Cat skipping

[Simon.O.]

I have a problem with my small Cat that just wont iron out with a lot of adjustments.
The best description is skipping. From the side looks similar to porpoising but from the rear you can see that it is different.
The rear lifts, blows out the prop and skips to the left. This cycle happens about 3-4 times a second. It looks very funny but i know that I am not getting the best speed.
Setup is
26" Balsa cat, my design. No steps. 3/4"wide ride pad and small spray rails at the outer edge of the sponson.
Feigao 8S on 2S lipo 432 Octura prop
Cg is at 30%
Strut is neutral and has been up to 3/16 above the bottom of the sponsons.

Since I went to lipo this has shown up. I have added weight to the front to get the Cg set.
I can fit a tunnel dam again as i had one in once for experimenting.

Some options please, before this hull becomes a dust collector on the rack.

[Fruity]

Simon,
it sounds like the prop torque pushing the back of the boat across, is is also refered to as prop walk, some cat struts have a small blade or fin like a turn fin below the strut to stop this happening, it may be worth just putting a small turn fin inside each sponson at the back to help keep it on track when it really gets aired out.

Check the link out and you will see the fin that I am refering too.

http://www.offshoreelectrics.com/pro...t-oc6str-130da

Good Luck....Fruity

[Fluid]

From the description most likely the problem is a prop that is too deep. This lifts the transoms and the boat comes out of the water - then prop walk can be bad. If this is what's happening then a skegged strut won't help. Raise the strut another 1/8" and see what happens. More may be needed but without seeing your cat I can't say.

You didn't happen to shorten the rudder recently did you? Prop walk in cats can usuallly be controlled with a longer rudder. Too, move the CG to 32-33%. Any angle to the strut?



.

[Diegoboy]

Fluid,
What I wouldn't give to pick your brain for just one day!

[ED66677]

yep too much lift and prop walk! try changing the prop angle so that it will point upward.

[Simon.O.]

From the description most likely the problem is a prop that is too deep. This lifts the transoms and the boat comes out of the water - then prop walk can be bad. If this is what's happening then a skegged strut won't help. Raise the strut another 1/8" and see what happens. More may be needed but without seeing your cat I can't say.

You didn't happen to shorten the rudder recently did you? Prop walk in cats can usuallly be controlled with a longer rudder. Too, move the CG to 32-33%. Any angle to the strut?
.

Jay, I was hoping you might tune in on this one.
The strut angle is flat.
If the rudder were any deeper then tthey would need to dredge the pond for it to run. It is a full prop dia below the lower prop arc. This is the deepest rudder in the fleet. I can get away with a lot less on a few of the others like the Canard.
Cg is at 30 and I can easily send it to 33 - 35% one more dead Ni cell would suit as ballast.

I'll raise the strut to a previously unseen height and give it an honest run in the morning. While I am there I can move ballast to move the Cg.
ED
I have enough run time to be able to get a strut angle change in there too.
You should have seen the Canard today.... a full 6 feet of air while backward fliping. MAJIC !!! It hauls a$$. It would give my 2S rigger a run for money.

[ED66677]

I have enough run time to be able to get a strut angle change in there too.

change only one thing at a time, as you have no step, I would set the COG around 25% try it and next lift the rear of the prop shaft 1mm at a time!

You should have seen the Canard today.... a full 6 feet of air while backward fliping. MAJIC !!! It hauls a$$. It would give my 2S rigger a run for money.

lol

[Jesse J]

Great thread! one of the biggest mysteries I have encountered yet. I have it in my 14" cat and in Dad's Fastech. I will tell him to raise the strut (as much as he can) and go directly to raise the strut on my minicat. Thanks for the thread Simon and posts by all - what a great forum! Just a bunch of willing and really smart people with a lot of experience!!! Rock on all

[Simon.O.]

o.k the shaft centre line is now a full 6.5mm or 1/4"above the sponson bottom
This a 24"cat with a 20mm or 5/8"ride pad.
There is also the issue of the spray rail that I added to give it more lift when I was still running Nimh cells up to 8 weeks ago.

I will now remove the spray rails and run it with the strut as set and see what happens.

This hull has had a tunnel dam fitted beforte as a part of testing and may get it again.

I will get it sorted one day and will let you all know how it happens.
The why I will work out later