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Prop Walk

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britinusa
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USA
5404 Posts

Initially Posted - 08/07/2012 : 16:35:25

Ok, I admit it, I'm dumb on this one.

One of the consequences of taking the boat our for many nights but only every six weeks or so, is that we don't get that familiar with docking/undocking at the same dock.

So I need help here.
Quoting from Bills blog.
<blockquote id="quote">quote:<hr height="1" noshade id="quote">We talk about the propeller being either “left handed” or “right handed”. This refers to the rotation of the propeller when in forward. If the boat is not moving and the propeller begins to turn, the rotation causes the propeller to “paddle-wheel” and move the stern of the boat to one side or the other. A “right hand” propeller tends to walk (prop walk) the stern to the right (starboard) in forward; and to the left (port) in reverse.<hr height="1" noshade id="quote"></blockquote id="quote">

And so that I can understand the logic of this phenom.

Is it a case that the blade at the top of the prop has less effort than when the blade is at the bottom?

When referring to 'right hand' prop, are we talking of clockwise of anticlockwise?

And is it a phenom only at low prop rpm?

fire away...

Paul

Joint Decision. (Sold)
PO C250WB 2005 Sail # 841.

Moved up to C34 Eximius

Updated August 2015

Edited by - on

jerlim
Master Marine Consultant

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Response Posted - 08/07/2012 : 16:59:01

I'll take a shot...consider viewing the prop of your OB from outside of the boat - in the water - as if you were directly behind the prop, in the rotational centerline of the prop. When moving forward, the prop spins clockwise and when in reverse the prop spins counter clockwise.

Now imagine that instead of a cork-screw prop, it were a paddle wheel...with the same rotational center line...at low RPM, when engaged in either forward or reverse, the prop has the ability to slide the stern in one direction or another (to the right when in forward, and to the left when in reverse).

As the outer edge of the prop grabs the most water, it has the greatest effect...the prop walk only happens when the prop starts turning, before it is effectively pulling itself through the water.

Make any sense???

Edited by - on

GaryB
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Response Posted - 08/07/2012 : 17:16:53

Propeller walk is a complicated effect which depends on ship geometry, direction of travel, propeller direction, vessel speed and depth of water. Three causes are identified for a vessel in deep water:-

1.Upward oblique flow at the propeller location.
2.Vertical wake distribution at the propeller.
3.Unbalanced lateral forces on the rudder (when set amidships) arising from the propeller slipstream impinging on the rudder blade.
The first of these results from there being a measurable difference in speed of water flowing close to the hull and that at lower depths which has not been affected by the vessel's motion. At low speeds the last effect is most pronounced and when going astern has even more influence.

In shallow water the upwards flow from under the vessel becomes much less strong and ultimately disappears. Model tests carried out show that, at a very small underkeel clearance, screw bias caused a ship to sheer to starboard (rather than port) when moving ahead and that there is an intermediate depth where the sheer from bias is neither one thing nor the other.

Finally, when moving ahead with the propeller moving astern, flow into and around the propeller is very confused. Generally the overall result for a single screw ship when stopping is a sheer to starboard, but this is not always guaranteed; sometimes it may go the other way, depending often on any yaw rate on the vessel when the propeller starts to turn astern.

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dlucier
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Virgin Islands (United Kingdom)
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Response Posted - 08/07/2012 : 18:15:27

What, no footnote?

[url="http://en.wikipedia.org/wiki/Propeller_walk"]Propeller Walk[/url]

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Voyager
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Response Posted - 08/07/2012 : 19:08:16

Paul - you really got me quizzing on this one.
Before you asked you question so skillfully, I thought I understood the phenomenon - the prop goes around and it pushes your stern either left or right depending on the direction of spin.
BUT!!!!!!! while the top of the blade is turning right (CW prop), the bottom is turning left! Completely baffled now.
No matter why it works, I use prop walk for getting on and off the dock. But now I will always wonder why.
I've read Jerry's and Gary's explanations - thank you. Still not getting why the bottom grabs better than the top.
Guess for me, this will be one of life's little mysteries.

Edited by - on

GaryB
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Response Posted - 08/07/2012 : 19:54:26

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 What, no footnote?

[url="http://en.wikipedia.org/wiki/Propeller_walk"]Propeller Walk[/url]
<hr height="1" noshade id="quote"></blockquote id="quote">

Edited by - on

GaryB
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Response Posted - 08/07/2012 : 19:57:52

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Voyager
 Paul - you really got me quizzing on this one.
Before you asked you question so skillfully, I thought I understood the phenomenon - the prop goes around and it pushes your stern either left or right depending on the direction of spin.
BUT!!!!!!! while the top of the blade is turning right (CW prop), the bottom is turning left! Completely baffled now.
No matter why it works, I use prop walk for getting on and off the dock. But now I will always wonder why.
I've read Jerry's and Gary's explanations - thank you. Still not getting why the bottom grabs better than the top.
Guess for me, this will be one of life's little mysteries.
<hr height="1" noshade id="quote"></blockquote id="quote">

The deeper you go the more dense the water?

Edited by - on

Stinkpotter
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Djibouti
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Response Posted - 08/07/2012 : 20:31:41

I didn't read the references here, but this is my theory:

The bottom of the prop has more "paddle wheel effect" than the top because the anticavitation plate (or the bottom of an inboard boat) upsets the outward flow from the top of the prop, while the bottom of the prop creates an undisturbed outward spiral flow in the direction of the blades. Clockwise rotation thrusts water to port off the bottom of the prop, pushing the stern of the boat to starboard. The flow off the top of the prop might even push against the outboard shaft or the skeg or rudder on an inboard, also pushing the stern to starboard.

When the boat gets moving in the direction the prop is pushing, the prop has a more efficient "bite" in the water, directing more thrust in the desired direction (fore or aft). When the boat is standing still or moving opposite the thrust of the prop, the blades generate more of the outward spiral of thrust (the paddle wheel effect), and therefore more "prop-walk". Any angled surface, like a prop blade, will tend to push water in a direction perpendicular to that surface to some degree. Propellers are generally designed with a slight "cup" to the blades to direct the flow aft more efficiently in forward gear, but that reduces their effectiveness in reverse--one of the reasons most outboards have less stopping power than starting power.

All I know for sure is prop-walk works for me--outboard or inboard.

Edited by - Stinkpotter on 08/07/2012 20:53:49

britinusa
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Response Posted - 08/08/2012 : 03:27:47

Phew!
Let me see if I have this right (after reading the above and the wiki link)

Our outboard's prop rotates clockwise as viewed from the stern when in forward gear.

When the boat is stationary and the engine revs are low in reverse, then the prop has a tendency to exert a force pushing to starboard and so walks to port.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">A right-handed propeller (which rotates clockwise [as viewed from the stern] when in forward gear) will tend to push the stern of the boat to starboard. When in reverse gear, the effect will be much greater and opposite. A right-handed propeller will now push the aft of the boat to port.<hr height="1" noshade id="quote"></blockquote id="quote">

So now to put that in real life terms.

When we launch, it's almost always port side to the dock (easier to see the dock in the driver's side mirror)

The docks are located on the side of a channel. When facing the ramps, the channel exits to the left.

Tied up prior to departure port side too.

Now I'm ashore managing the dock lines, Peggy is on board at the helm.

Assuming no wind at the dock that would push us onto or off of the dock, we are going to motor astern and turn the stern to starboard as the exit channel is to port of the docks.

If we put the engine in reverse and keep the revs low, then (according to the explanation) there would be a tendency for the stern to walk to port. ie. prop walk would hinder our turn astern to starboard.

Of course, I'm on the dock pushing JD off, so I'm not aware of how the boat feels.

If the prop walk is to port, then we need to helm to starboard more than we would if there were no prop walk.

How can we tell?

Now that we have moved out of the dock and swung the stern to starboard so that the bow is starting to point out of the channel, we drop the engine revs, shift into forward gear and steer to port.

The boat slows to a stop and changes direction forwards and to port, centering on the exit channel.

During this maneuver, prop walk would be to starboard, and thus aid our turn to port.

It only takes a few seconds to make each of these maneuvers, and Peggy is at the helm, she frequently asks if we are doing ok and is happy if I suggest changes to current situation, but that does mean that I'm not aware of the prop walk effect.

So, to summarize.

Stationary, move ahead, prop walk aids turning to port.(pushes the stern to starboard)

Stationary, move astern, prop walk aids turning to port. (pushes the stern to port)

Have I got the theory right?

Paul

Edited by - on

dlucier
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Virgin Islands (United Kingdom)
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Response Posted - 08/08/2012 : 06:27:49

An interesting theory I've read regarding prop walk has to do with the shaft angle of the prop. The greater the shaft angle the more pronounced the prop walk due to the difference in angle of attack of the blades from one side of the prop to the other. With different angles of attack, more thrust is developed on one side of the prop and less on the other causing it to act like a twin screw boat. This is probably why boats with inboards, which have greater shaft angles, experience this affect more so than my outboard which has virtually no detectable prop walk due to the prop shaft being horizontal.

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Steve Milby
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Response Posted - 08/08/2012 : 07:13:17

An outboard engine can be turned, to direct it's thrust in one direction or another, so that makes it much more easily maneuverable than an inboard powered boat, and for that reason, prop walk isn't usually much of a concern. But, where it matters a lot is when you are docked with the starboard side alongside the dock, and you want to leave. If you just put it in forward gear and give it throttle, the stern will kick to starboard, and, as the boat moves forward, the stern will continue to drag along the dock. There are two ways to avoid it. On my C25, when I was leaving, I would push the boat away from the dock as far as I could, so that, when I put it in forward gear and started to move forward, there was enough distance between the stern and the dock so that the stern didn't hit the dock. Alternatively, you can back away from the dock, in which case, prop walk is pulling the stern away from the dock, instead of toward the dock. When you get a sufficient distance from the dock, then you can put it in forward gear and drive away.

Because of the effect of prop walk, whenever you are backing out of your slip, make a practice of backing further than appears to be necessary, because when you move forward, prop walk will kick the stern to starboard, and it could drag the boat into the dock pilings or even into other boats in their slips.

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John Russell
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Response Posted - 08/08/2012 : 08:23:57

There are a lot of things in my life that I don't undersand. For me, this is one of them. I just believe and move on from there.

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Steve Milby
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Response Posted - 08/08/2012 : 08:39:30

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by John Russell
 There are a lot of things in my life that I don't undersand. For me, this is one of them. I just believe and move on from there.
<hr height="1" noshade id="quote"></blockquote id="quote">I quit trying to understand why prop walk works, and only have a vague idea. The only thing that really matters is to remember that, when in forward gear, the stern kicks to starboard, and when in reverse gear, it kicks to port. If you remember that, you'll be able to use it occasionally to help you get in or out of a slip or away from a dock. When you move to an inboard powered boat, it will matter alot, because you won't be able to maneuver the boat without it, but with the outboard, it is a useful thing to know, but not crucial.

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pastmember
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Response Posted - 08/08/2012 : 09:18:57

On a steerable outboard propwalk is the footnote.

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Stu Jackson C34
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Response Posted - 08/08/2012 : 09:23:12

The last two posts make a lot of sense. With the outboard on our C25, it wasn't an issue, since we could turn the engine and did. On our C34 it's a different story, and the comments about port kick in reverse and starboard in forward are ESSENTIAL to working a boat near a dock.

The most repetitive "complaints" about prop walk is that "the boat won't turn to starboard in reverse" and "the boat moves to port when I start backing out of my slip." In most, if not all cases, the skipper refuses to use enough throttle to get the water moving over the rudder. Like, always!

Correct use of prop walk combined with a midships spring line or NautiDuck's Doc-O-Matic are necessary to maneuver a boat properly. Used correctly, we can turn our boat literally on a dime.

Like docking, it's practice, practice, practice.

Even with outboards, it can be used to your advantage. But with our C25, I used to set the engine when going forward so that there was no pressure on the tiller. Can't do that with an inboard. :)

Edited by - Stu Jackson C34 on 08/08/2012 09:24:08

dlucier
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Response Posted - 08/08/2012 : 11:15:26

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Stu Jackson C34
 The most repetitive "complaints" about prop walk is that "the boat won't turn to starboard in reverse" and "the boat moves to port when I start backing out of my slip." In most, if not all cases, the skipper refuses to use enough throttle to get the water moving over the rudder. Like, always!<hr height="1" noshade id="quote"></blockquote id="quote">

Yep...When backing out of my dock, and before putting the outboard in gear, I shove off getting myself going astern with enough momentum to provide steerageway before engaging the outboard in reverse with just a bit more throttle than idle to gain a little more momentum, then I go back to neutral. As Stu mentioned, with steerageway, prop walk is less an issue.

Speaking of steering with your outboard, when backing out of my slip and nearing the boats behind me, I position my rudder tiller to starboard to begin turning the stern to port. To really kick the stern to port, I then push the outboard tiller hard to port and throttle up thereby kicking the stern over that much quicker while still moving aft to allow my bow to clear the stern of the boat in which I share the slip. Once the turn is complete, I slowly bring the two opposing tillers back to centerline and go forward.

Edited by - on

AkaBud
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Response Posted - 08/08/2012 : 15:43:23

Here's an article on how to use prop walk to your advantage.
http://www.skippertips.com/public/1347.cfm

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