Cavitation?

[RyanB72]

Hi All. I recently worked through some issues with my carb and got the ski out today on the lake and it ran great! However after about 45 minutes on the lake I noticed that occasionally it wouldn't pull out of the water onto a plane. Id give it full throttle and it just did not accelerate. At 1st this only happened a few times. Then as the day went on it got worse and worse. The motor spins up like normal it seems and there is a big jet coming out the back as normal but the ski make a weird vibrating sound. and just doesn't have that pulling power that is normal. By the end of the day it would only occasionally "grab" where it pulls right out of the water.

Now if I just stay on it the ski gains speed and maybe after about 20-30 seconds it finally gets up on a plane. Once it's up it pulls hard and operates normally.

I've recently replaced the wear ring. I haven't pulled the jet pump off to get a good look, but looking inside from the back it looks ok so far. No more then 2 hours on this new wear ring.

Looking for things to check. Thanks!

 

[Coastiejoe]

Based on your description it does sound like a wear ring or impeller issue.

You are going to have to take a very good look in there. See if you wrapped something around the shaft irbid the impeller is bent. Clearance also.

Bent impellers are tricky. All looks good but it isn't. If a blade is bent it creates a cavitation issue as some blades are pushing and one blade is not.

 

[LouDoo]

The other most common cause for cavitation is the carbon ring part no. 7, to check it, pull back on the bellows and put a dab of waterproof grease on the carbon ring, if this fixes the problem replace the carbon ring.

http://fiche.seadoowarehouse.com/se...ake=seadoopwc&a=233&b=18&c=0&d=-DRIVE-SYSTEMs

Lou

 

[RyanB72]

Thanks for the replies guys.

Coastiejoe, I'll pull the jet pump out again and take a look. Do I measure the blades somehow?

Lou, when I 1st got the ski one of the problems I ran into was water getting by the carbon seal. I pulled it out and there are no bad wear grooves or anything. so I put it in and re-adjusted it. No more water leaking into the engine bay. Are you thinking that air is now being sucked past the carbon ring into the jet pump? Or is there something else going on with the carbon ring? Just trying to understand how it would create the problem.

Thanks again!

 

[crisso]

What Lou said sounds very close to the mark.

 

[Coastiejoe]

Lou is likely on the right path. It can suck air from inside the hull and cause cavitation.

 

[RyanB72]

Hi all,

Before I go and spend money on a replacement carbon seal setup, just looking for some clarification. If the Stainless flange is right up against the carbon seal and I was not taking on water past it, could I really get enough air past it to cause cavatation?

Also, how much 'force' should the little bellows jacket the seal is in be applying to the seal against the stainless flange?

 

[RyanB72]

Sort of a bump .. Previous post I asked about how much 'force' the bellows jacket should be applying to the carbon seal against the stainless flange? I took a look at them last night. The carbon seal is in good condition, no grooves. Same with the flange. However I had it adjusted so there is very small amount of pressure against the flange. I was worried about too much pressure building up heat and melting or destroying things.

As mentioned before as well, there was no water intrusion past the seal into the engine bay. So I assume that by adjusting things so it applies more pressure would alleviate the air getting past it. Just do not want to go overboard.

 

[Coastiejoe]

Air is both thinner and lighter than water. So it will "leak" before water will.

Also, when you accelerate and travel forward it creates a Venturi Effect.

Meaning, you are traveling forward and water is passing under the hull. When it travels past the seal it puts a negative draw on the seal. So,,,, you have atmospheric pressure pushing down on and inside your hull at 14.7 psi or so. This only adds to the seal sucking air.

You don't have to leak a drop of water and you could still be leaking air from the hull, through the seal, then into the impeller creating a cavitation issue.

I'm not saying this is your issue, just explaining how it could be and not leak water into the hull.

 

[RyanB72]

Thanks everyone for the replies. I took the ski out this past weekend and had a full day of fun with it. Pulled the kids on the 2 person tube a few times and gave some rides. For all the trouble this ski has been it's quickly turning into my favorite over the Yamaha Waveraider I have. Much more stable, and nearly as fast as my friends XP and that has a new motor.

Anyhow, as far as how to adjust the seal... It was originally clamped around the leading edge of the seal. I let loose the clamp and worked the seal forward exposing the back part of the carbon seal which had a smaller shoulder. I clamped down tight to that with the hose clamp. That gave me enough forward movement to keep more pressure from the seal to the flange. It's not a ton, but it's more than I had before. It still gave me the option to move the entire piece forward more and clamp it at the back. So far so good.

thanks again for all the replies!