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steves86ta

  • Karma: +41/-0
Cavitation Talk
« on: March 01, 2016, 04:13:37 pm »
Since the boats in storage, and im bored and exercising my google-fu today. I wanted to make a thread asking a couple questions. More for knowledge/education than anything

I have a Rogers, 5.3 LS powered.

According to the impeller chart im making somewhere in the neighborhood of 400-425 hp on motor @ 5200 rpm, and about 600 on a 150hp shot spinning about 6200 rpm or so with a B- AT impeller.

The boat runs 70 on motor (repeatably have seen higher a couple times) and 85 on the bottle on GPS.

Heres where my Cavitation questions come in.

The Pump is a JC suction housing, Dominator Bowl, AT aluminum impeller. No inducer, no stuffer, everything is stock. Has a loader by GS i dont know what kind, and i believe a Backcut shoe. No Diverter, Full droop with a set of adjustable wedges. Wedges are setup at 3*up and i also have a rideplate. I dont know the angle of the rideplate somewhere around 3.75 degrees

Before the intake there is a pretty bad transition, think a Flat intake in a rounded hull. And not blended in. I have corrected this, but have not taken the boat out since.

I just put a brand new AT impeller in that was cut down for me to a B-, other than that the impeller is stock and non-detailed. I put a brand new non shouldered stainless wear ring in with this impeller. Clearance measured out to about .024" total. I did not check for runout.

What happens?

If i flat foot it from idle, the boat will literally bounce off the rev limiter as long as i keep my foot flat (rev limiter is 7200). The boat does move and accelerate)

If i Flat foot it from idle, here it come "unhooked" and back off the throttle, hold a steady throttle about half or so. Then stab it, its fine and pulls to my max RPM and holds

If i dont flat foot it, but get on it spiritedly wait til the boat planes then stab it she loads well and takes off pretty nicely. Will pull hard and run to WOT no problem

If i'm cruising from plane (say 3-3500 rpm or 45mph or so) and stab the throttle. The boat pulls to WOT and holds hard and accelerates very nicely

If i wait until the boat planes, and grab the Nitrous it hits hard as hell and pulls to the max RPM and the boat accelerates as expected.

I have not hit the nitrous from idle because i know the boats going to nail the rev limiter. Something i DO NOT want to do on the bottle.

Basically the boat blows the pump out from idle, but once im moving its fine.

I have a couple of thoughts, but id like to hear your ideas.

1. With no diverter, i go so bow up (from the droop/up angle) that it uncovers the jet intake, causing it to suck air
2. my pump is shitty and i need to stop being cheap and have it built
3. I need to buy a diverter (again stop being cheap)
4. The transition into the intake was the issue, everything will be fixed when you take it out
5. Im an idiot, and doing everything wrong
6. LS engines wont work in a jet boat

Last one is a joke, but id still like to hear everyones thoughts.
« Last Edit: March 01, 2016, 04:15:38 pm by steves86ta »
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jyeager

  • Karma: +62/-0
Re: Cavitation Talk
« Reply #1 on: March 01, 2016, 04:19:24 pm »
Your unloading, throw a diverter on there. You also may gain a few mph.

Also there's no need to grab the nitrous right off the hit. Wait a second or 2


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jyeager

  • Karma: +62/-0
Re: Cavitation Talk
« Reply #2 on: March 01, 2016, 04:20:06 pm »
Give her 2 degrees of down and 6 up 👍🏻


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steves86ta

  • Karma: +41/-0
Re: Cavitation Talk
« Reply #3 on: March 01, 2016, 04:30:56 pm »
Give her 2 degrees of down and 6 up 👍🏻


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You mean, wedge the droop down 2 degrees, and set the nozzle stop at 6 degrees up?
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jyeager

  • Karma: +62/-0
Re: Cavitation Talk
« Reply #4 on: March 01, 2016, 04:43:45 pm »

You mean, wedge the droop down 2 degrees, and set the nozzle stop at 6 degrees up?
Put a diverter on the thing. Give it 2 degrees of down nozzle on the hit. And 6 degrees of up nozzle once you are going.
Make a stop for the up nozzle( in red)
Make a stop for the down nozzle (blue)


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GT Jets

  • Karma: +192/-0
Re: Cavitation Talk
« Reply #5 on: March 01, 2016, 04:47:21 pm »
How new is the pumps packing? Is the cleanout tight?
  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
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If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray

steves86ta

  • Karma: +41/-0
Re: Cavitation Talk
« Reply #6 on: March 01, 2016, 04:49:11 pm »
Put a diverter on the thing. Give it 2 degrees of down nozzle on the hit. And 6 degrees of up nozzle once you are going.
Make a stop for the up nozzle( in red)
Make a stop for the down nozzle (blue)


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Any ideas on how to do a stop with a manual diverter?
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steves86ta

  • Karma: +41/-0
Re: Cavitation Talk
« Reply #7 on: March 01, 2016, 04:50:18 pm »
How new is the pumps packing? Is the cleanout tight?

Brand new rope seals. They drip water about once every 30 seconds or so

Clean out is very tight with a brand new o ring/seal thingy. Also permanently closed now that it has transom plates. If i suck up a rope im effed
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GT Jets

  • Karma: +192/-0
Re: Cavitation Talk
« Reply #8 on: March 01, 2016, 05:08:48 pm »
Brand new rope seals. They drip water about once every 30 seconds or so

Clean out is very tight with a brand new o ring/seal thingy. Also permanently closed now that it has transom plates. If i suck up a rope im effed

You're f'ed even if you had a cleanout. Lol

Just for shits and giggles,  tighten the gland nuts a little then back them off again.

GT
  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
  • Boat #2: 19' Bubble deck Jet BBC Berkeley
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If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray

steves86ta

  • Karma: +41/-0
Re: Cavitation Talk
« Reply #9 on: March 01, 2016, 05:39:32 pm »
You're f'ed even if you had a cleanout. Lol

Just for shits and giggles,  tighten the gland nuts a little then back them off again.

GT

I'll try that. But I think I'm being cheap. Need to save up and here me a diverter
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69kona

  • Karma: +13/-0
Re: Cavitation Talk
« Reply #10 on: March 01, 2016, 05:44:05 pm »
Hey Steve u don't think it's more of a slip in the pump ... Kinda acting like a stall convertor  in a automatic transmission?
Just throwing it out there..... From what I hear from sitting on the beach


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steves86ta

  • Karma: +41/-0
Re: Cavitation Talk
« Reply #11 on: March 01, 2016, 05:44:55 pm »
Hey Steve u don't think it's more of a slip in the pump ... Kinda acting like a stall convertor  in a automatic transmission?
Just throwing it out there..... From what I hear from sitting on the beach


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That's exactly what it is. But there's no gears or anything to "slip"
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69kona

  • Karma: +13/-0
Re: Cavitation Talk
« Reply #12 on: March 01, 2016, 05:49:19 pm »
Just leave it! ... I'm still trying to get my boat dialed in too pull u now lol..
U fix it I'm really gonna have to do my homework...😜


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steves86ta

  • Karma: +41/-0
Re: Cavitation Talk
« Reply #13 on: March 01, 2016, 05:50:32 pm »
Just leave it! ... I'm still trying to get my boat dialed in too pull u now lol..
U fix it I'm really gonna have to do my homework...😜


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LOL!  I wanna get this fixed then start to bump the nitrous up. Picked up 2 new sets of plugs both colder for more nitrous
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69kona

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Re: Cavitation Talk
« Reply #14 on: March 01, 2016, 05:53:57 pm »
Nice! I'm gonna have to step it up a little more lol..I curious the gap from face of wear ring to impeller is?



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steves86ta

  • Karma: +41/-0
Re: Cavitation Talk
« Reply #15 on: March 01, 2016, 05:55:29 pm »
Nice! I'm gonna have to step it up a little more lol..I curious the gap from face of wear ring to impeller is?



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Do you mean side clearance?

About. 012 on each side

Its a non shouldered wear ring
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69kona

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Re: Cavitation Talk
« Reply #16 on: March 01, 2016, 05:59:50 pm »
No from shoulder of wear ring  even if it's not a shoulder wear ring. If the gap is executive might make it slip just throwing it out there..


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steves86ta

  • Karma: +41/-0
Re: Cavitation Talk
« Reply #17 on: March 01, 2016, 06:04:42 pm »
No from shoulder of wear ring  even if it's not a shoulder wear ring. If the gap is executive might make it slip just throwing it out there..


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I'll be honest I didn't measure that.  It bottomed out in the case.

And I bottomed the impeller out or so I think? 

I guess I could pull the bowl and measure that?  If I was able to bottom the impeller out is it possible its to far from there?
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GT Jets

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Re: Cavitation Talk
« Reply #18 on: March 01, 2016, 06:24:38 pm »
So what you're experiencing is called low NPSH.

Look it up in engineering toolbox.

When the boat is idling at rest,  there is very little volume in the pump. This creates a fairly low pressure differential across the impeller.

When you hammer down, there is a very sudden and drastic increase in the pressure differential with the low side (suction side) being too low for the impeller to "suck" the water and the slightest little bit of air makes the impeller "blow a hole" in what should be a solid,  pressurized column of water.

The loader does "ZERO" to load the impeller without forward movement. This is where an inducer comes in.

A few things need to be checked before taking that drastic step.

Make sure there is no way air can be pulled in through the packing. I use the graphite impregnated seals and make sure it is snug. You should see approximately 1 drop every few minutes at rest. While running, it may not leak at idle,  but will drip as the intake pressure increases.

This should never exceed 10-15 drops a minute.

The cleanout is another potential air source.

One stupid trick is in the drivers control.

Double tap the throttle on take off, in other words,  blip the throttle a split second before hammer down. This purges any air from the pump.

Hull design, water conditions and overall pump set up all come into play when looking at these scenarios.

A Diverter can help, but it is still entirely dependent on forward hull movement.

Good luck
  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
  • Boat #2: 19' Bubble deck Jet BBC Berkeley
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If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray

steves86ta

  • Karma: +41/-0
Re: Cavitation Talk
« Reply #19 on: March 01, 2016, 06:32:12 pm »
So what you're experiencing is called low NPSH.

Look it up in engineering toolbox.

When the boat is idling at rest,  there is very little volume in the pump. This creates a fairly low pressure differential across the impeller.

When you hammer down, there is a very sudden and drastic increase in the pressure differential with the low side (suction side) being too low for the impeller to "suck" the water and the slightest little bit of air makes the impeller "blow a hole" in what should be a solid,  pressurized column of water.

The loader does "ZERO" to load the impeller without forward movement. This is where an inducer comes in.

A few things need to be checked before taking that drastic step.

Make sure there is no way air can be pulled in through the packing. I use the graphite impregnated seals and make sure it is snug. You should see approximately 1 drop every few minutes at rest. While running, it may not leak at idle,  but will drip as the intake pressure increases.

This should never exceed 10-15 drops a minute.

The cleanout is another potential air source.

One stupid trick is in the drivers control.

Double tap the throttle on take off, in other words,  blip the throttle a split second before hammer down. This purges any air from the pump.

Hull design, water conditions and overall pump set up all come into play when looking at these scenarios.

A Diverter can help, but it is still entirely dependent on forward hull movement.

Good luck

Thank you for the info.

My rope seals are the graphite ones. I can for sure tighten it up more

The blipping thing may help. Also the disturbance before the intake might have hurt the scenario as well.

I think I'm gtg on the clean out. Its very tight and. Brand new seal.  Buy I'll double check the tightness.

Its also silicones around it. 

I'm curious if the double tap will help.

Wouldn't the pump such air all the time regardless if I was at idle or moving? If that was the case?
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jyeager

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Re: Cavitation Talk
« Reply #20 on: March 01, 2016, 09:22:56 pm »

Any ideas on how to do a stop with a manual diverter?
You can put washers in between there to stop the travel. Or just adjust it t where the first or second notch back is 2* down.


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Re: Cavitation Talk
« Reply #21 on: March 01, 2016, 09:23:41 pm »
Also what kind of shoe do you have and what is the size opening?


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steves86ta

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Re: Cavitation Talk
« Reply #22 on: March 02, 2016, 06:07:38 am »
Also what kind of shoe do you have and what is the size opening?


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This I'm not sure of. I think it's a back cut shoe

The opening is have to measure
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jimpen

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Re: Cavitation Talk
« Reply #23 on: March 02, 2016, 08:55:33 am »
GT is right most of the stuff being discussed will only help with forward movement. Install a inducer and no NOS until wide open throttle. The work you did to the bottom may or may not help but from a dig there will not be a change. 


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Re: Cavitation Talk
« Reply #24 on: March 02, 2016, 09:19:37 am »
Since the boats in storage, and im bored and exercising my google-fu today. I wanted to make a thread asking a couple questions. More for knowledge/education than anything

I have a Rogers, 5.3 LS powered.

According to the impeller chart im making somewhere in the neighborhood of 400-425 hp on motor @ 5200 rpm, and about 600 on a 150hp shot spinning about 6200 rpm or so with a B- AT impeller.

The boat runs 70 on motor (repeatably have seen higher a couple times) and 85 on the bottle on GPS.

Heres where my Cavitation questions come in.

The Pump is a JC suction housing, Dominator Bowl, AT aluminum impeller. No inducer, no stuffer, everything is stock. Has a loader by GS i dont know what kind, and i believe a Backcut shoe. No Diverter, Full droop with a set of adjustable wedges. Wedges are setup at 3*up and i also have a rideplate. I dont know the angle of the rideplate somewhere around 3.75 degrees

Before the intake there is a pretty bad transition, think a Flat intake in a rounded hull. And not blended in. I have corrected this, but have not taken the boat out since.

Ride plate should be a negative.  (Up from keel line) in other words it would read -3.75°. It should not be down from the keel line, especially with a back cut shoe.

Do you have a bowl bleed?

GT
« Last Edit: March 02, 2016, 10:08:36 am by GT Jets »
  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
  • Boat #2: 19' Bubble deck Jet BBC Berkeley
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If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray

 


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