Rigger question...

[Brian MAhoney]

1., 2., and 3. help over come prop walk.  #2 really only makes it worse if the rudder/prop relationship is WRONG.  However there is only one place where it can be wrong and the other 90% of the locations are OK.
You guys are barking up the wrong tree with the moment arm and leverage ideas.

But keep thinking.....  B)

Good Boating,

Andy

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let me guess, the wrong place is inline behind the prop.

 

[Marty Davis]

Been Purty busy Preston, but I'll get to it.
One thing you mentioned that is partly correct Preston, is that "prop walk only happens in acceleration".   The prop is always trying to walk, but it seems most appearent during acceleration.  In reality the prop is always trying to "walk" with the same amount of force, but hull and hardware dynamics are constantly changing during acceleration giving the "appearence" of propwalk.

More later.  

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Ok, that leads me to what I always thought and that is

1) you need a long rudder, like yours

2) prop position relative to the rudder

3) and I go with Marty on this one. a properly sharpened prop

1 & 3 seem to work for me.

Yes, the prop is always trying to walk. But at a contsant speed there is no acceleration therefore the body is at rest and I didn't want to go there. Mostly because I can't.

You must be busy with Don's 21 again. :lol:

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Preston:

One additional item that contributes. Not only the sharpness of the prop, but the thinness of the prop. Imagine plunging a prop with a thick blade into the water. It acts like it is not sharp also. You obviously can not thin a prop a ton and have it hold up, but on the larger boats, the props are so thick that they REALLY contribute to prop walk just by being thick.

Marty Davis

 

[Preston_Hall]

Marty,

I have read almost all your tech papers. I tried the prop stuff a while back. It does work.

 

[AndyBrown]

Andy,
OK, the stub shaft (bearing surface) is behind the strut mounting center.

Still the wrong tree?  :huh:

Preston,

And now you took Don's boat. He has the ultimate excuse for not running now.  :lol: Man, I must have him shak'n pretty good. 

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Preston,

Don needs a break from racing boats this weekend after leading the Twin Shootout for the first two laps at Atlanta. B) Besides you'll need a head start on the points Preston.

still nothing about the outboards Andy.... ??? or is it in some cryptic code that I havent/cant seem to grasp?

~James 

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James it will be appearent soon.

1., 2., and 3. help over come prop walk.  #2 really only makes it worse if the rudder/prop relationship is WRONG.  However there is only one place where it can be wrong and the other 90% of the locations are OK.
You guys are barking up the wrong tree with the moment arm and leverage ideas.

But keep thinking.....  B)

Good Boating,

Andy

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let me guess, the wrong place is inline behind the prop.

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The water is spinning off the end of the prop blade as it exits the water on the up swing. If the trailing edge of the rudder is even with and very close to the prop, this water coming off the prop will put a big force on the rear of the rudder blade that most servos can not withhold and the boat will go right. This is not propwalk, just rudder deflection that is caused by the prop against the rudder.

We tend to think that only water at and behind the prop is in motion, however water in front of the prop is also in motion. The water motion is not always parralell to the shaft. This water in front of the prop is also begining to rotate. When the hull is in a high drag condition such as launch and going through turn this rotation becomes greater and effects everything it comes in contact with.

This includes what is in the water and what is just above the surface, including skegs, fins, tansoms, strut blades and so on.

Crank up Mama's "Mixmaster" , stick it in some water and observe what happens.

 

[Preston_Hall]

Besides you'll need a head start on the points Preston. 

If I was after points I would have had it easy when the boat would go 50 the other weekend. Double Besides, we scared away all the other 20 hydros. Nobody but him to beat up on. I tried to race against the bigger boats but they wouldn't let me. You would have thought I had a gas boat for crying out loud. :lol:

 

[topfuel443]

James it will be appearent soon. ~ Andy Brown

Hmmmm..... no words whatsoever? no little teasers or anything? Come on Andy!

~James

 

[Watercadet]

James it will be appearent soon.  ~ Andy Brown




Hmmmm..... no words whatsoever? no little teasers or anything? Come on Andy!

~James 

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I think this is relative to OB's too:

"We tend to think that only water at and behind the prop is in motion, however water in front of the prop is also in motion. The water motion is not always parralell to the shaft. This water in front of the prop is also begining to rotate. When the hull is in a high drag condition such as launch and going through turn this rotation becomes greater and effects everything it comes in contact with.

This includes what is in the water and what is just above the surface, including skegs, fins, tansoms, strut blades and so on.

Crank up Mama's "Mixmaster" , stick it in some water and observe what happens."

Right in front and below the prop is the skeg....

 

[Watercadet]

Andy, on the OB side, have you tried a sort of back cut or bar cut in the T/E of the skeg? Seems like that might clearence some of that pre-swirl and let the prop spin easier.

Wrong sapling?

Adam

 

[AndyBrown]

Adam,

I havn't done any Foot work on the OB. I did here of a very sucsessful OB hydro (full size) that used an extented prop shaft and bearing housing to move the prop away from the foot.

 

[K-ZAM]

Besides you'll need a head start on the points Preston. 

If I was after points I would have had it easy when the boat would go 50 the other weekend. Double Besides, we scared away all the other 20 hydros. Nobody but him to beat up on. I tried to race against the bigger boats but they wouldn't let me. You would have thought I had a gas boat for crying out loud. :lol:

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Preston,

Don needs a break from racing boats this weekend after leading the Twin Shootout for the first two laps at Atlanta. Besides you'll need a head start on the points Preston.

Hey Preston,

Not to worry I got your back!

Don has to send his to FL you can bring your next door. :lol: :lol:

Kentley

 

[Nigtmare]

Besides you'll need a head start on the points Preston. 

If I was after points I would have had it easy when the boat would go 50 the other weekend. Double Besides, we scared away all the other 20 hydros. Nobody but him to beat up on. I tried to race against the bigger boats but they wouldn't let me. You would have thought I had a gas boat for crying out loud. :lol:

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Preston,

Don needs a break from racing boats this weekend after leading the Twin Shootout for the first two laps at Atlanta. Besides you'll need a head start on the points Preston.

Hey Preston,

Not to worry I got your back!

Don has to send his to FL you can bring your next door. :lol: :lol:

Kentley

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Hey Kentley ever since Preston got dem' channel on the tv, he will not leave the house. I should of told him to buy it at his own risk. Poor boy, he might grow hair on his palms. :lol: Cya. Nikos

 

[K-ZAM]

Hey Nick,

His new nickname is Rosey.

KP

 

[ClayGlover]

Been Purty busy Preston, but I'll get to it.
One thing you mentioned that is partly correct Preston, is that "prop walk only happens in acceleration".  The prop is always trying to walk, but it seems most appearent during acceleration.  In reality the prop is always trying to "walk" with the same amount of force, but hull and hardware dynamics are constantly changing during acceleration giving the "appearence" of propwalk.

More later. 

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bump

 

[VansRacing]

Hey Gil, I'll ask you or anyone who knows, since I get a reply from Andy Brown about 1 in 3 times if I'm lucky  .  What makes the "anti-walk strut" work as an anti-walk strut?  The extra length?  Thanks

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Good question. It might be magic since the strut isn't in the water.

My boats quit walking when I learned how to set them up correctly.

1- how come there is no off set in the front spondons??? the boats turn to the right so how come no hull off set to the right???

With proper setup there is no need. Also, the weight displacent doesn't seem to be much of an advantage at our scale speeds. Ask this, why would you at this point?
Also, when things are not staright the boat seems to be in a bind going down the straights and then it just exhibits unwanted traits.

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Anti-walk strut? Good question. I will talk about why it does work sometime in the next few days when I've got a little more time.

Preston...It is Magic! hehe

I'll give you something to think about for now. If you run a hydro with the Drive Dog (not the prop) even with the back of the transom and then run the same boat set up with the drive dog 2" behind the transom you will see a difference in the way the boat handles. The effect of the prop is exerted on the bottom of the transom.

Even though the transom is in the "air". And even though the prop is BEHIND the transom in both cases.

Outboard racers could benefit from understanding the effects the prop excerts on the harware in front of it.

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We already have...

-Carl

 

[Preston_Hall]

Nick,

I'll shake your hand any day. :lol: :lol: :lol: You might get a warm fuzzy feeling though. :lol: :lol: :lol:

 

[Nigtmare]

Nick,I'll shake your hand any day.  :lol:   :lol:   :lol: You might get a warm fuzzy feeling though.  :lol:   :lol:   :lol:

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I don't care if is fuzzy!!!as long is not sticky :lol: How is the H/B comming is ready to do 70 mph?or you will have to get Kentley to set that thing up for you

Nikos

 

[Don Ferrette]

And now you took Don's boat. He has the ultimate excuse for not running now.  :lol: Man, I must have him shak'n pretty good. 

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The only shakin goin' on over here is mixing up some more fuel! :lol: :lol: :lol: :lol: Hey when the brains behind CMD can't see any obvious reasons why it won't go over 70 & says let me take the whole boat back with me that is a no brainer. If Stu Barr walked up to you and said he would take your Crapshooter back with him & straighten out those issues you're having what would you do?

Seriously though the problem with that boat was that you couldn't see it just looking at it or on the set up board, but the construction templates found it & she will be haulin' the mail now. So anyone who thinks set up on a 20 ain't critical better think again. B)

 

[Chris Wood]

on the prop walk issue..

has anyone seen that blade dia plays a big part in things. also. can you have a prop to far back behind the rudder??? inquiring minds would like to know

chris

 

[Preston_Hall]

And now you took Don's boat. He has the ultimate excuse for not running now.  :lol: Man, I must have him shak'n pretty good. 

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The only shakin goin' on over here is mixing up some more fuel! :lol: :lol: :lol: :lol: Hey when the brains behind CMD can't see any obvious reasons why it won't go over 70 & says let me take the whole boat back with me that is a no brainer. If Stu Barr walked up to you and said he would take your Crapshooter back with him & straighten out those issues you're having what would you do?

Seriously though the problem with that boat was that you couldn't see it just looking at it or on the set up board, but the construction templates found it & she will be haulin' the mail now. So anyone who thinks set up on a 20 ain't critical better think again. B)

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Well, at least I don't hide the CS in the trailer because it isn't at it's best.

 

[John Finch]

Acceleration and propwalk. Let me think. Prop turns counterclockwise. plus acceleration. Imagine grabbing the prop, much like what the water is doing and imagine what the boat would do. It slams into a clockwise rotation. In turn, the right front sponson is slamed into the water hard, which makes the boat turn right.

Is that too simple.