Rigger question...

[scott bouchie]

I would like to share a problem I had with my 1st boat prop walking that I fixed.This is a rookie mistake but was easy to do....Ill try to make it short,I tryed all the normal fixes,tow in,cockin' the rite sponson.2 1/2 deg left in the strut,dropin' the left sponson tip.None of these things worked,it got better but not 100%.So what it turned out to be was the rudder wasnt holding because the linkage was wrong,the rudder was hooked up close to the center and the servo was hooked up on the farthest hole out so there was no mechanical advantage for the servo,and it was this way because it was to easy to align things up and then take out the ATV.I was useing about 30% of the servo throw.Also I had the leading edge of the rudder 1/4 inch behind the pivit, so make sure it's on center.In the old days you didnt have ATV adjustment so you had to set it up rite or your servo would bind-up and kill the battery.After I fixed this my boat went straight.Now my new boat is set-up straight and it goes straight.I hope this help's,it's basic but easy to screw up.

 

[Brad Christy]

Andy,

Here's a thought, and you can tell me if I'm on target or not.

As the prop spins, it pushes the boat forward. Since only the bottom half is in the water, it also pulles the transom to the left, causing the boat to turn right. These two forces combine to create what I would call a "vectored thrust", which is obviously vectored at some angle other than 180 degrees from that of the desired direction of travel. The vector would be determined by the ratio of forward and lateral thrust created by the two forces mentioned above, and thus the vector would change as boat speed changed. For the most part, we are only concerned with, say, the top third of the speed range. If we could determine this vector (I know it would not be a constant, and it would be at least SLIGHTLY different with each boat/engine/prop), then prop walk could be virtually eliminated. Personally, I don't like the idea of setting the strut at an angle, as that puts the strut body at an angle other than that of the desired direction of travel. I would think, though, that a strut with a n angled shaft hole should be the trick, though. Maybe 2 1/2 degrees is not enough. Has anyone gone farther? How far?

Like I said, the prop is generating a vectored thrust. Set this vector 180 degrees from dead straight foward and the boat will go straight.

Yes? No? More complicated than this? To variable to be dealt with in a single step fix? Tell us..........

Thanks. Brad.

Titan Racing Components

BlackJack Hydros

 

[AndyBrown]

Brad,

I've tried shiming the strut ( 3-5 degrees) on boats that pulled badly with little or no effect. There are many times "bad" forces working on the hull that we precieve to be prop walk and most likely is propwalk but is multipled by the "bad" force.

The key is to eliminate all bad forces then the prop force ("vectored thrust") to the left becomes minimal depending on the prop.

Scott you are right on. The Rudder, linkage, servo, and battery power are very important. The bigger the Pitch/diameter ratio the bigger the resistance force is need from the rudder.

We had a customer that built a twin MAC 67 powered SG and used a "budget" 130 in/oz servo.

I spent a couple of hours working on his boat trying to eliminate a bad right pull.

As a frustated last resort, :huh: I removed the "budget" servo and replaced it with a "quality" 130 in/oz servo.

Next run went straight as an arrow.

For SAW speed runs we all would like to run smaller than normal rudders, but in reallity a larger rudder is needed on riggers to resist the "vectored thrust" created by the larger prop.

 

[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

( I guess nobody wants to try and give me a anwser )

 

[DON MAHER]

Chris,

Andy pretty much answered the blade dia thing ( bigger rudder)

As for the prop too far behind the rudder OH YAH!!! The father back the more sensitive the rudder movement becomes.

Don

 

[Chris Wood]

sorry don,

maybe i mis wrote that. ...

I was talking about the blade dia of the prop.

chris

 

[AndyBrown]

Chris,

Bigger diameter doesn't cause more propwalk. Big pitch/diameter ratio does.

Also the design of the blade.

An Octura 1967 will have more walk than a 1467 (same dia.) or even a 1475 (more dia.).

 

[Chris Wood]

thanks

chris

 

[Preston_Hall]

So there is no such thing as an actual antiwalk strut? It is just longer for boats that have rudders too short and/or servos too weak?

 

[DON MAHER]

Preston,

 

[DON MAHER]

By the way Andy,

What are "bad" forces??? :blink:

Don

 

[Mike Stewart]

Andy,

So are you saying that the water IN FRONT of the prop is getting "stirred" up? Which affects the ride of the hull. When you move prop back with the extended strut then the boat hull is running in "cleaner" water?

 

[Jeff Lutz]

I have laughed at many theories on this thread, but would like something good to come out of it. Although it's hard to teach someone something when they are being sarcastic.Some of ya'll act like you "Already know the antiwalk strut does nothing" so you ask questions you think can't be answered. One good example (HINT) will be a story I'm fixing to tell you. Me and Andy talked about this years ago, when a very good outboard man cut the bottom skeg off an outboard motor.

Years prior to this 2 guys here in Mobile did the same thing.They did this to eliminate the drag of the skeg, They said, Hey the boat will still turn when the foot and prop turns...What they didn't know is that was going to be about all that it did!

With the foot and prop dead straight (and the skeg cut off) the boat would not go forward straight anymore -it tried to turn itself! SOO WHY DID THIS HAPPEN? Now Andy has already talked about the water starting to turn in front of the prop...

So that is something to think about until the information is "HANDED 2 U". Remember Andy developed the antiwalk strut to help prevent 1 of the many things that factor in boats free movement to the right.