Cavitation-rocking back and forth while planinh out

[Hollianne97]

Both of my skis do this. and i just counter steer the direction but it isnt safe... Thats for sure, especially if someone is riding. When someone is riding along the nose goes way up... And then if it decides to rock back and forth too! Ug

 

[Coastiejoe]

What is the gap from the impeller to the wear ring?

The more weight the more crucial for things to be right in the pump area.


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[Sportster-2001-951C-Stock]

Stability relates to hull shape, short hulls tend to wander back and forth a bit while coming on plane, and at low speeds jet pump directional stability is reduced so steering correction is necessary at low speed. Wind plays a role as well. Not sure though, does rocking back and forth mean from side to side or front to rear? I wonder if you're describing "porpoising"? Rocker keels tend to porpoise more than a straighter one.

Changing pump shoes to a different shape such as a delta can help change the characteristics on plane, or installing an extended ride plate, even changing the angle of the steering nozzle can help (I moved my Sportster's steering nozzle to the up position to help reduce porpoising).

What you're describing might be normal, it's hard to know without further info such as top speed and RPM and if there are acceleration problems.

If the outer fiberglass layer has separated from the lamination, the hull could behave badly with bizarre planing characteristics, so check your hull bottom carefully.

 

[DavesNotHereMan]

Hollianne97:
I have a 2001 GTX RFI, so I believe it is the same hull as your 2000 model. Mine also does the "rocking" side to side while it is trying to get up on plane. It happens consistently at about 20mph. As soon as it planes out (between 20 & 25), the hull tracks perfectly. Don't try to correct it with steering, you will make it worse. You can use your weight. Think of a bicycle where you are standing on the pedals and trying to accelerate. The frame is really leaning to one side or the other and you counterbalance that with your weight.

The faster you accelerate, the quicker you get on plane and it will go away.


-Dave
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[Spimothy Leary]

we've been discussing this on the 4 stroke section as well

if its side to side, your likely experiencing chine walking, tends to happen when your close, but not quite on plane, and heading at a relatively consistent speed track or accelerating/decelerating slowly. The answer is more throttle, at least on my models.

if its up/down then its porpoising, and the answer is also usually more power.

cavitation might be a separate problem, and i'd start by taking a good look at pump/impeller.

 

[Sportster-2001-951C-Stock]

Chine walking is something that happens at high speeds, typically speeds the hull wasn't designed to achieve. And can be dangerous. Bass boats are quite susceptible b/c many anglers like to overpower their boats thinking going faster than anyone else gives them an edge. I see Bass boats chine walking all the time, exceeding the safe speed of their hull.

A short hull will tend to wander back and forth as it's approaching planing speed, that's the nature of a short hull. My Speedster's hull for instance is practically round thus at low speeds it takes a lot of steering control to compensate and if I cut the engine at no wake speed the boat will do a 180 due to the combination of short hull and engine weight in the rear.

Short hulls tend to porpoise as well (especially if they have a lot of rocker in the keel), will switch ends much easier than a longer (more directionally stable) hull would.

Buy a short kayak and try to paddle it some distance, it's difficult. A longer kayak is the way to go if you aren't just goofing around and actually need to get somewhere.

Chine walking happens when the boat chine bounces off the water at high speed and begins uncontrollably oscillating from chine to chine. Main reason a hull is limited legally, to how much power can be hung on the rear. Look it up.

 

[Hollianne97]

Okay im attempting to respond... The correct way...

The ski rocks back and forth, just like if it is still..... And you lean from one side to the other.. So...when it picks up speed (about 20 or 30 with rpm of +- 4430/4580. Approx.

my other ski (in shop )does this too but, yes i rode it out as it isnt as extreme (it is in shop for driveline pto boot issue. I dont know the cause yet but if something is bent that could cause it i guess).

But This gtx ski, it does it different, it does it extreme and feels like it raises way up off the water (seems that way..) and after it planes out it will start, thats why it is different, because you gotta deal with the rocking at a higher speed.

And i do countersteer slightly, as i decelerate a little... The fix is different every time-it behaves diferent every time. I can see there is a nick in the prop... And i dont know the space between the impeller and warering, i have no gauge, and have never taken off the stuff on the back to get to it.... (assuming the ring thing - is that thick - round thing that has to be pounded in around where the prop is....??)

What is the gap from the impeller to the wear ring?

The more weight the more crucial for things to be right in the pump area.


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[Coastiejoe]

If the gap is thicker than the width of a dime, then the wear ring needs replaced.


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[Hollianne97]

yup, i actually knew that part.. It is fixed. Now. Was a slight bend to driveline, cant see with eye. But i guess it had a bend.

 

[Coastiejoe]

yup, i actually knew that part.. It is fixed. Now. Was a slight bend to driveline, cant see with eye. But i guess it had a bend.

It doesn't take much of a bend for there to be an issue..Glad you are back up and running,,