Twin prop walk

[Buckshot]

Do most of the twin pilots experience prop walk?

I'm trying to get a grip on my twins handling and one of my issues is that it pulls right down the straights. I have checked the tub and it is straight. I have double and triple checked the front sponson alignment and it is spot on. I have checked the turn fin for correct alignment and straightness and it too is all good. I have installed a new S5301 servo and a fresh 6 volt 4200mAh receiver pack and I'm using 1/8" SS rod for my push rods and I'm sure I'm not getting any deflection in the linkage. Props are AB sharpened and balanced so no issue there.

I have remedied the problem by adding .020" shim behind one side of the struts to get about 1* angle in them but can't help to think that this is just a band-aid for some other problem.

What do the twin guys think about this?

-Buck-

 

[Don Ferrette]

What rudder blade Buck and how long?

 

[Mark Bullard]

Do most of the twin pilot experience prop walk?
I'm trying to get a grip on my twins handling and one of my issues is that it pulls right down the straights. I have checked the tub and it is straight. I have double and triple checked the front sponson alignment and it is spot on. I have checked the turn fin for correct alignment and straightness and it too is all good. I have installed a new S5301 servo and a fresh 6 volt 4200mAh receiver pack and I'm using 1/8" SS rod for my push rods and I'm sure I'm not getting any deflection in the linkage. Props are AB sharpened and balanced so no issue there.

I have remedied the problem by adding .020" shim behind one side of the struts to get about 1* angle in them but can't help to think that this is just a band-aid for some other problem.

What do the twin guys think about this?

-Buck-

Hi Buck,

This is a problem that has always been in a twin. Some boats are worse that others. The key is to get the boat to run as loose as you can and stay on the water. And then the driving comes into play. By letting off the rudder sooner from the exit of the turns you can let the boat run it's arc in the straights. The arc will not be that bad if the boat is loose but if not you will be all over the pond. My boats use a 1" blade not that deep and and they will run pretty straight with very little arc.

Mark

 

[Preston_Hall]

What hull is it?

I have seen this fixed on a RR by offsetting the struts 3/8" to the left. (torward the outside)

 

[moparbarn]

without counter rotating props, there will always be propwalk. looser will make it better, but then you have to "walk that line", i think letting off a little sooner coming out of the turn, and running an arc (if not too wide) is best. this is all related to full size boats, but pretty much directly relates to your boat, imho.

 

[Buckshot]

What rudder blade Buck and how long?

1.1" x what ever lenght John wrote in my instruction booklet. I can measure when I get to the house tonight. I put a 1.2" on it two weeks ago but it spun out and bent it. Yeah it was a new 7075 blade and I still bent it.

By letting off the rudder sooner from the exit of the turns you can let the boat run it's arc in the straights. The arc will not be that bad if the boat is loose but if not you will be all over the pond. My boats use a 1" blade not that deep and and they will run pretty straight with very little arc.
Mark

Man that sounds a little extreme to me.

What hull is it?
I have seen this fixed on a RR by offsetting the struts 3/8" to the left. (torward the outside)

SGX 60/80 with 80 motors

 

[ALLAN YUEN]

Hi Buck I was really frustrate with SGX Twin same problem drag right down so I decided to changed little modified struts( left negative 1 degree and right 0 degree) and replaced CMDI blade 1.1" long 4"( end sharp flash flat) I really glad better perfect ride fastest straight no more drag

Hope to help B)

 

[Terry Keeley]

What props?

 

[Buckshot]

What props?

H-38's cupped up.

-Buck-

 

[PHM Racing]

Do most of the twin pilot experience prop walk?
I'm trying to get a grip on my twins handling and one of my issues is that it pulls right down the straights. I have checked the tub and it is straight. I have double and triple checked the front sponson alignment and it is spot on. I have checked the turn fin for correct alignment and straightness and it too is all good. I have installed a new S5301 servo and a fresh 6 volt 4200mAh receiver pack and I'm using 1/8" SS rod for my push rods and I'm sure I'm not getting any deflection in the linkage. Props are AB sharpened and balanced so no issue there.

I have remedied the problem by adding .020" shim behind one side of the struts to get about 1* angle in them but can't help to think that this is just a band-aid for some other problem.

What do the twin guys think about this?

-Buck-

Buck add more shim to the struts to get them to 2* and the boat should run as stiaght as an arrow and you should be able to leave the rudder straight

 

[Tom Foley]

sounds like the rudder is too short a others have mentioned . This problem is amplified by the lack of rear sponsons to add tracking . I bet if you go to a wide profile full depth rudder it will get much better .

 

[Terry Keeley]

What props?

H-38's cupped up.

-Buck-

I was running H32's on my single and got away from them because of that, 1662's run a lot smoother and straighter. Ever try some 1667's with a little cup?

 

[dr gary turner]

Increase the angle of attach of the right sponson and flatten the angle of attach of the left sponson. I have done this for 15-20 yrs and it works virtually for all hydros. I learned this from Bill McGraw many years ago!!!!!!!!!! GTT

 

[AndyBrown]

Buck,

A couple of guys have been correct about one or two items, but I will give you the shopping list of everything to do to your SGX.

#1. Set the right sponson tip 0.100" higher than the left sponson. You can adjust them up or down, but always maintain

the 0.100" differential.

#2. Set the right strut at 1 1/2 degree negtive.

#3. Set the left strut at 1 degree negitive.

#4 The stuts could be a little more or less, but maintian the 1/2 degree defferetial. The bottom of the struts must be lower than the bottom of the center sponson. About .030".

#5. Set the break lines of your turnfin parallel to the deck line of the tub.

#6. We have always used the 1.1" blade. 5.3" long is enough. Too long will glue the front sponsons down in the turn.

This can cause the back end to come around as you accellerate out of the turn.

This area is important. Too much strut angle will cause the props to lift sooner than the front sponsons lift upon accelleration out of the turn and cause the boat to pull hard right.

Out of sync. engines will also cause this. If the engines are not together you will be chasing your tail, so be sure to keep them in sync.

#7. Good servo power is a must.

#8. Keep the hull running clean, never too tight. Keep Sharp sponsons too! Sand the bottoms!

#9. Make sure the drive dogs are not pressing against the struts when under load. This will transfer torque to the hull.

#10. No thick shaft grease. Same effect as above!

#11. Use only stock SGX turnfin. DO NOT round off or smooth off the leading edge bevel. This is very IMPORTANT!

#12. Keep the Struts Square to the transom. Do not angle them.

I may have forgotten a few things.....it's late.... I may add more later.

Good boating,

Andy

 

[Buckshot]

Buck,
A couple of guys have been correct about one or two items, but I will give you the shopping list of everything to do to your SGX.

#1. Set the right sponson tip 0.100" higher than the left sponson. You can adjust them up or down, but always maintain

the 0.100" differential.

#2. Set the right strut at 1 1/2 degree negtive.

#3. Set the left strut at 1 degree negitive.

#4 The stuts could be a little more or less, but maintian the 1/2 degree defferetial. The bottom of the struts must be lower than the bottom of the center sponson. About .030".

#5. Set the break lines of your turnfin parallel to the deck line of the tub.

#6. We have always used the 1.1" blade. 5.3" long is enough. Too long will glue the front sponsons down in the turn.

This can cause the back end to come around as you accellerate out of the turn.

This area is important. Too much strut angle will cause the props to lift sooner than the front sponsons lift upon accelleration out of the turn and cause the boat to pull hard right.

Out of sync. engines will also cause this. If the engines are not together you will be chasing your tail, so be sure to keep them in sync.

#7. Good servo power is a must.

#8. Keep the hull running clean, never too tight. Keep Sharp sponsons too! Sand the bottoms!

#9. Make sure the drive dogs are not pressing against the struts when under load. This will transfer torque to the hull.

#10. No thick shaft grease. Same effect as above!

#11. Use only stock SGX turnfin. DO NOT round off or smooth off the leading edge bevel. This is very IMPORTANT!

#12. Keep the Struts Square to the transom. Do not angle them.

I may have forgotten a few things.....it's late.... I may add more later.

Good boating,

Andy

#1 - I'll try this, it is set up were both are the same and they are 0* across the top of the sponsons.

#2 thru #4 - Both struts are currently set at 2* and the end of the struts hit the table. I'll take .5* out of the left (outside) strut and drop both struts down so that the ends of them are ~0.030" below the bottom of the shoes.

#5 - All my SGX's are set up this way (always re-checked after sponson adjustments)

#6 - I have been using the 1.1" blade set 3.75" below the tub at the transom as per instructions. I tried the 1.2" blade set to the same length but everything else was still out of whack so no noticeable change could be seen. As you can see from an earlier post that blade got bent, and I ordered another one yesterday under the advice of another fellow boater. I guess I'll keep the 1.1" blade on it for now at the current length till it is deemed necessary to go deeper or larger. If it still tracks hard right I'll take some angle out of the struts. (From 2* & 1.5* to 1.5* to 1*?). The motors are synced up nicely! They sound strong and I think I have them tuned up well. (Sound like one motor on crack coming down the straights B) )

#7 - I don't thing I can get anything stronger and I'm using the biggest batt. pack I have ever seen. But what do I know.

#8 - The front end will dance a little at speed - it is far from tight but not crazy loose. All my rides have nice sharp wet sanded running surfaces.

#9 - Always!

#10 - I don't think the lube that I'm using is too thick but could be wrong. I'm using Mercury marine lube called 2-4-C for this boat and my 90/100. (Thinner stuff on the smaller boats)

#11 - I've never modified any of the SGX's turn fins. The only thing I did do is add a stiffener behind it like the one you guys sent me for my 90/100 boat. The twin didn't have one and I felt that this boat really could use one if I ever got it running at full song. What are your thoughts on this addition?

#12 - OK I'll go back to square and try everything else that has been discussed here. When I did this a few weeks ago (add the shims to the struts) it helped a bunch but like I said earlier I couldn't help but think this was just a band-aid for other issues.

Any other advice is much appreciated. I probably could have just called you or John but I hate to bother.

Regards,

-Buck-

 

[Buckshot]

sounds like the rudder is too short a others have mentioned . This problem is amplified by the lack of rear sponsons to add tracking . I bet if you go to a wide profile full depth rudder it will get much better .

Umm.....It dose have rear sponsons. They might not be as big or in the same location as yours.

-Buck-

 

[Buckshot]

Thanks for all the input. Ran the boat again yesterday and it is running great!

-Buck-

 

[Tim Kish]

Thanks for all the input. Ran the boat again yesterday and it is running great!
-Buck-

How did you fix the problem Buck?

Tim K

 

[Buckshot]

Thanks for all the input. Ran the boat again yesterday and it is running great!
-Buck-

How did you fix the problem Buck?

Tim K

I set the boat up as Andy stated in the earlier post.

It is amazing that 0.100" on the sponsons and 1/2 deg. at the struts along with 1/2" longer rudder will tame an big, heavy, 80 mph boat.

-Buck-