I look at diameter as being the independent variable. What I mean by that is, diameter is diameter. Everyone knows how to calculate this value. When a prop makes a full circle diameter is the outer sweep from the tip of the prop. This value is very easy to visualize and understand.
But what is pitch? How do you calculate that? Pitch is commonly determined by a ratio where the value depends on diameter. For a typical application where we want to travel at a decent speed, a very small pitch ratio will be quite inefficient. A very large pitch ratio will also be inefficient.
Pitch of prop = Pitch ratio * Diameter
example for 40mm prop with 1.4 as a pitch ratio
Pitch of prop = 40mm * 1.4
Pitch of prop = 54mm
This means we have a standard pitch ratio range that is best suited for our application. Every application may have a different range that is best suited. A scale type hull would use props that have very low pitch ratios. We would probably not use these in our hulls.
Diameter for a power system is a lot easier to visualize, and select a prop from. But why not calculate which prop to use instead of making a judgement based on experience???
Gas full size boats are a lot more predictable than electric boat. And especially electric RC boats. I have made several programs that calculate which prop would be best suited. Before I achieved any accuracy I had to have a large amount of variables to be input. Anything from size of hull, type of hull, down to the power system specifics.
These calculated values the selection close but was never right on. It can't and won't ever be. Full size or Hobby size. But for electric, overshooting the diameter can ultimately lead to electrical failure.
This is the reason I use diameters in the chart and greatly emphasize to always ask about prop size if unsure. The wrong kv motor on the wrong cell count, with the wrong prop is going to be very destructive. Just one of these variables incorrectly matched may experience failure.
Once diameter is selected you can start with a small pitch ratio. From here it is just a matter of experimenting with greater pitch ratios/diameters ect.
You could then calculate speed based on slippage, Loaded Prop RPM and pitch used. I would strongly advise against calculating pitch from speed desired by re-arranging your equation. Reason is, we back track to where I mention failure if the pitch you select is too great for the motor.
Greater Pitch may affect:
- Prop Slippage
- Prop Lift (Hydros typically have higher pitches ratio's for greater lift)
- Motor Load ( A motor will turn the load you hand it whether it melts from ehaustion or not / A gas engine will not turn an increased load if it does not have the power to do so)
- Speed ( Easy to understand more pitch may increase speed. But keep in mind more pitch can also slow you down. This is harder to visualize)
- Thrust ( Greater Pitch ratios may decrease thrust. Thrust is needed for maximum acceleration/getting out of the hole)
Ryan