Octura props: "lifting" vs "pushing": X & other types

What do the various designations for any given octura prop really mean? Which are "Lifting" or "Pushing" types? And which type is preferred for Hydros. Which for cats? And how can I tell from Octura charts what diameter the shaft hole is? JIM

Take a look at this thread: http://www.jrcbd.com/showthread.php?...-lift-question

Lohring Miller

Lohring getting into Jims is tougher than getting into the White House. Any chance of copying the info?

this explains it better! Scroll to the bottom.

http://www.fastelectrics.com/octuraprops.htm#The Octura Propeller Definition Table

That's a good explanation of Octura props. This is an explanation of what prop mods do from a post on Jim's:

Brian, that severe curl on the trailing edge ( arrow on pic #1 ), is the trailing edge cupping that's on just about every surfacing prop. Its purpose is to provide better thrust in a water/air mixture. It contributes to the average pitch of the blade, but doesn't have an effect on lift.

A semi circular cut in that area right next to the hub, is a Barr cut. It can be very small to quite large. When the hull is getting on plane, or milling, much more of the blades are in the water. By reducing the blade area next to the hub, the eng. "sees" less load, and can accellerate quicker. Once at full song, the last two thirds of the blade do all the work, and the Barr cut is out of the picture. No real lift issue there.

A prop that throws water off its blades at more of a 45 deg. angle ( star tips / flattened blades ) is usually a lifting prop because that water discharge is also being pushed down at a severe angle as well ( pushing the transom up ).

There are three good mods that can alter the amount a prop will lift.

You can change the rake of the trailing edge by radiusing a 1/8" cut at the hub ( just like a Barr cut ) but tapering the cut up to 0" at the top/tip of the blade. Because you are removing some, or most of the trailing edge cupping, you will lose some pitch in the process. This Back Cut doesn't allow so much water to climb up the blade and spill off the tips at a high angle. ( You are simulating the effect of a 20 deg. prop )

Rounding the tips ( a little or a lot ) allows a generous cup to be bent into the blade's end, where most of the thrust is produced. Again, this keeps the water moving back, instead of spilling off at an angle ( producing lift ). It will give back a little pitch that was lost from the Back Cut.

If you slightly curve the blade from perhaps 1/3 from the tongue, gradually more to the tip, you are Spooning the blade. Take a look at a kitchen spoon - THAT'S the idea. This works very well to blend into the cup of a rounded tip blade. And again, more water held on the blade and directed to the cupped tips to thrust rearward. A spooned blade can also offer a little less resistance to the blade entering the water.

When choosing a raw prop to mod, keep in mind, you may lose some pitch in the process. But then if done well, it will likely be more efficient, and handle better.

You probably know most of this Brian, but maybe it can help others.
Gary, Doug is right; The Barr cut is usually on the trailing edge, but as well, whatever floats your boat ...

Another mod that can help unload the prop and help the top end as well, is when the tongue is removed from the water entry of the blade. This is the part of the blade that has a lesser pitch and can help a heavier boat jump on plane quickly. But it can also act as a governor, holding the prop back on the top end. Removing it, involves forming a very smooth curve to the blade starting at the hub ( by the drive dog ) and fanning out to the trailing edge. PropShop ( UK ) props come this way from the factory.

What YOU have done is a leading edge Barr cut to an extreme. It's unusual, but not unheard of. The ABC G20 ( and some others ) has a slight leading edge Barr cut.

By making your cut so large, you have somewhat simulated tounge removal by cutting some of that lesser pitch off. But mostly, you have reduced blade area to help unload the prop on the bottom and mid range.

If it works for your setup, 'nough said!

Good stuff Lohring! Thanks!

Thanks for the info Lohring, I wonder who the source was. :wink: IME the famous "Barr" cut is not a very good idea on FE boats. First, it was developed for fuel boats with limited low-end torque - it helped to keep the engine on the pipe when the prop ran too deep. FE has plenty of torque so this advantage is moot. Second, I have had two different props with "Barr" cuts, each one done by a different well-known prop expert. Both props cracked at the bottom of the cut over time, rendering them useless. No more "Barr" cuts for me.


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lohling: Great info!!! You should be elevated to a SENIOR member. THX, JIM

Thanks, but I'm a very junior FE boater. I just met Stu Bar at this years NAMBA Nationals. He has some of the fastest riggers I've seen.

Removing the hub area cup, either with a Bar cut or by back cutting, unloads the prop. In electrics that reduces the current draw and might allow more cup at the tip. That lets the boat run faster and lowers the current spike when the prop drops into the water. The data logging from the ICE controllers or an Eagle Tree is very eye opening. I've been amazed at electric motor torque and how much current they actually pull since we started data logging a 1/4 scale model of a proposed full size electric boat in 2006.
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The old GPS had a slow update rate so the speed lagged real time. It was still informative. Data from the full size boat's 2008 record runs with an outboard hydro design is below.
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Lohring Miller