Prop walk thoughts.

DougMc · Dec 16, 2012

With out prop walk I would need to have bow thruster. Boat is big enough that I have to plan all docking procedures with wind and prop walk. Would love a thruster but need a 40+ sail boat to mount. No prop walk would be a bad thing.

capejt · Dec 16, 2012

not too sure

DougMc said:
With out prop walk I would need to have bow thruster. Boat is big enough that I have to plan all docking procedures with wind and prop walk. Would love a thruster but need a 40+ sail boat to mount. No prop walk would be a bad thing.

I don't know, I could do without it.

shemandr · Dec 16, 2012

capejt said:
I don't know, I could do without it.

Really?
Prop walk makes it possible for me to exit my slip backing to port with a sharp enough turn to be able to turn the bow out without running out of water. If I had to go the other way I'd be F'***ed.
When I'm away I usually exit the slip and enter the fairway backing to port. If I have to back out of the fairway then so be it.
I can say if I had to back into my slip it would be more difficult. My rudder would also get stuck in the mud and be subject to damage so I don't want to do that.

DougMc · Dec 16, 2012

Well I think a lot was to with you and your boat. I am 66 and my boat weighs 15,000 lbs. Grabbing a dock and pulling it in is not a option, this is impossible with low floating docks. I am single handling 99% of the time so stopping the boat right at the dock is a must. Not such a big deal in my slip. Going into a crowed fuel dock where you have to parallel dock with just a boat length now thats the problem. Getting out with the wind pinning you against the dock and the gas attended leaves and you have the same problem.
With out prop walk I would need thruster.

Tom J · Dec 16, 2012

I think what you are referring to is called a "nozzle" on fishing boats. It improves thrust at lower speeds, but adds drag and does not improve maneuverability at all. A friend of mine took out a channel marker when he backed a 60' commercial fishing boat out of her slip, and the wind blew him sideways. He is an experienced captain, and he said those boats are notorious for having no steerage in reverse.

Bill Roosa · Dec 16, 2012

I use my prop walk all the time too.
On a related note, why do you think a shroud (you called it a hoop) will in any way effect your prop walk?
Shrouds are used to raise the efficiency of a prop by eliminating the tip vortexes which are one type of drag. Course the shroud causes drag also and usually more then it gets rid of soooo.......... that is why you don't see them much on inboards. Also why you see a two blade being more efficient (not effective flamers, efficient) than a three blade prop, one fewer vortex.
None of any of that effects prop walk which is caused by the prop rotational axis being at an angle to the fluid flowing by it

Bill Roosa · Dec 16, 2012

A common misconception. The water at the "top" is not very much different than the water at the "bottom" 12" of water does not change its density and the pressure of the water does not effect the thrust (ie the thrust equations does not have a pressure term) The technical reason is that if the rotational axis is inclined to the fluid flow direction the (in boats that is usually a vertical and not a horizontal angle so I'll talk to it that way, the principal is the same for horizontal angles though) angle that the blades strike the water is greater going down than when it is coming up. Greater angle of attack create greater thrust resulting in an asymetrical (port-starboard wise) thrust which creates a torque which we all call prop walk. It is greater with greater prop shaft angles. A horizontal prop shaft does not prop walk.

FourPoints · Dec 16, 2012

Bill Roosa said:
A common misconception. The water at the "top" is not very much different than the water at the "bottom" 12" of water does not change its density and the pressure of the water does not effect the thrust (ie the thrust equations does not have a pressure term) The technical reason is that if the rotational axis is inclined to the fluid flow direction the (in boats that is usually a vertical and not a horizontal angle so I'll talk to it that way, the principal is the same for horizontal angles though) angle that the blades strike the water is greater going down than when it is coming up. Greater angle of attack create greater thrust resulting in an asymetrical (port-starboard wise) thrust which creates a torque which we all call prop walk. It is greater with greater prop shaft angles. A horizontal prop shaft does not prop walk.

That is 75% of the reason, but not what actually creates the prop walk. The "walk" is caused by this torque in the water creating an unequal speed of water flowing over the hull on either side of the prop. In situations where the prop shaft is also offset, this usually means the flow over the hull is only on one side.

This difference in flow over the hull creates a low pressure area on the side of the hull with the faster moving water, thus the boat gets pulled to that side, and creates the effect known as "prop walk".

kloudie1 · Dec 16, 2012

Chris, here is a pretty good link to Kort Nozzles.. pretty much what you are describing.
http://www.solarnavigator.net/kort_nozzle.htm

Nodak7 · Dec 16, 2012

FourPoints said:
That is 75% of the reason, but not what actually creates the prop walk. The "walk" is caused by this torque in the water creating an unequal speed of water flowing over the hull on either side of the prop. In situations where the prop shaft is also offset, this usually means the flow over the hull is only on one side.

This difference in flow over the hull creates a low pressure area on the side of the hull with the faster moving water, thus the boat gets pulled to that side, and creates the effect known as "prop walk".

Four that is a very good explanation but I have one question... I have a three bladed vari-prop and have always wondered why I do not have "prop walk" in reverse? Your explanation sounds as if it explains why all boats should have this but why not on mine? Is there something different in respect to variable pitch props? I am not trying to hijack the thread but your explanation seems to be inconsistent with all situations.

Roger Long · Dec 17, 2012

Bill Roosa said:
A horizontal prop shaft does not prop walk.

That's not quite true. The hull makes the flow regime at the top of the prop different from the bottom so there will still be some side force or prop walk.

You are correct though that the shaft angle is the primary culprit. You can really feel this flying a single engine airplane. As you raise the nose higher and the angle of attack increases, you have to keep pushing down on the right rudder pedal to compensate.

My boat probably has the highest shaft angle ever put in an production boat and amazing prop walk in reverse. I love it. As long as dock portside to, I can make the boat almost go sideways.

hewebb · Dec 17, 2012

Chris Patterson

I have been thinking about the same thing. I am not sure what causes the prop walk as I have had two power boats with single engines in the past that did not have the pronounced prop walk that the sailboat has. I have actually made a sketch of a ring to attach but not sure how long to make it. I am also concerned about hitting something with it that would cause damage making the propeller inoperative or structural damage to the hull. Basically I have decided not to pursue that.

Jackdaw · Dec 17, 2012

Shaft angle is 95% causal. Ask anyone with a saildrive, which is has its shaft parallel with the waters surface. Anyone could back our 367 doing figure 8s in the fairway. No skill needed at all, just stand backwards facing aft to the wheel and drive it like a car.

OakHeart · Dec 17, 2012

Primary cause of prop walk is the angle of attack of the prop blade to the fliud passing through the prop. In aviation, this is called P Factor. There are several effects making up P Factor, you can find a good explanation in any Private Pilot Exam study guide. Direction of prop rotation will effect which direction the boat walks.

The shroud you mention equates to a ducked fan in aviation or thruster in marine applications. The concept was tried with conventional props without much success. Now the fan jet engine is another story or in marine terms, a jet pump.

For the record, I do not like prop walk.

Skipper · Dec 17, 2012

Shift to reverse.
4\5 throttle in reverse (11.2 of 14 hp in my case).
This gets reverse speed up to around 2+ knots in about 6 seconds.
Back to idle, shift to neutral. No more walking.
Use the rudder.

With the quick application of power, the boat doesn't have time to start its walking. The reverse thrust is far greater than the sideways torque. If you saunter backwards at
1\4 throttle, the sideways pull has more time to influence the direction of travel, and less rearward pull and speed (rudder needs) to compensate.

RichH · Dec 17, 2012

The functional cause of 'prop walk' is the close proximity of the propeller tips producing the 'radial and rotating' shedding vortices which impact (at a rotational angle) on the hull 'under surface'. How 'close' the prop blade tips come to the hull is the 'major' factor of the 'intensity' of prop walk.
When the shed vortices impact against the hull, the viscosity of the water then transfers the energy to the hull via friction or by direct impact in the case of deeply vee-ed hulls (one side of the vee receiving greater amount of impact than the 'shadowed' side).

If you dont want prop walk, simply keep the prop rpm at a minimum to minimize the shedding of tip vortices onto and against the hull.
Better to use and learn how to control prop walk, especially at the slow speeds where there is no water flow across the rudder and the rudder is functionally useless. You just have to learn how much rudder offset to use when backing down to counteract 'prop walk'. .... much cheaper and more reliable than applying ducted flow propulsion, thrusters, etc.

Jackdaw · Dec 17, 2012

RichH said:
The functional cause of 'prop walk' is the close proximity of the propeller tips producing the 'radial and rotating' shedding vortices which impact (at a rotational angle) on the hull 'under surface'. How 'close' the prop blade tips come to the hull is the 'major' factor of the 'intensity' of prop walk.
When the shed vortices impact against the hull, the viscosity of the water then transfers the energy to the hull via friction or by direct impact in the case of deeply vee-ed hulls (one side of the vee receiving greater amount of impact than the 'shadowed' side).

Rich, you're going to have to help me out here.... I've heard this line of thought as you present, but it runs counter to my (and many others) large base of empirical evidence that says that saildrive motors have no prop walk. I've been on dozens. No walk. Wadaya think?

I've always gone with the p_effect type of explanation:
http://www.diysailor.com/index.php?...lk-an-explanation&catid=9:technical&Itemid=12

philwsailz · Dec 17, 2012

Chris Patterson said:
Sunday evening, and I'm bored. So here is a thought that I've had for quite some time. At least it's a thought that won't get me locked up. On another thread about bow thrusters, of which I didn't want to sidetrack a relevant thread, but what would be the thinking on putting a loop on the main prop, to prevent prop walking, and conceivably a more efficient thrust in the process? I've seen these before on outboards, and it seems a simple matter to incorporate one on an inboard using a strut. If it were substantially fastened to the hull, (as it would have to be regardless), I would think it may even have the added benefit of preventing some prop fouling, at least to some degree.

I know there are some hydrodynamic engineers out there, as well as somebody who may have experimented with this, and others that may just have some thought on the subject.

One more thing, please spare me the rhetoric about how I should be able to "use" the prop walk, as I've been doing it for years. It just seems to me that some folks would like to throw 'er up in retreat, and the boat backs up straight. Shoot, I wouldn't mind it from time to time..

There is a device that was developed some time back in time called the Kitchen Rudder. Designed by some guy named Kitchen apparently.

It was a tubular-style rudder that mounted behind the prop and provided some crude vector-thrusting of the prop wash at low speeds, as well as acting at higher speeds more like a traditional rudder. What set it apart was the fact that the tube was split vertically, such that it could be articulated as two halves. The trailing edge of each half was miter-cut in such a manner that the two halves could be completelly closed behind the prop. Doing so provided a reversing function similar to that found on jetboat nozzles, and provided a reverse "gear" while eliminating any clutch or reversing transmision. Also, just as a jet, the rudder could be partially opened, providing a 50/50 split of forward and reverse thrust, effectively creating a "neutral" that could be achieved with the motor still runnig and the prop still spinning.

Here is an image showing an example:

Here is a link to some patent information and a few more pictures: http://www.rcgroups.com/forums/attachment.php?attachmentid=2957348

The nice thing about the Kitchen is that in theory it would/might/could provide positive steering control in reverse..... More than simply eliminating prop walk....