Thread:

Prop Design Variables

I was talking with my Dad the other night about props, and found we had some questions about some of the basic variables involved. Your basic prop has two main variables- diameter and pitch. When calculating your theoretical max speed, you only need to know pitch- diameter doesn't come in to it at all. My understanding from reading the boards is that a bigger diameter prop is more efficient- it will slip less- but takes more HP to turn at a given speed. A bigger prop will also generate more lift (so I've read).

Obviously, you can play with these variables to tune the boat. A smaller prop might have more slip but allow you to spin faster and get in a better part of your power curve, and thus go faster. There are also lots of smaller variables involved, such as material/thickness of the blades (and how much the prop bends under power) and cupped blades (more effective pitch) and cleaver props (generates lift). Last, there are things like burning the prop on launch and other dynamic effects.

Mainly I'm interested in the basics of the main two variables- diameter and pitch. It seems the new thinking is towards smaller diameter props- a lot of recommendations for 11" and even smaller props, even with the same diameter. Can anybody give us the real scoop on this?

Well you have a very good handle on things to start with. Your boat and your combination are going to be a huge part of the variable. Lets look at just one aspect of diameter. I run a small 10 3/4" prop on my 9 second runner bottom. My overall combination is fairly light and it likes it. Now put that prop on an 800# plus ex fuel hull with an iron block and it's probably going to dig a hole in the water. Diameter has to do more with your hole shot, and how much ear you need to hook up. To much ear, and your going to hook up to much, maybe bog the motor down to an rpm thats less then desirable. Getting the ideal prop for your boat is alot like finding Cinderella's glass slipper. It can be done, but you may need to try a few to find it. Lunch breaks over......

Originally Posted by Factory1

Well you have a very good handle on things to start with. Your boat and your combination are going to be a huge part of the variable. Lets look at just one aspect of diameter. I run a small 10 3/4" prop on my 9 second runner bottom. My overall combination is fairly light and it likes it. Now put that prop on an 800# plus ex fuel hull with an iron block and it's probably going to dig a hole in the water. Diameter has to do more with your hole shot, and how much ear you need to hook up. To much ear, and your going to hook up to much, maybe bog the motor down to an rpm thats less then desirable. Getting the ideal prop for your boat is alot like finding Cinderella's glass slipper. It can be done, but you may need to try a few to find it. Lunch breaks over......

Cool, at least I have the basics down. So my next question is, are the newer props more efficient at smaller diameters? Again, the trend seems to be towards smaller props. Even in the thread on cruiser props, 11"-11.5" seemed to be the goldilocks range.

Also, what about prop lift, or, if I'm using the wrong term, the tendency of a prop to "climb" in the water? A cleaver prop I know is a "surfacing" prop, in that it wants to run on the surface of the water, and will rise in the water even parallel to the direction of travel. Good for hydros (and whackers, it seems), bad for anything that wants to keep the back in the water. Do larger diameter props inherently have more lift?

Thanks for the info!

Originally Posted by VDRIVERACING

A cleaver is designed to be part way out of the water, which isn't going to work on a vdrive hull.

Small diameter racing props and extreme rakes are for racing applications, and those guys stay up all night thinking of the smallest increments and their effects. You have bigger fish to fry and your engine and hull requirements really do not need the type of scrutiny that racers do. There's an excellent chance that a good off the shelf steel 11.5 X 15 from Kindsvater, et al., will have all the rake(lift) you require and along with your carbs dialed in will really wake your boat up.

Get that boat anchor off your prop shaft and try a 11.5 X 15 and we can then dial in the gears. I guarantee you your slippage will be 20-25% once the engine can spin.

Thanks for the info. I wasn't actually trying to carry over from the other thread, we were just thinking about props in general and I wanted to know more (not necessarily about my prop)! I just want to have an understanding of the subject in general.

Prop design and theories is a huge subject that could fill a book. A few paragraphs isn't going to do much. And I'm clearly not the one to be writing it. I have some basic knowledge, and thats it. Where's Palhal when you need him?!! My point I was trying to make was simply one prop can act differently on two seemingly similar boats. There is no one prop fits all wonder prop on the market. I don't think you can draw any absolutes on props either. I think it would be wrong to say all newer props are more efficient, and therefore should have smaller diameters. Here's an example. My prop came to me as an 11"x16. It didn't burn enough at the hit, so we cut it down to 10 3/4". Now it burns killer. I bought another prop to try a while back, different brand, and I bought another 11"x16. Why not a 10 3/4" ? Because I know it's going to act differently and it's easy to trim it down, not so easy to put it back. Always start a little big if you have the choice. Kinda like buying a new suit. Find one off the rack that fits well, but needs to be taken in a little here and there. A good prop man can get you close off the shelf, and tailor it to perfection after youv'e tried it and have some feedback for him. This could be a good thread, hope more people chime in with constructive input~

I have a 20ft Brendella Tunnel V-Drive. The bare hull weighs just over 450lbs. When I bought the boat 21yrs ago it had an elephant ear with rake that blew the tail. WRONG PROP !!! You need a Flat Bottom prop that doesn't have rear lift. I do agree with the information earlier in this post. A lighter boat doesn't need as much diameter as a heavier boat. Back in the day I talked to Tom Black he got me up to speed about props. He was familiar with the hull. He needed to know power, wieght, motor RPM range, NOS or Blown and what I was using the boat for Lake or Race. I tried 3 - 11 3/8x 15 props. All were way different from each other. I bought the one that seem to work best. The diameter was big and moved the boat around and not enough slip. This prop ended up at 10 1/2x15. I ran this prop with 400 shot of NOS and Blown Alcohol and it worked great. I just gear it to what power I am using. That's one of my prop stories. Good Luck RACERICK Tom Black-RIP

So, here's something I've wanted to talk about for some time, but the last good prop thread died out before I got this far. Now this pertains to flat props, however the theory could work with a prop for any type of hull I suppose. From what I can tell, I see two main schools of building flat props. There's a larger thinner eared variety made by Menkens and Grose, and a smaller thicker eared variety that was made by Tom Black/Precision then later by Braaksma. Not long ago I saw a new prop from Harold that had it's own distintive shape, but if I had to decide, I would say it fell into the smaller thicker catagory. Now I've been running an old Precision for years thats been serving me very well. Trying to run the 9's, it's been providing me all the et I need and more. But it has a very distinct performance to it. It leaves the line hard, goes like a bat out of hell to half track, than hangs it's tongue out. At that point I'm on cruise control, I check out the ladies on the shore, do some texting and what not. I know the boat wants more gear, and that might help that, but the last thing I have needed was more et. I do attribute it characteristics to it's design though. If I had to draw a comparison, I would say the small thick eared prop would be like a baseball bat hitting line drives, but falling short of the fence. While the thinner larger eared prop would be more like a golf club, thinner, flexing more, releasing more stored energy. It seems like it would flex noticably more at the hit, making it act like a variable pitch prop. Flattening out a bit, loosing some pitch, getting the boat going quickly, than as boat speed increases, it regains all of it's pitch allowing it to march down the track better. I 've heard people say the Menkens pulls all the way down the track, gaining mph the whole way. Of course every aspect of the boat will influence that as well, however seems to me the two schools would have two different personalitys. Anybody wanna take a swing at it? Hmmmm...

Paging palhal .....

nobody better to explain this than that man IMO

Originally Posted by Moneypit

The "pitch" of the prop is a measured average. Starting at the base of the "ear".... As the measurement moves outward, the pitch increases. So the "15" in an 11x15 could start off as a 13, and could end as a 17.. If it was originally an 11 1/2 diameter, cutting off the last 1/2" could reduce the pitch average as well as the diameter... How the prop ear is shaped, where the 13 starts to increase, and where the 17 is "all in" can drastically affect the "bite". As mentioned, there is no ""one size fits all" prop. Different brands have different shapes, different boats like different rakes/cups, different HP to weight ratios require different designs.... All this was laid out to me in 2004 by Clyde, Duff Daily's "wheelman"...... I may have distorted his actual words, but I think you get the picture...Two identical boats with identical power may not like the same prop. I have also heard over the years that no two props are exactly the same, even when they come of a precision machine.,.....That may have changed with the advent of all the CNC stuff out there today....
Ray

Ray, Does the "15" refer to 15 revolutions per foot in distance?

Good stuff Ray and Vince. As usual I have alot to say (whats new huh?) But not enough time to get started in. Will add some tonight. I know there's some sharp guys out there not stepping up to the keyboard. C'mon lets have good discussion!