Lifting Or Non-Lifting Prop - How Can You Tell?

[John Finch]

Hi John,

If you look at a prop shaft with a prop on it (left hand rotation) directly from the back like looking down a rifle barrel - positive rake is when the blade is tilted to the left and negative rake is when the blade is tilted to the right. You end up with negative rake when you thin a blade to much and it opens up when you run it on a boat and that causes the tail end of the boat to lift or hop out of the water. Very hard to repair - most times I threw them away. I like to use smaller rake by cupping the blade from the tip down toward the nose or lead rather than the whole blade. My reasoning is due to the high RPM's we operate in compared to other applications.

Thanks, John

So when you make the modification the blade cuts into the water smoother rather than slapping it? Thus less lift.
John

 

[Terry Keeley]

Might havta re-tweek my prop jig...

 

[John Finch]

Might havta re-tweek my prop jig...

Terry,you come up with some of the nicest tools and hardware.

John

 

[Terry Keeley]

Might havta re-tweek my prop jig...

Terry,you come up with some of the nicest tools and hardware.

John

Thanks brother John, you're E Hydro record is still there tho!

 

[Andy Brown]

I remember Tom Prezentka years ago was asking us how his 21 series props were working on our SAW boats, he was interested because this profile has some rake angle (as well as nice progression). Not sure if any of his other props are "raked".

Funny how that works...

All Octura props have rake From negetive 5 degrees (competition series 2.0's , 2.4's, ...ect.) to positive 25 degrees. X series props have 10 degrees of rake, but are still low lift props. The reason is that the leading edge pitch is very low compared to the diameter of the prop. Remember that low pitch/diameter ratios also create low lift, so even if the rake is low which normally causes more lift, the lift can still be low if the P/D is low enough. In the case of an X470, the L.E pitch is 2.9" and the dia. is 2.76" so the P/D ratio is 1.050 which is much lower than the Octura claim of 1.4.

Lohring described the proper method to determine the rake of a prop. A straight edge must be placed against the face of the blade from hub to tip and rotated until it contacts the blade surface from hub to tip. There is no other way to determine the rake of a prop. Even I can not guess what it is by just looking at it with out using a straight edge. Back cutting more angle in the trailing edge does not increase rake. Although it can reduce the lift of a prop with high trailing edge pitch.

Cupping the tip increases rake at the tip. Cupping the leading edge reduces L.E. pitch. Both of these modifications serve to reduce lift.

Some props have variable rake. Octura V900 series, copied from a Mercury racing prop is a prop with variable rake. It has about 40 degrees of rake at the L.E. near the hub and only about 10 degrees at the T.E.

Terry don't Re-Tweek your prop jig!...Just go out and set a new E hydro record! We've been waiting!

 

[John Finch]

KP gave Terry the templates for the boat that set the record. Terry's boat is fast too, but the stars have to be aligned just right, perfect air and water etc. You know the drill. Thanks for the prop explanation.

John

 

[John Beardslee]

Andy,

Great explanations. Do you think Tom Prezinka (Octura) used any contacts with OMC or Mercury when he started making props? With both being outboard manufactures I would think one of their design criteria had to deal with having the full weight of the engine on top of the prop and neg. rake is a way to add lift. The competition series Octura props 2.0, 2.2 etc. were the only props available when I started in the hobby and I couldn't get my 40 hydro to run any of them so I started first by cutting them into "Elephant Ear" style props and finally got some to run good on my boat. Soon after many of the Detroit Club (Wolverine Model Boat Club) started asking me to do props for them. Ron Walker had the first IMPBA boat over 60 mph in 1972 at the internats in Flint with his E-Hydro rigger. Not sure about NAMBA we didn't speak to those folks back then. The speeds seemed to progress fairly quickly with the calls coming that I'd done the prop or the first boat over 70, 80, etc. I recall a call from Ron Walker after he'd moved to Chicago that he'd run 109mph in a one way pass with the "Black Russian" geared twin. By then he was running a pitched and modified 1667 but on a 3/16" shaft. The Chicago pond was short and wouldn't allow a back up run the other way - so no record.

I was surprised a few years ago how many people thought "back cutting" the trailing edge was the same and changing the rake.

Thanks, John

 

[lohring]

I see what you are saying now. Nice equipment you have there.

John

Thanks. Another point. The common practice of cupping the edges of the blade is a way of adding rake. From Props Part 3:

The final common modification is curling the outside edge of the prop along the lines shown in the drawing. This is done to decrease the tendency of water to flow radially off the prop. Rake does much the same thing, and edge cupping can be thought of as adding rake to the outside edge of the blade.

Edge Cup.bmp

Lohring Miller

 

[Andy Brown]

Andy,

Great explanations. Do you think Tom Prezinka (Octura) used any contacts with OMC or Mercury when he started making props? With both being outboard manufactures I would think one of their design criteria had to deal with having the full weight of the engine on top of the prop and neg. rake is a way to add lift. The competition series Octura props 2.0, 2.2 etc. were the only props available when I started in the hobby and I couldn't get my 40 hydro to run any of them so I started first by cutting them into "Elephant Ear" style props and finally got some to run good on my boat. Soon after many of the Detroit Club (Wolverine Model Boat Club) started asking me to do props for them. Ron Walker had the first IMPBA boat over 60 mph in 1972 at the internats in Flint with his E-Hydro rigger. Not sure about NAMBA we didn't speak to those folks back then. The speeds seemed to progress fairly quickly with the calls coming that I'd done the prop or the first boat over 70, 80, etc. I recall a call from Ron Walker after he'd moved to Chicago that he'd run 109mph in a one way pass with the "Black Russian" geared twin. By then he was running a pitched and modified 1667 but on a 3/16" shaft. The Chicago pond was short and wouldn't allow a back up run the other way - so no record.

I was surprised a few years ago how many people thought "back cutting" the trailing edge was the same and changing the rake.

Thanks, John

John,

Yes, many people STILL THINK back cutting can change the rake.

Yes,on the transom weight. Also power to weight is important. Our nitro, electric and new gas models have a high P/W compared to full sized gas burners. It is easy for us to blow the prop right out of the water. The new electric systems even more so.

You made great props John! The first and only set we got from you went on our Twin Mongoose in 1987 and the boat won US-1 at Huntsville.

Tom P. told me that he talked directly to Mercury about using the V900 design. He did not say, but I believe that the 1400, 1600 were copies of old outboard props also. The X series props are the only props that do not look like some version of a full sized props. It has the cupping along the L.E. as you and Lohring mentioned. I have never seen a full size prop cupped that way, so I want to think that Tom P. got the X prop from an r/c boater. It used to drive Tom crazy when I talked to him about cupping props. He always said, "why not just use them stock?

The H667 is a scaled up copy of the ABC H-32. I know, because Tom called me one day and asked me to send him an H-32, stating that he wanted to make a 67mm version of it.

For years I've heard tales of r/c boaters running 95, 100, 109 mph "back in the day" (late 70's Early 80's). What happened to them? When the old records were retired for the new electronic system it took 10 years before the records got back to 90 mph (1993) and 5 more years (1998) before someone went 100. But only 3 more years to go 125mph (2001). Some of those fast guys from the 70's and early 80's are still running boats today. Maybe it's like John Finch said! "the stars have to be aligned just right, perfect air and water etc."

Back in 1977 I watched our much missed friend Tom Pretzfeld making consistent 75 mph passes with his Twin OPS. 60 Gator in Miami. The fuel was Randy's 40%. The props were stock 1667's. Suddenly he made a 2.50 second (90 mph) pass on the bomb sights. It became big news across the r/c boating world. Tom was a great r/c boater with great running equipment. Over the following 30 years Tom built Twin .67's , 80's, 90's and ran 60% nitro. I never saw or heard of Tom making another 90 mph pass, not even an instantaneous 90 mph pass on a radar gun.

Ron Walker called me back in the late 80's when I had started setting new records in the 85 mph range. He said he could bring his Geared Twin out and easily run over 100 mph, but that he would not, because, his words, "I do not want to bury the record forever". I kind of wish he would have showed us slow guys some fast passes. Oh well! His "buried" record would have gotten dug up nearly two decades ago!

I'm not trying to put anyone down here. I just think that there were a lot of bomb sight timing errors that were telegraphed throughout the r/c boating community out of shear hope that maybe it was true. But it irritates me a bit when these old speeds are quoted as if to say the current record holders are not that far advanced over 35 years.

Today we have great timing equipment and it is satisfying to know that the great speeds that people are running today are true and accurate .

As I mentioned above John, you made some great props back in Early 80's. We took the first Twin Mongoose with Picco 67's to the 1987 Internats at Huntsville. Our only set of props were just a little too big for the Huntsville altitude. We got a set of props from you at Huntsville and won the 1/3 mile oval event and US-1 in F-hydro with those props. I think they were a set of cupped 1470's.

 

[John Beardslee]

Thanks Andy,

Hi Andy,

Yes I agree the old bomber site equipment was inconsistent - mainly because a human was on each end of it. I also agree that there is always an element of luck as far as weather and water conditions are concerned but the fast guys are always fast. They work at it to be consistent and usually refine their equipment on there own. The current group of record chasers across the board from nitro, gas to electric is very impressive. It would be nice to have a time machine to go back and snatch one of those old boats and just see if it was really that fast or the stars aligning just right. Some of the guys have tried to get a hold of Ron Walker, the last I heard he was flying r/c planes in the Chicago area, some of the guys I fly with in FL in the winter are from that area and they think he passed away. Huntsville was really a challenge the first time, hot, muggy weather, learned to prop down and needle in like never before. One of the things we seem to do much of the time with prop work is to compensate for hull design problems. I can remember a well known racer that added rake to a prop by cupping the edge from the tip down toward the nose. When he tested it ran great but the boat would now blow off the water at high speed. I think the positive rake helps to reduce the thrust cone and make for more efficient axial flow. I'm sure that's why it was so successfully on the impellers in the jet boat pumps and jet skis.

Thanks, John

 

[John Finch]

The easiest way for me to understand rake are these drawings I got off the internet.......rake is the fore and aft slant of a blade with respect to the hub. The whole blade is rocked back on the hub looking at the prop from a side view to get more rake. See attached diagrams.

When I got home today I pulled two ABC props out of my supply and put them on a shaft so folks could see the difference on props we use. The prop on the left is a 2716- 15 degree rake. The prop on the right is a 20 degree rake 2716.

20150501113237266.pdf

20150501131812750.pdf

 

[Chris Wood]

Could john Andy someone explain how to put pitch in the l.e?i have wondered and attempted several times and failed. Also I guess for te question. How or what do you look for when doing l.e. Pitch vs say trailing edge pitch ?

Thanks

Chris

 

[lohring]

The way we pitched up props for SAW runs was to cut the blade at the hub from the leading edge almost through to the trailing edge. We then added pitch to the whole blade (especially the leading edge) and rebrazed the cut. This was on a 2170 prop with "normal" cupping.

After years of attacking the gas records, it became obvious that all the conventional ways of bending the blade were only adding pitch to the outer part. The slip kept getting worse, probably because the inner section of the blade was running at a negative angle of attack. Gas engines are lower rpm than nitro. Modified Zenoahs had little power after 19,000 rpm and Quickdraws died before 25,000 rpm. That limit set the maximum speed with modified 2170 props at around 100 mph for modified Zenoahs and 110 for Quickdraws. Increasing the pitch let the Zenoahs get up to around 106 mph.

In the beginning we thought power was the issue and worked hard on peak power. In the end we realized that we were running way past the power peak and that the prop was the problem. Gearing up the prop is another approach that we never investigated fully in models. Others are trying that approach. We used it in our full size outboard hydro where the motor was limited to around 5,000 rpm, but the available props for IC engines were designed for nearly twice that. Standard quick change gears allowed cut and try matching.

Lohring Miller

 

[Andy Brown]

The easiest way for me to understand rake are these drawings I got off the internet.......rake is the fore and aft slant of a blade with respect to the hub. The whole blade is rocked back on the hub looking at the prop from a side view to get more rake. See attached diagrams.

When I got home today I pulled two ABC props out of my supply and put them on a shaft so folks could see the difference on props we use. The prop on the left is a 2716- 15 degree rake. The prop on the right is a 20 degree rake 2716.

John,

On those two example the trailing edge and leading edge is cut on the same angle as the rake angle. That is the case for most ABC props and also Propshops. However, some props have the trailing edge cut at a much different angle than the rake. Octura 2200's are one example and Octura 1400 series another. They both have much more rake than what they visually appear to have.

 

[Chris Wood]

So if a person wants to bend up on the leading edge I guess my question is where on the blade? Tip middle edge? And more importantly what are you looking for in regards to lead edge pitch

Chris

 

[Dan Pralle]

Now now Chris your whip will be fast enough with a square prop

 

[Chris Wood]

Lol fast is the first loser[emoji41]

 

[Andy Brown]

So if a person wants to bend up on the leading edge I guess my question is where on the blade? Tip middle edge? And more importantly what are you looking for in regards to lead edge pitch

Chris

Chris,

Increasing L.E. pitch is very tricky and can only be done a very small amount on some props. The problem is that the overall progression gets compromised.

Best to work the full L.E.

 

[Terry Keeley]

Terry don't Re-Tweek your prop jig!...Just go out and set a new E hydro record! We've been waiting!

lol! Just kidding, would be pretty complicated to angle the hub (or each blade) to adjust the rake.

The props I'm working with from my jig are showing some promise, but as you know you usually take two steps back to go one step forward. The extra pitch makes more lift, I doubt adding more rake would fully compensate for it. I've had to make changes to the boat but that's effected other areas, and so it goes...

KP gave Terry the templates for the boat that set the record. Terry's boat is fast too, but the stars have to be aligned just right, perfect air and water etc. You know the drill. Thanks for the prop explanation.

John

Yup, you and KP have been very open about what you did to go fast, I have no secrets either as you know. More fun for anyone that is willing to put in the effort to chase records with us.

The new boat has a 36" tub just like your record boat and drives like a Cadillac, it's a bit over build tho and a pound heavier at 6 3/4 than the one before. Might build a light weight version next winter.

 

[shoboat]

Terry,

I doubt very much any extra pitch is causing any lifting problems.

Read your PM from me and try that suggestion.

A 2818 with 20 degrees of rake angle reduced to 64 mm in diameter.

I think you will see what more rake angle will do for your set up?

Thanks,

Mark Sholund