Help: Bouncing in an old Allison GT(R)-20

[Slider]

We just finished our GT-20 project and would like some setup advice. It is an old Allison R-20 (Original GT-20 Stringer boat) with a 2.4 Merc on a 15" mid section. Depending on the definition of setback, the engine has about 11" from the transom and 15" from the pad of setback.

The problem we are having is that around 6,000 RPM, the bow just starts jumping up and down. If trimmed up high enough, we can drive through it with a 3 story rooster tail... The prop is about 1/8" below the pad. We really don't want to raise it any further. It started at 1/2" below, then was moved to 3/4" below and got worse.

As of last weekend, the boat with driver weighed in at 1,500 lbs. The bow is EXTREMELY light... All of the wieght is in the back. We have ran with a 29 Spinelli, a 26 (don't remember), and a 24 drag (don't remember). The boat seems to like rough water. Any help would be appreciated. The boat can be seen at http://www.mayonet.net/finished.htm .

 

[JR]

bouncing A boat

There was one that was running very well at the Eustis and a Critchfield that was identical to it also. You might ask Scott or Kevin who they belonged to. It really sounds like someone has reshaped the pad on your boat, though.
You also might be able to get some good info from Bill Scates at Allison.
JR

 

[Evin Rude]

Slider, first of all, your boat is very nice. Your resto job looks fantastic.

I had a GT-20 back when they were new, in 1979. I kept it for 8 years and ran it with several different motors on it. I never had the problem you note.

I did run my motor a bit higher than you are trying, about split shaft to 1/2" above the pad (with the LWP inline gearcase). I had a hydraulic jack plate with 6" of set-back. I really liked a Hoss Triton 1 on that boat. Good holeshot, and it was always the best on top end. I guess nowdays, a Lightning ET might be a good prop for a GT-20, the Tritons cost WAY less though.

At first, I had a very early 1979 2.4/200 on it that was mostly stock. It would run about 75-78 mph and the gearcase would blow out. This was WAY before the LWP nosecones were available.

As soon as they were released, I bought a 1980 2.4/225 Merc. I then extensively modified the 2.4/225 and that is what I ran on the boat most of the time I had it. I adapted an inline 6 150XS gearcase to the V6 and with the 2:1 gear and the LWP, it would run around 95-98 mph with a 28" pitch Hoss on my Keller speed-o if you really strung it out.

These boats were built before Allison put the lip on the trailing edge of the pad. I am thinking adding one would be easy and might improve the problem you have along with increasing your performance at high speed.

 

[ALLISONMAN]

dumb question but have you check your swivel bracket for play??? grag motor at cavitation plate and pull up and down!!! I ran in a ss2000 that had a detweiler jack plate and those plates have aton of play and it wanted to do the same thing you are talking about!!!!

 

[SSTOM]

GT 20

I had a 78 GT 20 w/a 93 2.5 200 10" of set back and ran the motor 1/4 below to enen w/ the pad. I did get some bouncing and fuond that the bottom was not level the pad had a few high and low spots after I got them out the boat ran fine. If you would like to contact me I might be able to help you with your boat my # is 1 502 803 5638 if no awnser leave a message and I'll call you back. Thanks Tom 8)

 

[Slider]

JR:
I was driving the Red Critchfield and the blue GT-20 is the boat that I am talking about. Richard is the proud owner now. The Allison likes rough water so that is why it appeared to run well at Eustis. Keep in mind, we had less than 8 hours on the engine and less than 4 days of testing before the Eustis race

The boat was flipped and blueprinted prior to rigging. The edges are sharp. All of the props that we are running came off of a Mirage. We are going to try the 26 Hoss Trition that I am running on the Critchfield this weekend. It was worked for an Allison before I purchased it. I think we will raise the engine a little more.

We hit 81.1 GPS this past weekend. It was squirly at best.

Thanks for the suggestions. We will give them a try this weekend.

Mark

 

[Slider]

Well, things aren't looking too good. The best performing prop is a small ear 26P chopper. And that isn't saying much. We tried a 28P ET and it faired well, but we still can't get over the bouncing. What should be our next move? More or less setback??? Move battery to front? Set fire to the whole thing?

 

[Rocket]

Ok now,, take for grated that I'm a,,, "rookie" Allison guy,,, But,,,,

I was just reading this post and was wondering,,,,,,

if you might NOT have a "HOOK" or "Warp" or "something" in the bottom of you hull???? :?


Could it do this? :shock:

It does on Tunnels!!!!! :roll:







Rocket

 

[Slider]

It could be done. I have heard a lot about this hook, but never seen it. I don't think that the Allisons this old ever had a hook. But, this is an unusual boat. I would be interested in some pictures of a late model pad if anyone has some.

 

[Rocket]

It could be done. I have heard a lot about this hook, but never seen it. I don't think that the Allisons this old ever had a hook. But, this is an unusual boat. I would be interested in some pictures of a late model pad if anyone has some.

No,,, I was talking about your boat having something wrong with it and causing it to bounce! :shock:

With the age of the boat,,,,I was thinking that if it hung in a boat house for a long time (I'vee seen it to other boats) on straps!
Or sat funny some how,
Or maybe somebody fixed the bottom from damage but did'nt do a good job! :shock:
Or maybe somebody tried to make it BETTER! :?



I was jsut think'n!

 

[Slider]

Oh No, the boat was flipped and blueprinted this winter. It is just extremely light up front. All of the weight is in the back. We are trying to figure out our next move seeing as props aren't going to solve the problem by themselves.

 

[ALLISONMAN]

I WOULD TRY AND MOVE THE MOTOR IN OR OUT AND SEE IF ONE HELPS ITS OR IF ONE HURTS IN SOUNDS LIKE THE BOAT IS NOT WANTING TO CARRY THE WEIGHT AT SPEED

 

[82alli]

GT-20

Slider, Talk to SSTOM. He had a GT-20 with 10in set back. He can help.

 

[froggy]

Slider,

When your boat was designed and built, a 6" offset pate was about as big as it got. I've seen several of the old Allisons, when they were new, running in the high 80's to 90's with no more than 6" of offset plate. You just might want to bring your motor up some.

froggy