Scary High Speed Nose Wobble, Please Help

[DeputyDawg]

I wrote a thread about a buddies boat breaking a new Aggressor droop the other day and tearing up the boat. Well, he got the fiberglass work done(hasn't painted it yet) and the boat is now sporting a Berkeley Droop and back on the water. We went to test it today and found an all new problem. When the boat hits 100ish or so mph the nose starts to wobble/make little circles and figure eights and it is a little discomforting. The boat is a Kachina Tunnel very similar to the Southwind and Placecraft Tunnels and has an 8-71 blown 509BBC in it and is turning a "worked" stainless AB cut Dominator impeller 6600rpm. We tried the ride plate from one degree up to one degree down and all in between but couldn't get rid of the wobble. Also played with the diverter a little and it didn't get rid of it either. The intake pressure is 35psi when the boat is in the 95-100 range and doesn't seem to change when the boat starts moving around. The boat also has the little plates over the tunnels which are set at two degrees up and I was told not to mess with them way back when I stilled owned this boat before I sold it to my buddy. I never had that much power in the boat and it only ran 83mph when I had it but never did the nose wobble thing. We got a best GPS speed of 105mph out of it today, and the boat set down nicely on shut down and never acted wiered or anything other than the high speed deal. Any thoughts fellas?

[flat broke]

I'm not a tunnel expert, but I could swear I remember BP talking about that little "dance". I'd PM him, or do a search for threads he's posted on about tunnels
I found a quote from him that sounds very similar to what you experienced... Thread ID 19192
originally posted by BP
i don't know about that stuff jer, but around a hundred, with the boat up on the center sponson, so it's flopping back and forth from one outer sponson to the other, it kinda feels like it's doin' the hula. wakes ya up a bit the first time, but after awhile, it ain't goin' fast unless it's dancin just a bit
brian, i'm turning 62-63 with a B.
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bp
Hope that helps,
Chris

[Duane HTP]

Sounds like you may have a hardware or pump loading problem at those speeds. It you do, it can break up just about anything you put back there. Droops, Snoots, Pump Suction Housing, top loader, or impeller. Be careful. Give me a call, and I'll see if I can help you. This is too involved for typing at the speed I type. 316-794-8616 Duane HTP

[moneysucker]

Good news, Both mine and my brothers SWTDs do the circling of the nose from 98 to 100+ it is when the boat is flying and rocking back and forth on the outside sponsons. It definately took some getting used to.

[Nucking futs]

Just my 2 cents here, but the side to side hula is normal and that is when the boat is starting to work. The "other"dance with the nose is a different animal.Get the nose down a tad bit.Sounds like a hardware chang is in store.
Tim

[DeputyDawg]

Today we took the back cut shoe out and put in a regular cut shoe and that helped a little. We then wedged it two degrees down and it still rocks side to side but no more wobble or figure eight moves but we lost three miles an hour after making the shoe/wedge change. The boat ran a best 106mph today before the changes, and ran a best of 103mph after. We are going to try dropping the shoe down a little bit tomorrow and running it again.
We also played with the little plates over the tunnels and I honestly can't tell that it makes a damn bit of difference what you do with them. We didn't get real crazy with the plates but we went from two degrees up to one degree down and could tell no difference in handling or speed.

[bp]

you're not gonna get rid of it changing the shoe. as duane mentioned, you don't want the boat unloading at that speed. the 2 deg b/c shouldn't be creating too much lift at that speed, but too much attitude combined with too much lift could result in driving the boat out of the water. you really don't want to do that, but it doesn't really sound like that's what's happening.
it would be interesting to see a pic straight on of the transom. what you're feeling what is "somewhat" normal. i say somewhat, because there -should- be a way to minimize or eliminate it, and i've done that to a degree on my boat. the "hula" isn't a good thing, because that will slow the boat, and i have eliminated that. the last time i felt that was 3 years ago.
basically, you get to a certain speed where the boat lifts to the point that it is solely riding on the center sponson, and the outter sponsons act like outriggers. one sponson will touch the water and that tunnel will fill with air. when it's pressurized enough, it lifts, causing the other sponson to drop into the water. the other tunnel fills with air, and you go back and forth this way until you slow down. each time a sponson hits, drag occurs. if you have enough attitude, the sponsons will really dig in when this happens, causing some real drag to the point you feel the boat going backwards (at 100+), hence the hula. the boat will continue to go straight as long as you have flow through the pump, but it's not an entirely comfy ride. too much nose up attitude makes it really uncomfortable.
if you look at the IHBA pics thread, sunday finals, and scroll down a ways, you'll see a pic of me near the end of the track. the attitude is about as high as i ever run it; this is around 104 and change. the only thing holding it back is the drag of the bottom of the sponsons you see in the pic, and can be easily addressed with setup. if you go to "dragboats.com", randy krohn's site, click on the picture section, look under the name Lance Matty. about half way down, there is a pic of Lance placing the nose of his boat about 1' past my nose at the finish line at the phoenix opener this year. look at the attitude of my boat, the water under the boat, and the water under the sponson. very different setup for firebird than chowchilla, due to the water. in that firebird pic, i was running 106 and change, and if i ran that setup at chow, it would have been significantly faster, but i run ME :cool:
the dancing has been all but eliminated on all these passes, that i felt up through the end of last year. i get just a -slight- bit of wobble past the stripe, but nothing like before. things can be done to make it better.