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swing, wing sming?

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 11/04/2003 : 15:30:32

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 I thought prop walk had to do with the direction of rotational torque, similar to what happens to my spinning golf ball when it leaves the tee...takes a ninety degree turn to the right!

<hr height="1" noshade id="quote"></blockquote id="quote">
The rotational torque of the prop would try to turn the lower unit in the opposite direction--without much success (unless you have a very weak bracket

). The force that turns your golf ball (I'm way too familiar with that) has to do with air pressure. For a right-hander, when your hands come through ahead of the club head or you swing from outside in with the face open, the club imparts a clockwise rotation (looking down) on the ball that builds up pressure on the left side of the ball, and you know the rest. Club before hands (or inside-out swing face closed)... counterclockwise... hook. You should see Chi Chi Rodriguez demonstrate that!

You could think of the prop as having more traction at the bottom than at the top (because of the plate), building pressure below and to port of the engine in forward gear, pushing it to starboard (turning the boat to port)--a little like the golf ball.

Now, if somebody could just get me to hit the ball straight...

Edited by - on

Steve Milby
Past Commodore

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USA
5902 Posts

Response Posted - 11/04/2003 : 17:09:42

<blockquote id="quote">quote:<hr height="1" noshade id="quote">I thought prop walk was a result of water flow from the prop being driven unevenly across the rudder. A boat with a perfectly aligned prop (in relation to the center line of the rudder) would, theoretically, have no prop walk with the helm held at dead center.<hr height="1" noshade id="quote"></blockquote id="quote">
J.B., if the boat is at a dead stop, and the rudder is centered, and you shift into forward gear and give the engine a shot of throttle, the stern of the boat will kick slightly to one side (usually to the left). That is prop walk.

If the propwash from your propeller is directed past your rudder, and if you turn the rudder, the boat will turn accordingly. That is not the same principle as prop walk.

If you want to turn within an extremely tight radius, you can combine the two principles. If the prop walk causes the stern of the boat to kick to the left, then turn the rudder hard to the right. From a dead stop, give a short, gentle burst of forward throttle, until the boat moves forward about 4-5 feet, and then shift into reverse and back up the same distance (Keep the rudder turned hard to the right while backing up.) Repeat that procedure, and the boat will rotate almost as if it is on a turntable. This procedure is more helpful to an inboard boat than an outboard, because the thrust of the outboard prop can usually be turned, whereas that can't be done with an inboard motor with a fixed prop.

This procedure will also help the C-250s that can't turn their outboards to make a right turn, although it won't work as well as with an inboard, because the prop wash on the C-250 doesn't flow past the rudder.

Edited by - Steve Milby on 11/04/2003 17:13:26

osmepneo
Past Commodore

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USA
1420 Posts

Response Posted - 11/04/2003 : 18:46:25

If Frank sails Nationals this year someone (me) ought to bring him a bone!

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 11/04/2003 : 20:11:12

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Steve Milby
 
...This procedure will also help the C-250s that can't turn their outboards to make a right turn, although it won't work as well as with an inboard, because the prop wash on the C-250 doesn't flow past the rudder.
<hr height="1" noshade id="quote"></blockquote id="quote">
Just one problem... A normal, "right handed" engine's prop-walk in forward gear pushes the stern to starboard--meaning it's trying to turn "left." So C-250 folks still have a problem. Backing is the opposite--prop walk pulls the transom to port, turning the boat to port.

To the extent that you can turn the engine to counteract prop-walk, that's great. Otherwise, one remedy in tight quarters is to use short bursts of throttle to get moving, and then idle down and use the rudder to steer. Another is to alternate between forward and reverse, using (for example) reverse to get the boat moving, and a short burst in forward to push the transom around as its moving, as I do to counter the prop-walk that tries to prevent the turn to starboard when backing into our very tight slip.

Either way, understanding prop-walk is key to maneuvering around a dock or slip. For most of us, prop-walk will turn the boat to port, both in forward and reverse.

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Steve Milby
Past Commodore

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USA
5902 Posts

Response Posted - 11/05/2003 : 09:01:22

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Just one problem... A normal, "right handed" engine's prop-walk in forward gear pushes the stern to starboard--meaning it's trying to turn "left."
<hr height="1" noshade id="quote"></blockquote id="quote">
You're right. That isn't the way I remember it, but it goes to show that the memory is the first thing to go.

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 11/05/2003 : 14:33:52

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Steve Milby
...but it goes to show that the memory is the first thing to go.

<hr height="1" noshade id="quote"></blockquote id="quote">
I thought it was the second thing...

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Doug G.
1st Mate

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USA
49 Posts

Response Posted - 11/05/2003 : 17:45:39

This discussion sent me to the books in my limited little home sailing library. Bond's Handbook of Sailing ('92) gives credit to the'density of water at different depths' argument. Chapman's ( 62nd Ed. '96) agrees with the water density story in one intro reference and then has a better explanation later with the idea of unequal blade thrust due to an angled prop shaft giving the lower and upper blades different angles of attack. I buy the second explanation as more plausible. The difference in density of water over 6 to 10 inches seems minimal. The concept is similar to auto engine torque and 'lift throttle oversteer' that was a characteristic of older Porsches. I could not find any reference to the effect of the anticavitation plate. Fixed prop boats have no plate and seem to have more propwalk than outboards. It sounds like all theories have some support.

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 11/05/2003 : 20:36:38

When you think that tiny forces can't make a difference...

Watch the water spin when going down the drain... all due to a miniscule difference in rotational velocity between the north end of the sink and the south end of the sink.

Hard to imagine.

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 11/05/2003 : 21:15:16

Basic fact: Water does not "compress" under pressure. If it did, you wouldn't need the air columns in your home plumbing system to prevent "water hammers" when a valve slams shut (as in a washing machine)--the water would absorb the shock. Water is almost like steel--it's volume and density is very constant as pressure varies--even more so than the hydraulic fluid used to power construction equipment, brake systems, etc. Even if it weren't, what's 8-10" of depth going to mean?? And if angle of attack were the issue, you could reverse prop-walk by changing your outboard trim from forward of vertical to the opposite. Try that and let us know...

The issue is the flow off the prop and its asymetric surroundings. That water flow, both back and outward, creates some VERY significant forces that will affect anything in the area, such as the anticavitation plate and the structure above it. After all, it's starting and stopping a 5000+ lb. boat in a pretty big hurry. A good way to appreciate that force is to run your motor in a trashcan full of water, and see how fast you can knock several hundred pounds of water out of the can at a fairly low throttle.

Correction: Water density may increase with depth because salinity increases with depth--due to the fact that higher-salinity water is denser and tends to sink. You can decide how much that affects an outboard propeller--particularly in fresh water.

Lots of very good sailors who write books never actually studied fluid dynamics, and sometimes it shows.

Edited by - Dave Bristle on 11/05/2003 21:32:02

Randall
Navigator

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123 Posts

Response Posted - 11/06/2003 : 08:58:28

What I meant by "compressed" was more like "less turbulent" or "unruffled" or something. In my original (misguided) vision of lift, I saw the keel creating turbulence as it moved through the water, with the deeper side being less turbulent, thus denser, while the shallower side had more turbulence & air infusion, making it less dense. Doesn't really matter, though, since it was wrong.

Twenty years ago, I was the commander of an Army rapid-deployment deep sea diving unit, and spent five months at the Navy Dive School. If I remember correctly, earth's atmosphere exerts 14.7 PSI on us at sea level, so 14.7 PSI equals one atmosphere of pressure. Every 33 feet you descend below sea level, pressure increases by one atmosphere, so at 33 feet you are at 2 atmospheres, 66' = 3 atmospheres, 99' = 4 atmospheres. etc.

If a large ship has a propellor 33 feet in diameter, the top edge would be at 14.7 PSI and the bottom at 29.4 PSI. I would think that would cause significant prop walk, and explain in part counter-rotating propellers. I know, I know, my vision is again flawed and overly simplistic-- the top of the prop isn't going to be at the surface, and counter-rotation does other important things like balance prop wash over the rudder, reduces turbulence, etc-- still it has to be a factor.

I agree that the amount of pressure change on a 9"-12" diameter outboard prop would seem negligible, that's why I remember that long ago thread-- I was surprised & intrigued by the very concept that it mattered.

Randall
'79 TR/SK dinette #1459

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ronrryan
Admiral

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USA
561 Posts

Response Posted - 11/06/2003 : 09:54:04

Technical complexities of physics aside (they are beyond me) I found the simplest way to describe the phenomenon when teaching Power Squadron courses was: visualize the propellor as a wheel at the end of an axle. The other end of the axle is on the ground. Rotate the wheel to the right, wheel wants to go right. Rotate wheel to left, wheel wants to go left. Think of prop as if you were looking at it from behind the boat, prop rotates right, stern wants to go right, etc, etc...Naturally many other factors (rudder placement, underbody design, blah, blah) create effects, but this is a handy place to start. Fair winds, ron srsk Orion #2343 SW FL

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 11/06/2003 : 10:09:17

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Randall
 In my original (misguided) vision of lift, I saw the keel creating turbulence as it moved through the water, with the deeper side being less turbulent, thus denser, while the shallower side had more turbulence & air infusion, making it less dense. Doesn't really matter, though, since it was wrong.
Randall
'79 TR/SK dinette #1459
<hr height="1" noshade id="quote"></blockquote id="quote">
There is a phrase that is the controlling concept in all of these issues relating to density.
"Nature abhors a vacuum" "abhors della natura un vuoto"
It is such a dramatic phrase that it is easy to remember. Lift is of course generated as a result of that phrase and the sucking effect of turbulence is also a manifestation of it.

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