Rigger question...

[Dan_Cousin]

Thinking about what Andy mentioned about the prop thrust deflecting the trailing edge of the rudder and causing what appears to be propwalk.

If the rudder is on the opposite side, would this effect counter (somewhat) the propwalk you normally get from torqueroll and paddlewheeling?.....

 

[gil sonsino]

Trust cone produced by the propellor will affect both sides so doesn't matter where your rudder is located propwalk will be there!

Propwalk is innehrent of surface propellor dynamics so what counteracts this effect is sometimes an offset of your strut and the turn fin.

Gill

 

[Dan_Cousin]

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.



It seems that if the thrust cone is pushing outwards from the prop and imparting a force on the rear of the rudder blade, the blade will deflect away from the prop. If the blade is on the right hand side (looking fron the back) this deflection will cause the boat to run to the right.

If the blade is on the opposite side, it would deflect the rudder the other way and make the boat turn left (somewhat). This would somewhat compensate for the propwalk caused by the prop alone. Am I missing something?

 

[Preston_Hall]

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.

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I think mostly yes. Maybe. The right sponson will not go down but so much and as acceleration increases the sponson would steadily have to sink into the water. But wouldn't the left sponson try to come up the same amount as the right tries to go down? I don't see left sponsons coming up.

I see two situations, prop walk and pulling to the right. Andy takes care of the pull with the "blunt" edge on the turn fin. I also think the pull is corrected by left rudder.

I want to know the real answer since it doesn't appear to me that prop walk has been an issue for "us" in a long while. Also, would like to know why some Eagles still apparently walk. You can't blame it on a turn fin because there wasn't one. Or was it walking or pulling.

I'll go a step further at looking at the prop. Imagine it was a paddle wheel. It is going to walk the back of the boat with zero forward motion. This is where I got my theory of forces in previous posts.

 

[DON MAHER]

Hi Preston,

Do you think that some of the eagles that "walk" are not setup with the left sponson tip 100th. lower like Andy says to do.

Also toe-in the turn fin will help with right pull.

Don

 

[Buckshot]

Hi Preston,
    Do you think that some of the eagles that "walk" are not setup with the left sponson tip 100th. lower like Andy says to do.

    Also toe-in the turn fin will help with right pull.  

    Don

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I've got my Eagles set as square as possible all the way around.

I have never had handling issues (going straight or through the corners) B)

Never have hear of setting up an Eagle with sponson offset or adding "wedge" into the turn fin. :huh:

-BUCK-

 

[Preston_Hall]

Hi Preston,
    Do you think that some of the eagles that "walk" are not setup with the left sponson tip 100th. lower like Andy says to do.

    Also toe-in the turn fin will help with right pull.  

    Don

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I'll have to agree with Buck on this one.

I have no Eagles but they are all set with sponsons parallel and little if any AOA offset and have no walk. I do think the walk is more prone to the larger boats where I only have 21's and 45's.

 

[DON MAHER]

Buckshot

Dropping the left sponson tip 100th changes AOA to make ride surface wetter (more drag) , less right pull

Adding toe-in (wedge) to the turnfin will push the front of a hydro to the left helping to overcome right pull from prop walk.

Several Eagles here in Los Angeles have this done to overcome right pull w/o adding left rudder.

Don

 

[Preston_Hall]

I think these are band aides. They may be necessary but still a fix. A straight turn fin and rudder would seem unargueably faster.

Just thinking now,

Pull due to the prop as John suggests would only happen if the prop didn't walk. Now if the boat doesn't pull then it prop walks. Just a matter of where the torque of the prop happens to go.

 

[DON MAHER]

Preston,

I agree a rudder & turnfin that are straight will be faster. The changes I mentioned have all been done on 80 & 90 powered riggers that have a lot of torque & pulled to the right, either because of torque roll or prop walk.

If you know of any other method to correct this please share.

 

[AndyBrown]

We do recommend the 2 mm to 2.5 mm Differential on the front sponson tips in many cases but it is not always needed.

The turn fin toe-in is not recommended at all and from my experience causes other problems

 

[Buckshot]

Buckshot
    Dropping the left sponson tip 100th changes AOA to make ride surface wetter (more drag) , less right pull

    Adding toe-in (wedge) to the turnfin will push the front of a hydro to the left helping to overcome right pull from prop walk.

    Several Eagles here in Los Angeles have this done to overcome right pull w/o adding left rudder.

      Don

                 

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I'll second the Band-Aid notion.

My big riggers don't have angle offset in the sponsons, strut offset, or turn fin wedge and I run CMB 90's and 100's with as much torque as any thing out there and swing 1470's and 1475 cupped up all day long in a straight line. B)

I do beleive in prop walk though, but with todays struts(extended) they all but cure the old problems. So now we get striaghter set ups and faster speeds.

There are however some props that I have tried that walk really bad but they where straight away props and I expect that kind of thing with huge amounts of pitch / cup.

Not tring to knock your set up or anyone else, but I would table the boat to make sure everything is kosher.

-BUCK-

 

[gil sonsino]

I've been running eagles and my own designs(21/45/67/80) with the same building set up.aralell sponsons and all them run straight with very little right overveer produced by propwalk. Super sharp props are a must to minimize propwalk i this issue.

off set struts can partially solve this problem but with a loss of final speed.

IMO for those who run riggers propwalk is not a problem just a particular way these models react to their dynamics.

Gill

 

[gil sonsino]

Hey Buck ...Kosher....Shalom boy!

Gill

 

[Buckshot]

What can I say B)

-BUCK-

 

[DON MAHER]

Hey Buckshot,

Do you have any left trim in your rudder & if so how much?

Don.

 

[Buckshot]

Hey Buckshot,
      Do you have any left trim in your rudder & if so how much?

    Don.       

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Yea there is left trim on the radio. It's right next to the TH trim. :lol:

No there isn't any dialed in and if there is it's soo min. that you can't see it. Maybe to dial in the push rods, but that's about it.

-BUCK-

 

[impba3548]

The antyiwalk strut is bored at an angle where the prop shaft goes thru about a degree or two off to the left, they work!!, I do mine with a couple of large cresent wrenches, this isnt recommended for the non machinist type as you can ruin it too.

I dont use them on my riggers as they run straight with the fin sharpened and angled a bit, I use it on my gas seaducer with very good results, a rudder with any left in it scrubs speed bigtime.......

mike

 

[scott bouchie]

Any left will also cause blow-over's.....

 

[AndyBrown]

The Original "anti-walk" strut that we make and sell at CMDi is bored straight. No angle at all. Angled struts on riggers have little effect.