Prop Blade Rake

[GrandpaRacer]

Is this a high rake prop? Probably not. I'll show you a cheap trick to figure it out in the next post.

[GrandpaRacer]

Fill a deep bowl with dirty water, I like to use old coffee. Put the prop in it like this so the prop shaft is centered at the water surface. Now look at the wet line on the blade. The angle between the wet line and the prop shaft will tell you whether the blade is square with the shaft or at a high rake angle. This one is pretty low, about 82 degrees.

[GrandpaRacer]

Cleavers are pretty easy to tell but round ears are much more difficult but this trick works equally well with round ears.

[ZUL8TR]

Cleaver way to see rake

[GrandpaRacer]

Here's a round ear.

[GrandpaRacer]

Rocket... I don't know. I suppose you may not get a straight wet line unless you move the prop shaft deeper or shallower.

[Smitty]

What are you flying (in the photo)? Ercoupe?

[Smitty]

Whoa, an RV-7 is a long jump up from an Ercoupe! Did you build it? Do you have a build thread on it somewhere?

[GrandpaRacer]

Yes, I built it from 2001 to 2004. The photo of me was taken after my first flight in 2004, hence the big smile. They are fantastic airplanes with a 200mph cruise speed.
John Adams

[Ram4x4]

Love the Vans aircraft. I've always wanted to build my own, but anything aviation related is EXPENSIVE...way out of my meager budget. My alternative is ultralights (although I have time in a lot of things). I ended up buying a powered paraglider, affordable, very compact (no hangar needed for storage), transportable and just incredibly fun to fly...downside, it doesn't get you from A to B very fast.

[GrandpaRacer]

Here's a J prop. BTW, you don't need coffee but it helps when photographing.

[Tunnelman801]

GrandpaRacer....Tell us how rake efects the handling and the preformace of a boat......William

[Stu Ford]

Another way my father looked at rake was to set the blade like photo 2 and see how high the back of the hub was from the table. On the table = low/no rake, 1/2 - 3/4" off the table mid to high rake.

[GrandpaRacer]

Tunnelman, you have to pretty much determine that by testing but generally high rake props will lift the bow more. These shown here would be what I consider low rake. I think high rake makes the back of the boat more squirrelly too but what works on one boat may not work on another.
John Adams

[Ram4x4]

Here's some info on blade rake as per Mercury Racing's website:

"A greater rake angle generally improves the ability of the propeller to operate in a ventilating situation. Ventilation occurs when blades break and re-enter the water’s surface."

"On lighter, faster boats with a high prop shaft, increased rake often will improve performance by holding the bow higher. This results in higher speeds due to less hydrodynamic hull drag. However, on some very light boats, more rake can cause too much bow lift. That will often make a boat less stable."

[ZUL8TR]

Rake

Imagine a plane perpendicular to the propshaft axis, with a pitch the same as at the blade's mid-section, e.g. 'Rake' is the angle of the blade relative to this plane. A zero-rake surfacing prop, typical on the old inboard hydros, tends to climb toward the surface, lifting the transom. This is how hydros 'prop-ride'. A blade with positive rake tends experiences a downward force that makes the gearcase want to go deeper. This effectively increases the trim angle and thereby increases the water pressure at the transom dynamically. With increased positive trim the boat runs with less surface in the water and the bow rides higher. In common but wrong parlance, the positive rake generates 'bow lift'. But as Louis J. Baumann says, and I agree, there's no such thing as 'bow lift', there is only a downward force on the prop blade.

Ref: http://www.mccauleyandson.com/html/props.html#improve

More rake focuses the thrust water backward in a tighter cone compared to less rake. Imagine an adjustable hose nozzle from a fan (less rake ) toward a jet (more rake).

Reading if more inclined for performance tech details on props:

http://www.mccauleyandson.com/docume...d_on_water.pdf

But as usual accurate testing is the only way to find out what works

[GrandpaRacer]

Zul8tr, good comments. I did not know about the focusing effect! Such an effect should provide more thrust. Now, consider what happens when a CSH runs kicked in with a high r.ake prop. That focused thrust line would tend to be more parallel with the surface of the water and contribute more to go fast thrust... And less lift on the bow as a result. On a Yamato you can not go very far with this because that exhaust snout is there and the prop may hit it ( only for a short time).
John Adams
P.S. Discussions like this that help racers understand are good for our Sport!

[ZUL8TR]

John that is exactly right as rake increases tuck become more effective since the submerged blade is more 90 degrees to the water thus water push is more parallel to the water, but there is a limit. Ex. as rake is increased and say the diameter remains the same then the blades are longer and there is more blade area to thrust an equal amount of water backward therefore more prop drag. So rake requires a balance with diameter, pitch, blade area, thickness, airfoil shape, etc etc,........ to achieve a certain performance. Not an easy task, part science, part experience, part guess, part ,,,,,,?

Accurate consistent testing always tells the tale.

[ZUL8TR]

Here is one (of many) who believes the marine prop operates like a rotating airfoil. There isn't much on research literature on partially submerged props ( If interested I have some from the Society of Naval Architects and Marine Engineers) but the principals still apply, fluid change in momentum thru the prop and circulation yield the thrust.

http://web.mit.edu/13.012/www/handou...rs_reading.pdf

[Ram4x4]

Good stuff, so sure if you others, put the links in!

I have no doubt an airfoil shape works to increase thrust, it works on air, which is just a less viscous fluid. However, everything I'm reading on racing props says "make it as thin as possible". I've looked at a lot of props this year and not a one had any real airfoil shape on the blades, they were thin though!. So what is the advantage to eschew a true, good airfoil cross section in favor of thin? Without any actual math or study data on high speed props in a ventilating condition, I can only guess that the thin blades present less drag and the trade off is greater than any induced thrust created by a thicker, airfoil shape with more drag.

I'm sure since our racing has been around so long, surely someone has tried thicker blades with more pronounced airfoil shapes. The fact it isn't talked about now as one of the "prop" tricks is telling.

Still, the science of ventilating prop performance is an interesting subject.

[ZUL8TR]

You are correct thin blade sections are low drag and that is what race props are using and SS and other metals are used for strength to get these thin sections. If you cut a race surface piercing round ear prop at a given radius the blade section has variable thickness. The leading edge is sharp to cut the water air mix, the center of the blade along the cut arc is thicker for strength, the trailing edge is thin with cup added. Also the blade section is curved with variable increasing pitch from lead edge to trail edge. It is a thin airfoil with different pressures on the front and rear face that produces lift (and lift induced drag) and the change in momentum thru the prop converts engine torque to prop thrust. The air water mix the surface piercing prop runs thru is compressive to a degree because of air bubbles and the progressive pitch takes advantage of this by increasing the pitch from lead to trail edge and compressing the air water mix as the water is accelerated along the blade arc to the trail edge.

Here is a pic of a blade section of a prop with progressive pitch compared to a straight pitch prop.

http://www.boattest.com/images-galle...rtech721_9.jpg

Cleaver surface piercing props have a similar progressive pitch cross section except the trail edge is blunt and that provides increased strength and performance increase on certain applications. Both designs have blade rake.

Here is an article from Mercury Marine on cleaver prop development. There is a video at the end on the cleaver development.

http://mercuryracing.com/virtual-tou...er-revolution/

Cleaver surface piercing props have a similar progressive pitch cross section except the trail edge is blunt and that provides increased strength and performance increase on certain applications. Both designs have blade rake.

Here is a link on slip and pressures/forces on the prop blade

http://mercuryracing.com/prop-school-part-6-slip/

Here is a link on discussion of race props by Brinkman and Dewald

https://www.bing.com/videos/search?q...18&FORM=VRDGAR

[Ram4x4]

On the other end of the spectrum we have the cleaver prop with it's wedge shaped cross section and flat plane at the rear of the blade. Cleavers do the opposite of high rakers, they lift the stern. I can imagine a hydro that already has a light front end that likes to fly slapping on a cleaver. Sounds like a recipe for a nearly totally flying boat...and probably scary handling to boot.

[Ram4x4]

Ha! Look what I found!

https://arxiv.org/ftp/arxiv/papers/1602/1602.00900.pdf

[ZUL8TR]

I posted that paper link previously under my 1st post on Rake. It is an interesting paper. I have had email discussions with the author on various specifics. His paper essentially is a method for determining predicted speed with similar propellers based on affinity formulas.

[Smitty]

Those with one or other position on whether a raceboat propeller gets part of its effect by means similar to that of an aircraft propeller should understand (if you don't already) that some of the old ideas about how an airfoil actually gets its lift (Bernoulli effect, etc.) have more recently become a subject of controversy. (I just read stuff of this kind, and don't know nearly enough to take a position on it) (not publicly, anyway).

[Ram4x4]

Yes, the old idea is that due to the airfoil shape, air has to travel faster over the top surface in order to meet up with the air under the bottom at the same time. This is known as the "equal transit theory" and has been proven incorrect.

In a foil where the upper surface is more curved than the bottom, what actually happens is the air flows faster than it needs to meet up with the air underneath and the effect is a "turning" of the airflow. Air does not just come together at the trailing edge and continue in the same plane of attack as it originally met the leading edge, instead it turns downward so that the air flow after leaving the trailing edge is focused downward. It is this turning that determines how much lift is generated.

Using Bernoulli's equations always fail to predict the actual lift generated as it is almost always more, due to the turning of the air and the higher speed over the upper surface than the equations say it would be.

There are actually some modern airfoil shapes where the bottom is longer than the top, yet it still creates lift.

[ZUL8TR]

True the equal transit theory was disproved back many decades ago with equal time pulse snap smoke bursts in wind tunnel experiments. The simple Bernoulli equation for energy conversion (of just pressure and velocity conversion) does not give accurate lift/drag results because it does not include non reversible terms like surface friction effects and turbulence that contain difficult to estimate parameters. When these terms are added the equation is beyond what Bernoulli imagined and is the 1st law of Thermodynamics. The lift/drag mechanism of an object in a moving fluid is related to pressure differences, flow circulation and change in momentum and can be explained by either the 1st law of thermo or change the momentum from Newton's 2nd law. A fabric sail on a sailboat is one example of a very thin cambered object in a flow field that when trimmed right provides lift to move the boat forward. Our props have a thin cambered cross section shape like the sail but the operation is definitely much more complected to predict.

As always testing tells the tale.

[Smitty]

Weelll, now, the idea that boat sails (fore-and-aft rig, close-hauled) get some significant portion of their thrust from the same Bernouli-style action as airplane wings were thought to have, sixty years ago, when I was learning such stuff, was yet another controversial proposition even then, with lots of doubters saying no, a sail in that configuration pushes the boat entirely as a function of a parallelogram of forces, not at all as an airfoil.

As to (ordinary low subsonic) airplane wings, when I read about the old Bernoulli explanation being discredited, one of the things that made it sound credible to me was the demonstrated effectiveness of producing more laminar flow and reducing the boundary layer (over the top surface). It just seemed to me like that worked better with the downward-directed flow explanation than the up-suction explanation . . . .

I don't see Wartinger posting here much, but when he and I were going to the UW in the mid-Sixties (along with Rick Sandstrom)(and all of us racing BSH), Bob was spending hours in the wind tunnel on the way to an aerodynamics engineering degree. Bob, if you see this, tell us about it.

[SeaBat]

Grandpa Racer do I see a rake gage in the near future?

[GrandpaRacer]

I was thinking about a simple way to make a gauge when I came up with the water idea. It is so simple to do that I dropped any further thoughts on a gauge.
John Adams