Prop walk thoughts.

[FourPoints]

Four, now that makes sense to me! Excellent explanation! Love the animation by the way! That explains why I do not have prop walk. My hull in that area is very flat! See picture attached. Thank goodness! Again excellent explanation! BTW did you make those animations??

NP, and yes that is my artwork. Instead of getting any work done today after lunch, I hand drew (wit the mouse) both of those animations instead.

 

[Bill Roosa]

Not buying it FourPoints. How does the water know to go sideways (to port in both diagrams)? Seems to me that the water would just continue forward along the hull on both sides of the boat. not at all sure what structure is causing a force on the water to make it turn AFTER leaving the prop. Assuming the hull is symmetrical and the prop shaft centered on it
Now if the prop was some how forcing the water to one side by being not in line with the CL of the boat that would make sense but that is just a misaligned shaft.
Also does not explain no prop walk in a sail drive or power boats with props that are not at an angle to the fluid flow.
Please be technical cause I'm still thinking it is the different angles of attack on the rising and falling blades causing more or less force on that blade.

 

[FourPoints]

Look at the prop wash off any outboard and it is clear as day. The water doesn't come out the back of the prop straight like out of a hose, it comes out with a corkscrew like rotation that does many revolutions. On an outboard this is clear to see since the exhaust exits through the prop hub and the bubbles make this twist visible, on inboard you don't see this, but the same thing is happening.

I tried to find some examples of this, it appears video taping prop wash from under water isn't a popular oats time, but I did find a couple.

First check out this one. You can see the exhaust trail twisting and corkscrewing with the surrounding water column.
http://www.youtube.com/watch?v=stI74T0sr3g

Here is a close up, you can clearly see tip cavitation, but that isn't the focus, look at the spinning and twisting inner exhaust column. You can see the way it is distorting that the column of water is twisting.
http://www.youtube.com/watch?v=QODJ8e3Nag4

Edit: Found a third clip. It's short but it shows both forward and reverse. Watch it close and you can see the corkscrew in the water both directions.
http://www.youtube.com/watch?v=1fIlTbYfyxo

 

[WayneH]

Oh, Great, Brian. Sucked me into the youtube time tunnel and now I've lost 3 hours.

Shipwrecks, scuba divers, prop wash, propeller inspection windows. Why in the world would you purposely put a window under your ship just to see the propeller go round? Yeah, I know, it was a ULCV and dry docks are few and far between for them.

 

[alexco38]

Life boat rudders

Two kinds of life boat rudders.

 

[Chris Patterson]

Well FINE.

I've now over thought this until my ears are on fire, and like previously said, watched you-tube videos on every prop scenario imaginable. Including some on Chinese midget mud wrestling. So...

I contend that the prop wash in retreat is deflecting off of the hull, in a weird comparison that of a paddle wheel boat it seems, in that when the prop starts off, while the boat is not making way, the tip vortex is going out, as apposed to back, (or to the front to be accurate). Makes sense. However..

I still have seen no evidence of why the hoop would not prevent this, as there is no way it can fling water UP the hull, only back,(front, whatever). And of course a hoop would present drag. The keel, hull, even the strut induces drag, and with your finger stuck down in the water, it will too. But is it significant? Or acceptable? With a top hull speed of around 7 knots, give or take, could it really make all that much difference? I understand why you wouldn't want it at 35, or 50 knots, but...

I sure would like to see a tank test of a prop shrouded. The cover on Alex's lifeboat pic looks simple enough. I'm sure that they are put there for protection of the others in the water that DIDN'T make it into the boat, but still. And the pic on the right has counter rotating props like most larger boats, so it's a moot point.

And oh yeah, that WAS a slick animated drawing, way cool.

And did you see what was said about us engineers in Phil's survey request post? The nerve. He's....well....right I guess, haha. It's more fun than Craigslist Rants and Raves. And relevant.

At least that's what I keep telling myself.

 

[FourPoints]

I believe a thrust ring around the prop would help. Not because it would reduce prop walk much, but because it does increase forward (or reverse) thrust, and would get the boat moving quicker, thus allowing the rudder to do its job.

As for drag, my research has turned up that they are usually only beneficial under about 7kts, any faster and they create more drag than they enhance thrust.

Coincidentally that speed is about perfect for a sailboat. I am replacing my prop for next season with a 4 blade high thrust, and plan on adding a thrust ring if that does not give me the improvements I am looking for.

 

[FourPoints]

Oh, Great, Brian. Sucked me into the youtube time tunnel and now I've lost 3 hours.

Don't blame me for that! I only got sucked in for 2 hours, and that was so all of you didn't. I spent those 2 hours watching boats crash and big ship propellers and crazy octopuses so none of you needed to.... :naughty:

Hmm, I wonder if the IRS will let me write that off as charity work?

 

[Old Skool Neil]

Indigo Electronics has developed an interesting propeller concept and marketed it specifically for Atomic 4 direct drives. The only thing that makes it specific to A-4's is diameter and pitch, the concept could be tried on virtually any boat/engine combination. It attempts to realize the advantage of the Kort Nozzle without the drag through the use of winglets at the prop tips.

http://www.atomic4.com/propeller.html

We've had a few discussions about it on the Moyer forum and by my faulty memory tally the reviews are mixed. Interesting idea though.

 

[Bill Roosa]

Well sure the bubbles come off the prop in a corkscrew!!!!
The water just comes off in a stream. another way to say that is there is no lateral or vertical component to the stream.
I note that the stream of bubbles forms a cylinder and does not show the water diverting either left, right, up or down.
I'm still listening though

 

[Bill Roosa]

Lets be clear. The water has bubbles in it from prop cavitation. Those have to be in a spiral because they are produced over time as the prop moves. The water stream just leaves the prop, perhaps with a rotational component centered around the prop axis. But the water is leaving the prop straight.
So, in reverse, the port side prop contribution goes down the port side and the starboard side prop contribution goes down the starboard side. These would effect the hull identically. Going forward they just leave.
Unless there is some hull-water interaction in reverse I can't see any way water that has left the prop effecting the boat.
Another way of saying this is once the water has been acted on by the prop and moved away there is no way for it to create a force on the boat. this is totally clear when going forward and I would submit unless you can demonstrate some latterly moving water on some under water part........
your explanation HAS to explain both forward AND reverse. So what does the water push on in forward that causes the boat to be forced sideways????

 

[philwsailz]

Let me put this out there. I do potentially understand this from a slightly different perspective.

Consider a helicopter:


When hovering, and assuming any wind is negligible or zero, the velocity of the air across all blades through 360-degrees of rotation are the same.

As the helicopter changes attitude and starts forward motion, there immediately comes a situation where a blade on the left of the aircraft has an airflow across the blade that is different than the airflow over a blade on the right side. As a result the side of the helicopter with the higher velocity blade, (the one swinging forward, as opposed to rearward) will have greater lift.

As speed over ground increases, the inequality will increase, and at some speed, the rearward blade has so little air flow over it that it stalls. I thik we can all visualize this as the lift is created by air flow across the blade, and where there is no airflow due to the blade moving in the same direction as the air at the same speed.


Now leaping, (a long way) to our props in the water...

Take a boat with forward motion and put a spinning prop on a totally vertical shaft. Spin the prop such that one side of the prop is "stiill" with respect to the water, like a paddle wheel. That side of the prop will not create any force in any vector because it is simply not moving appreciably with respect to the water, BUT the other half of the prop has a water flow at 2X hull speed, and therefore it is creating force. The boat will try to tilt, if even imperceptably, due to the lift created by the prop.

Now rotate this same prop so that the shaft is fully horizontal. The effect totally goes away.

Now tilt the prop shaft to a point in-between like is the case with most of our boats and you will see the force manifest itself as prop walk if I have made a proper leap of an analogy.


This is accounted for on higher-speed planing power boats, as there is usually a little trim fin mounted just aft of the prop to correct for this when the trim is tilted way high, as when the boats are planed and trimmed out for max speed.

We usually don't notice it in forward motion, and I bet it is due to the engine and shaft being installed with a very slight angle with respect to the boat's centerline, skewed port to starboard or vice-versa... Look around your marina and I bet you will find direct drive vessels with this slight left or right drive angle....

I hope I got it or am close, otherwise I have wasted 5 minutes of a few people's lives that they will never get back....

 

[FourPoints]

Well sure the bubbles come off the prop in a corkscrew!!!!
The water just comes off in a stream. another way to say that is there is no lateral or vertical component to the stream.
I note that the stream of bubbles forms a cylinder and does not show the water diverting either left, right, up or down.
I'm still listening though

Bill, re-read my previous post, I specifically said to look past the tip cavitation in the video and focus on the exhaust column in the middle. Look closely and you can clearly see that the inner column is getting twisted and spun. If the water was not spinning around that exhaust gas column, the bubbles would not stay in that cylindrical shape, they would immediately start to rise, but it is the spinning of the water that is maintaining that shape.

As further evidence I can watch stuff floating in the water like a submerged leaf get sucked into the prop, and watch it spin in the corkscrew of the prop wash.

So, in reverse, the port side prop contribution goes down the port side and the starboard side prop contribution goes down the starboard side. These would effect the hull identically. Going forward they just leave.

No, since the column of water coming off the prop is in fact spinning, the port side and starboard side of the propwash have different angles they hit the hull, see the diagram I drew for clarity.

Another way of saying this is once the water has been acted on by the prop and moved away there is no way for it to create a force on the boat. this is totally clear when going forward and I would submit unless you can demonstrate some latterly moving water on some under water part........

The water isn't imparting a force on the hull, it's the fact the water moves over the different sides of the hull at different speeds that causes the effect. Basic principle of lift;when a fluid (or air) is moving at high speed on one side, and at a slower speed on the other, the faster moving side will result in a low pressure area. This is known as Bernoulli's principle. It is this low pressure area that actually causes the surrounding water to suck the boat to that side, it is never being pushed by the propwash as that would be canceled out and achieve nothing (Newton's 2nd law explains that).

In short, "prop walk" is the phenomenon witnessed when the prop and hull design combine with the result of uneven water flow over the hull in reverse resulting in a low pressure area that results in the boat being pulled / pushed toward the lower pressure on the higher pressure side. It has nothing to do with the prop wash or tip vortices's pushing the boat anywhere.

 

[Bill Roosa]

Spinning water, I got it. assume a RH prop, are you trying to say that the water on the starboard side is moving at a different speed than the water on the port side????!!
If we assume a symmetrical flow (up, down, port an starboard) and the only component that is non-symmetrical is the rotation then what you are saying is the water on the starboard side is moving up with respect to the water on the port side. That might heal the boat but not move it sideways.
Alternately you could be saying that the water on the upper half of the prop is moving to port and the water on the lower half is moving to starboard. Kinda cancels out in my mind.
An all cases the hull is (ideal case) symmetrical around the fore-aft axis and any "lift" forces on one side would be the same as those on the other since the water is moving at the same speed on both sides.
How is the water coming off one side of the prop with greater speed than the other side.
Be careful how you answer as you are dangerously close to saying that the angles of attack are different resulting in a torque on the prop resulting in the water leaving with more or less velosity..........
See we are in agreement.
don't you just hate engineers?

 

[Bill Roosa]

try googling "p-factor"
or
http://answers.yahoo.com/question/index?qid=20070712055710AA0ogVR

 

[FourPoints]

Spinning water, I got it. assume a RH prop, are you trying to say that the water on the starboard side is moving at a different speed than the water on the port side????!!

the water isn't moving at a different speed, but relative to the hull it is. Look at the following diagram again. The water flows differently over each side of the hull, not at a different speed, but over different places.

The original drawing leaft out one important aspect, the dissapation of the moving water away from the hull vs against the hull. I have (crudly) updated the animation to better demonstrate this.

If we assume a symmetrical flow (up, down, port an starboard) and the only component that is non-symmetrical is the rotation then what you are saying is the water on the starboard side is moving up with respect to the water on the port side. That might heal the boat but not move it sideways.

getting warmer, almost there...

Alternately you could be saying that the water on the upper half of the prop is moving to port and the water on the lower half is moving to starboard. Kinda cancels out in my mind.

If you only look at it as a force diagram it does cancel out (Newton's 2nd law), but a force diagram doesn't include the low pressure area created by the asymetrical water flow over the hull (Bernoulli's principle)

An all cases the hull is (ideal case) symmetrical around the fore-aft axis and any "lift" forces on one side would be the same as those on the other since the water is moving at the same speed on both sides.
How is the water coming off one side of the prop with greater speed than the other side.

It depends on hull shape, but in the more extreme cases, one side of the hull will experience the prop wash flowing over the hull, while the other side of the hull the prop wash will flow down and away from the hull. The prop wash is moving the same speed on each side, just different directions, along the hull on one side, and away from the hull on the other. The different direction is the key here, when it flows away from the hull, it doesn't have the same effect of creating a low pressure zone near the hull the prop wash flowing over the hull does. Thus the boat is pulled to the side the prop wash stays near.

Be careful how you answer as you are dangerously close to saying that the angles of attack are different resulting in a torque on the prop resulting in the water leaving with more or less velosity..........
See we are in agreement.

I'm saying the angle of deflection of the water off the prop is different relative to the hull, not that has different velocity.

EDIT: after giving it more thought, the flow being directed against the hull may also result in a higher velocity on that side, but I'm not certain about that...

don't you just hate engineers?

no, they can carry on a healthy debate over the facts as opposed to their emotions, something far too few americans are capible of doing (just look at the number of people right now who think "gun control" will result in safer streets and fewer crimes, they ignore all the evidence and go purly off emotion).

I'm going to stop now before I fully commit this thread to the war room!

 

[Bill Roosa]

OK,
RH prop operating in reverse and angled shaft. ALL the water is moving upward ward toward the hull. Why does the starboard side water not do the same thing as your animation shows the port side doing. Also, same question for the water shown moving downward on the starboard side.
When I look at the videos of the spiral bubbles i do not see the spiral turning in the water. I see it moving away from the prop as a solid (mostly) mass that just happens to have bubbles carved into it in a spiral. I do not see the spiral unwinding or winding up. The water does not leave the prop with a lot of rotational component.
A great example is airplane contrails. A prop driven aircraft leaving a contrail (off a nacelle is best example) does not show the contrail getting "wound up" along the contrail. I'm not talking wing tip vortex here!!
So all the water is moving up at pretty much the same speed and would get deflected outward from the keel (port and starboard) and downward along the keel (again port AND starboard)
seems pretty symmetrical to me.

And yes guns don't kill people, people kill people! I've had one for most of my adult life and the only time any of them ever killed anybody was when I pulled the trigger.

 

[KingAirDriver]

Gonna throw my hat in the ring here.

As for the prop shroud. Unless the props axis of rotation is parallel to the direction of travel it will do nothing to increase performance and only increase drag. It should however eliminate prop walk.

The purpose of the duct around turbo-fans on aircraft is to accelerate the incoming air charge using Bernoulli's principle (venturi effect) and thus making the fan more efficient and reducing/containing noise and flying engine parts.

As a pilot, the ultimate most efficient propeller has only 1 blade. Proven. The more blades you add, the worse your low speed high power setting performance is going to be. On aircraft, the higher blade counts allow a smaller diameter and this allows the prop tips to be under the MACH (speed of sound) making the noise levels more neighbor friendly.

A pilot's dream is a variable pitch, reversing, counter rotating, full feathering, ducted propeller.
All those thing fix many problems with propellers.
The Soviets used this prop design on the TU-95 Bear Bomber. With the exception of the Duct.

My dream is a brushless electric motor driving a ducted AziPod with power coming from a diesel power generator. With a full feathering prop.

I think I can build it for under about 2 grand to power most 30 foot class boats.

And I can put a bow thruster on a 22 foot boat. Not that hard. Can probably do it for under 500 bucks too.

 

[Chris Patterson]

Ah HA!! Now we're getting somewhere. My Westerbeke is coming out tomorrow in anticipation of an electronic counter-rotating sail-drive.

Send it to me Neal, with a Cat motor.

 

[KingAirDriver]

Get a sail drive unit, and we will get the right prop and duct on it. If you want electric, find a motor with the rated horsepower you want, and we can do that too.

Actually, what we need to develop is an affordable Z drive...that is what it all boils down too...