stv cavitation problem

[jerremy powery]

guys anybody out there that can help would be appreciated have a stv with a v8 on the back of it been playing with engine height set up and differrent props ,also put on extention plate on the exhaust some one said to try it has'nt helped so anyone with a idea let us know thanks:)

[Techno]

Whats the cavitation problem?

One wild guess is someone filed the blowout ring off the LU.

[JAGPILOT]

Techno:
The problem with this rig is that it cavitates heavily off the line even w/ a 24 ET, 22 or 24 worked Yamaha.
This is a F1 V8 on a 6 inch midsection with a Galen Burden adapter bolted to the Bobs noseconed CLE you see in the photo. You can't see it in the photo, but the blowout ring is still on the L/U.
The Hull is a lightweight (475Lb.) STV and the splitshaft is currently about 1/4 to 1/2 inch above the pad but we've tried it higher. Setback is about two inches with the Bobs Jack plate turned inside-out.
The current thinking is that the tuner, which is swept back to blow the exhaust out the snout at the back, is blowing exh. gas around the prop. They put a cavitation plate extension on but the problem is still there. Someone said that since the adapter between the foot and the mid is solid and doesn't allow gas to blow into the foot, then the problem might be an air vacuum around the L/U (a large bubble of air?) on the launch causing the prop to lose bite.
This one has us all stumped. Let us know what you think might be the problem. Thanks.

[Raceman]

I don't believe your problem is exhaust related. Post more pics of the rear, showing the whole motor.

I haven't seen the problem you're describing, but it seems obvious that it's got to be setup related.

[jerremy powery]

race man heres another pic of the setup,thanks for the help jag on giving better details on this problem:)

[jerremy powery]

another shot raceman

[stvhelm]

I dont think your exhaust is the problem either. I have a plate I made that completely covers my exhaust in the offshore housing. It lets all of the exhaust out down through the gear case. I notice very little difference in planing off with the plate on. I basically made the plate for noise reasons just in case someone gave me a hard time about it.
One thing I do notice. If I want to plane off, I find that if you try to ease onto the gas at the very begining and then start to mash it, it will cavitate the hell out of the prop. But if I dump it hard then immediately back off, it will jump out and plane off with out ever going over 4k rpm's. Its possible your motor isnt giving you enough snot at the second you hit the gas, but rather starts making power just when the prop breaks loose. kind of like a hesitation. I think if my motor were to hesitate it would cause it to cavitate too. just a thought. I could be wrong though. I remember seeing other v8 boats that would cavitate alot too. I think my Merc with my overweight hydraulic jackplate weighs pretty close to your F1 v8 so I doubt weight is an issue either. thats a tough one.

[AnthonySS]

WOW...not sure what to say....but most boats with a over hub exhaust designed props will boil water off the start.

Maybe you don't have enuff prop (blade area / pitch)?

When the boat does finally get going...what percentage of slip are you getting???

STV's are VERY freindly out of the hole and the techniques HELMUT mentioned also work.

You can even feather an STV onto to plane with very little, to no cavitation.

ANYWAY...lets keep talking!!!

[JAGPILOT]

Thanks for the help guys.
We're not sure what kind of slip it's getting because it doesn't have the right pulse tach so not sure of exact RPM's but the boat has run 109-110 MPH at the end of the 1/4 on GPS so we think it's biting pretty good. Also, when it hooks up you can actually feel the boat launch forward. Feels like a 30 cleaver off the line except when it hooks it accelerates real good.
Just a thought about the Blowout Ring. I have the same L/U (CLE/Bobs) on my rig (a Lt. weight Mirage RR with a F1 Mid/Drag P/head) and I cut the blowout ring off and my rig hooks like hell off the line. Do you think we should try taking the blowout ring off?
I'm thinking that we should bore a hole in the adapter to let any trapped air in the foot out as the exh. cavity in the footend is completely sealed off from the midsection by the adapter. What do you think?
Best case scenario: (and I've already given this advice) ditch the V-8 and put a 2.5 Drag on the back, Amen.;)

[AnthonySS]

...GLAD to know I am not the only one with those "Black" thoughts...

anyway ..trying to have an open mind here:D

[JAGPILOT]

Anthony:

"Black" thoughts often win wars... Hee! Hee!

Anyways, I have a correction to make. Apparently, the adapter does NOT seal off the exhaust cavity in the L/U so I guess that eliminates that theory. Probably just needs a chopper or a four blade on it to get proper bite.

[AnthonySS]

...hoepfully it is just a prop issue.

Any pictues of the full rig and what does that OMC weigh?

[JAGPILOT]

Anthony:

I don't actually have a photo of the entire rig. Jerremy, if you do could you post it? The motor I think weighs around 400 Lbs. F1 V-8s are quite a bit lighter than the Looper V-8s.

If you used to follow DSRA racing you would probably know this boat. It was the Monty Racing blue, orange and yellow rig and Gordon used to run a Looper V-8 on it. Gordon rerigged it with the F1 V-8 and sold it to friend of mine who collects race boats as a hobby. This guy at last count had five Drag motors mounted on his various rigs, so I thought it would be a no-brainer to bolt one of them suckers on the back. That would cure the problem for sure. Then all he'd have to do is sell her to me! :D

Dale, You reading all this old boy? Put that V-8 on a stand in your living room and throw the "remains" my way! Heh! Heh!

[jerremy powery]

heres a shot of the bottom looking back at the gearcase

[jerremy powery]

one from the back

[jerremy powery]

from the front

[Techno]

One thing I noticed is the water hose is throwing a large shadow in front of the prop, don't know how else to say it. Most are horzontal above it. May not be the instant launch problem but would seem to start turbulance soon after.


Wonder if you could move the fitting to the top of the LU rather than the side.

Another idea is to have someone take a picture right back there on launch, maybe the cause will show itself. Different boat of course.

[trbocharge]

Just an observation: Jeremy, what kind of power is your motor making?..........

On a hull that light it seems that your top speed at the end of the 1/4 would be much higher........something doesn seem kosher there.........maybe a slip problem?..........

Just an observation

[DaveR]

I hate to say something stupid here, but could it be ventilation instead of cavitation? Maybe a larger plate above the prop to keep water around it would help.

[Propman]

Just a guess but, I think it's your skeg, Looks like someone increased the size and put quite a torque tamer on it. On your boat it doesn't need a huge flare to counteract torque steer. Another thought, on the OMC equipted drag boats I've done the prop on they need a big cup, the bob's cone on the OMC isn't as clean as a Merc setup.:cool:

[jerremy powery]

thanks for the info guys i'm in the process of rebuilding all carbs and replacing intake gaskets on the motor and will be testing it this weekend:cool: trbcharged :im not sure what horse power the engine is making because we dont have a dyno here on the island,but the engine has turned a 24 et 9600 rpm on 1.75 gears Propman:ive had that thought on the skeg will try a next gearcase next time around if things dont work out this weekend

[ShipBear]

Man, thats a nice looking Rig, like that paint..
Think you could post a few more pictures of the Hull..?

Thanks, Larry

[AnthonySS]

That is one special rig...congrats on it and thanks for sharing those pics...

Do me a favour and STOP BY STVowners and post some pics in the reader rides section...we would welcome them too..

sounds like a bunch of decent advice guys and my hope is that a prop will solve things also...JACK that motor up...with the power you have....it may run real sweet in the 1 1/4" to 1 3/4" split shaft height.


The higher you go closer to 1 3/4" you'll need more rake angle to hold the boat but at 1 1/4 to 1 3/8" a low rake prop (10-15°) should work fine

KEEP US INFORMED THANKS

[Flat Out]

V8s are great. The guys are right on. From the pic of the lower unit from the front it looks like you can still raise the motor.
Do one thing at a time in small incraments. There is no reason that you should't get a great holeshot with that rig. Fine tunning your set up is the key. That motor should be approx. 475hp.
Nice paint job.
Good Luck, keep us posted.