Prop walk thoughts.

[kloudie1]

I firmly believe that prop walk is caused by underwater gnomes .. The nasty little buggers look at the intended "back-in" slot and do everything possible to get the boat to not go there..Sometimes when no dock committee is looking at you, they don't affect the boat at all and it goes where you intend.. but if anyone is there, they have on their little bitty frog flippers and push push push.. Jackdaw.. the little buggers don't like saildrives (just as I don't) so they stay away!..

 

[Nodak7]

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.

Phil is this Kitchen Rudder used as a replacement for a complete rudder or does it augment the rudder?

 

[philwsailz]

Phil is this Kitchen Rudder used as a replacement for a complete rudder or does it augment the rudder?

I don't know. I have never seen one implemented on a sailboat. I have never seen one in person, just read about them a little. Not sure how widely they have been implemented, although I see a newer US patent from 1990 citing several improvments to the original design.

I can visualize one adapted onto the leading edge of a sailboat rudder... Assuming the rudder is close enough to the prop to make it work. I can't help but think that fully openend, it would not hinder forward motion under sail too bad. Fully closed and you could use your tiller or wheel to better(?) control reversing in either direction. The force might be a bear to manage though.... I don't have a gut feel for that.


In thoery, you might be able to get rid of the reversing gear.....

All things to ponder. Maybe someone should try it out....

 

[Jackdaw]

Phil is this Kitchen Rudder used as a replacement for a complete rudder or does it augment the rudder?

Please! ;^)

That thing is a dream of someone that has never been on a sailboat. NO WAY it could take the place of a rudder.... think about the surface area, and the load your rudder is under in a good blow. I've seen 2 inch carbon fiber tubes destroyed by the loading.

 

[RichH]

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

Its the same with aircraft propelled as with boat propellers - torque and "paddle wheel' effect.

Its what makes a propeller driven aircraft yaw off to one side ( and the port side wing dipping slightly) just as soon as the aircraft wheels lose contact with the ground ... and was the reason so many especially multi engined craft of WWII had 'very short' noses --- to eliminate/reduce the 'paddle wheel' effect. The paddle wheel effect can be reduced by 'ducted flow', winglets on the prop tips, etc. .... and giving sufficient 'clearance' between the tips and hull.

The discussion of angled down prop shafts, is probably valid as such can enhance impact of the prop wash TO the hull when in 'reverse' ... and the shaft is now angled 'up' w/r boat direction - and now forcing 'the rotating wake' from the prop against the hull.
The sail drive has no angled props shaft .... but you can still feel the action of 'torque' ... i.e..: the boat still want to always turn to the left when going forward, just like how a propeller driven aircraft usually always yaws slightly to port when the exact moment the wheels become free of the ground .... torque and paddle wheel effect. A sail drive with insufficient prop tip to hull clearance WILL have prop walk - paddle wheel effect.
On my heavyweight cruising boat with the engine sitting in a deep bilge, and with nearly zero angled propshaft, I simply closed down on the aperture size until I had sufficient prop walk in order to better maneuver in tight spaces, especially when there was little to no water flow across the rudder.

Re: tip clearance
From the Michigan Wheel website - FAQ (propeller mfg. w/r to prop walk): "The clearance between the propeller blade tips and the hull bottom should be at least 15% of the propeller diameter and ideally, 20% or more. So for example, a 20” diameter propeller would have 3” minimum clearance and better still 4” or more. A common guideline for recommended propeller to strut or keel clearance is 20% of propeller diameter measured between the propeller blade edge and strut leg or keel. This is often measured at a point on the propeller blade edge about 70% of the distance from the shaft centerline to the blade tip."

Other explanations:
http://www.physicsforums.com/showthread.php?t=596845

 

[philwsailz]

Please! ;^)

That thing is a dream of someone that has never been on a sailboat. NO WAY it could take the place of a rudder.... think about the surface area, and the load your rudder is under in a good blow. I've seen 2 inch carbon fiber tubes destroyed by the loading.

It has been employed on power boats where the forces under power would potentially far exceed those on a sailboat. 'Not sure your argument re: carbon fiber tubes applies.... For a recreational boater, busted 2" cf tubes would point to poor design and severely un-balanced forces. We discuss all the time how an unbalanced rig will affect rudder post loading. One needs to fix the imbalance if the rudder tube is subject to high torsional loading.

Let's not forget similar buckets ate used on the aft end of jet airplane engines to brake the plane on landing. Talk about big forces... And they are used successfully...

 

[Jackdaw]

It has been employed on power boats where the forces under power would potentially far exceed those on a sailboat. 'Not sure your argument re: carbon fiber tubes applies.... For a recreational boater, busted 2" cf tubes would point to poor design and severely un-balanced forces. We discuss all the time how an unbalanced rig will affect rudder post loading. One needs to fix the imbalance if the rudder tube is subject to high torsional loading.

Let's not forget similar buckets ate used on the aft end of jet airplane engines to brake the plane on landing. Talk about big forces... And they are used successfully...

Phil, I agree that the CONCEPT has been used in other areas. But THAT thing looks like someone made it in their garage. ;^) And in any case, it could never take the place of a rudder and still serve it purpose to divert flow from the prop. Too big for one purpose, too small for the other.

If you want to talk design with the broken Melges 32 rudder shaft, I'd sure the engineers at at Reichel/Pugh would be interested in your thoughts re their rudder. ;^)

 

[Stu Jackson]

OK, Chris, you've gone & done it now!!!

Loop? Hoop? Kitchen?

It's a loopy idea. [Couldn't wait to get THAT one in!:cussing:]

Actually it has merit, and this has gotta be one of the very BEST discussions and links about prop walk I have read, ever. Even if you didn't want to get into that discussion. :naughty:

Anyway, I forget who mentioned it, but getting the boat moving backwards has its charms. I always just love the look of horror on the face of the skipper in the 32' Bayliner behind me when I goose it in reverse to get out of my slip. Priceless!

And I think Roger has kinda little teeny background in boat design. :dance:

Now, Phil, OTOH, has redone the "kitchen" on his boat in grand fashion, right, so he must know what he's talking about.

I think the nice part about prop walk is that almost all of us have port prop walk, but it'd be weird, really weird, to get on a boat that has starboard prop walk. Anyone have one?

Great discussion, thanks, Chris. You still have too much time on your hands.

 

[philwsailz]

Phil, I agree that the CONCEPT has been used in other areas. But THAT thing looks like someone made it in their garage. ;^) And in any case, it could never take the place of a rudder and still serve it purpose to divert flow from the prop. Too big for one purpose, too small for the other.

If you want to talk design with the broken Melges 32 rudder shaft, I'd sure the engineers at at Reichel/Pugh would be interested in your thoughts re their rudder. ;^)

It was/is homemade in the picture submitted. That is on an RC model......

I too believe it cannot be used to replace a rudder blade, but your sweeping gesture dismissing it had to be addressed . If only for sport.

I do see it as a possibility when appended to a blade rudder and designed as as total system, where size, distance from prop, and other mechanical issues were addressed. I can see it being largely invisible to the water flow when sailing forward and open and attached to a traditional rudder, and at the same time effective primarily as a reversing device not unlike jet buckets when reverse is desired, something I think Chris was pondering in his original post.

I'm not going to argue with someone about their Melges rudder design. If it broke it saw forces beyond design. Period. I will argue that I am not sure your use of it as a talking point is relevant. By inference you suggest a Kitchen will break a rudder post and that is quite a leap...

 

[Jackdaw]

Stu, for sure Phil has chops... His C27 has gotta be the nicest on on the planet!

And lots of Beneteau owners have left hand props. my 260 included. Those all walk right!

 

[Jackdaw]

I

I too believe it cannot be used to replace a rudder blade, but your sweeping gesture dismissing it had to be addressed . If only for sport.

All good!! ;^)

Note I ONLY dismissed it as a rudder replacement!

 

[Nodak7]

Actually I do believe the "kitchen rudder" is in wide use right now. All the "Jet Ski's" or "wave runners" (you know those noisy irritating things) use that methodology for steering and stopping.

 

[philwsailz]

I drove an old Chris Craft cruiser where the V8 was installed backwards. I seem to recall it walked to Starboard.....

Yup, the flywheel was forward and the Paragon reversing gear was connected to what we know as the front of the motor... It's still awkward to think about... Then I realize my A4 is similarly "backwards". Oh well : D

 

[Jackdaw]

I don't think this is a too OT, because its a starboard walking sailboat that could use a kitchen! ;^)

The First 260 has dual rudders on a very wide transom. Great while sailing, but a BEAR when an low/no speeds or while reversing power. With NO prop wash hitting the rudders, until the boat is going several knots you got nutun. If anyone wants to design a kitchen for this baby, I'm all ears! ;^)

 

[philwsailz]

Actually I do believe the "kitchen rudder" is in wide use right now. All the "Jet Ski's" or "wave runners" (you know those noisy irritating things) use that methodology for steering and stopping.

Similar and close but not exactly. For a jet power plant the nozzle is vectored, and the bucket moves with it. This creates a helm that is unnaturally backwards in reverse. It drives me nuts in my little jet boat. turn the wheel to starboard in reverse and the boat backs to port.....

For the Kitchen the propulsion is fixed and the bucket is vectored. The Kitchen will have a familiar helm response in reverse. It will respond more like a stern drive, i.e. it will back like a car. Turn to starboard and it will back to starboard.

 

[philwsailz]

All good!! ;^)

Note I ONLY dismissed it as a rudder replacement!

Weebee on da same page den!

 

[philwsailz]

Now, Phil, OTOH, has redone the "kitchen" on his boat in grand fashion, right, so he must know what he's talking about.

Uh..... Nope...

 

[philwsailz]

Now, Phil, OTOH, has redone the "kitchen" on his boat in grand fashion, right, so he must know what he's talking about.

*Doh!!!!!*

Yes, I did redo the kitchen, (galley). Literally... Hah hah hah. Good one. Just got it!

 

[FourPoints]

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.

3 blade, 2 blade, 5 blade, they make no real difference to this pice of the equation. if your prop shaft (when thinking about reverse) is not pointing back up at the hull, or your hull is basically flat where the reverse prop wash hits it, than there is very little prop walk.

On the other hand if your shaft is at a large angle so that the prop wash directed at the hull then it is likley to induce a lot of prop walk.

I made these two crude illustrations to show how the water flow goes over the two common hull types. The side profile is a typical fin keel boat that will have mild prop walk. The rear profile is a typical full or 3/4 keel boat where the prop shaft comes out of the back of the keel, and the prop sits onle a few inches behind the keel.

EDIT: the images are not previewing with the animations, but if you click on them they will display the animation properly

 

[Nodak7]

3 blade, 2 blade, 5 blade, they make no real difference to this pice of the equation. if your prop shaft (when thinking about reverse) is not pointing back up at the hull, or your hull is basically flat where the reverse prop wash hits it, than there is very little prop walk.

On the other hand if your shaft is at a large angle so that the prop wash directed at the hull then it is likley to induce a lot of prop walk.

I made these two crude illustrations to show how the water flow goes over the two common hull types. The side profile is a typical fin keel boat that will have mild prop walk. The rear profile is a typical full or 3/4 keel boat where the prop shaft comes out of the back of the keel, and the prop sits onle a few inches behind the keel.

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??