Swapping the rear at high speeds.

[Bahner tunnel]

Has any one ever had this problem in a jet boat? Today at the lake , I was testing a new intake grate , and while running at about 80 the rear of my tunnel V hull started to swap sides back and forth about 2-3 feet. Besides my co-pilot almost having an accident in his shorts as I slowly backed off of the throttle and it stopped. Is this caused by to much intake pressure , making the back of the boat want to lift out of the water? It has never done this prior to today but this is the third trip with my new motor and at these speeds . (old motor only ran around 65 mph)Any ideas ? Also with a "blocked center" loader grate , how do you know how much of the center is to be blocked? ( how much from the trailing end of the blocked ceter to the leading edge of the shoe)? Thanks.

[Unchained]

I had a similar problem caused by too much intake pressure. The boat wasn't moving side to side but just hopping up and down at speed. Take some shim out of the shoe. Try removing .060 at a time. What is your intake pressure gauge reading? I am getting 30lbs at 103 mph with a Cheyenne tunnel, and the boat is real stable.
I have a V blocker loader with 1/2 of the blocker removed.

[ChetCapoli]

hey unchained,
how did that A/T snoot work compared to the actual droop you had?
thanks--chet

[greg shoemaker]

With blocked hardware shoe depth becomes very critical. To much hardware will cause a nose dropping , lifting the tail problem. Not enough hardware as well as blocked hardware will cause a skating problem. This problem can be solved by adding shoe in small amounts or opening the blocker. Most of you tunnels need very little help lifing the boat out of the water so blocked hardware is usually used sparingly. Greg

[disco_charger]

When my Dad had too much shoe in his Bahnner Tunnel, it tore the transon off of the back, so take Greg's advice about adding it in very small amounts. Be careful with that hull, we had to move the motor forward a few inches to keep the nose from coming over. That hull is probably pretty light, so keep that in mind as you creep towards 100.
Disco
[ September 30, 2002, 08:52 AM: Message edited by: disco_charger ]

[Bahner tunnel]

The shoe is a back cut stlye with about .100" removed from the leading edge ,going to nothing at the ride plate side. The loader is blocked 2/3 the way back, around 4 1/2" open for water flow. I'm running a "B" impellor . With the open loader , it would rev. 6100 rpm. Now with it blocked off it revs. 6800 rpm. At this rpm the top speed is 76 mph. This boat is proof that the set-up is everything(or lack of). I'm in the 700 h.p. range (13-1 roller 496)and it runs with my buddies open bow 21 Advantge. Being the shoe is already cut , does the answer lie in the loader ? Or should more shoe height be removed as well ? I need to install a pressure gauge to "know" what is happening at speed. Greg S. do you have a loader in stock that would work for this , or is each boat it's own animal? It looks like it is going to be a long winter at the lake.
[ September 30, 2002, 09:26 AM: Message edited by: Bahner tunnel ]

[Bahner tunnel]

Unchained, When you had this problem , did your top speed really suffer (besides being a little dangerous to drive)?
[ September 30, 2002, 09:57 AM: Message edited by: Bahner tunnel ]

[Unchained]

Bahner tunnel, You need a pressure gauge. I got a liquid filled one from Northern Hyd. for $16
Don't get a gauge that's not liquid filled as they'll bounce all over.
Yes, my top speed was way off with too much shoe. It was like the phrase I heard from the old RJB site about " dead cow dragging "
I think you need to put in a thinner shoe. I don't know about the back cut.
ChetCapoli:
hey unchained,
how did that A/T snoot work compared to the actual droop you had?
thanks--chetChet, I'm real happy with the all the AT equipment that I installed from the bowl back. It's hard to tell the difference in hard numbers because I didn't run the drags this year until I
had already switched.
From my best assumption I would say that I am getting the best accelleration ever with the
AT equipment. Maybe that is directly related to the fact that I'm not bending the water as
much as with the droop.

[Bahner tunnel]

Thanks Unchained . I'll try to get a picture of the botton of this thing so you can check it out.

[bp]

Bahner tunnel:
Thanks Unchained . I'll try to get a picture of the botton of this thing so you can check it out.bt, interesting to look at the back of your boat; looks similar to the back of my sw.
at top speed, i'm getting up on the center sponson, and it feels like it's just moving around back there a bit, going from one sponson to the other. if you're getting that much lift, one sponson lifts and air is released. then, the other side lifts and air is released, and back and forth you go. my data shows that i'm loaded while in that situation (35-40lbs), so that is not a problem for me.
i'm also running a b/c shoe and center blocker. my blocker was built by mpd, with the changeable block.
from what you say, you may be getting more lift than you want at the speed you're running. you may want to try less b/c, or head toward a flatter shoe. since you stepped up, you probably need less lift at the new speed.
good luck.