Added rear ballast, had first ever "shudder" experience under load. Thoughts?

[Joshua J Rowe]

Hey folks. I am still a boat noob so I am looking for opinions on something that happened last night while wakeboarding.

1999 Air Nautique. Boat has run flawlessly since I bought it last summer. I have put maybe 50 hours on it.

I recently added a 600lb fatsac in the rear locker and had it bursting full for the first time with 4 people in the boat worth about 800 lbs, plus the 400lbs factory ballast full.

So here is what happened. When pulling up a friend, who is about 250 lbs, as we got near getting on plan there began a harsh shudder type vibration. I reduced throttle and it stopped so increased throttle again and it started to shudder again. I dropped the throttle to idle, shut it down, and had my boarder swim back to the boat. He felt the rudder and propeller for anything, and there was nothing. I had him get in the boat and we decided to call it a day. As we headed back down the river there was no issue with the shudder. Got up on plane and cruised no problem.

The boat felt like something was slipping.

I am wondering if its possible the prop was creating a vortex and sucking air as we got near plane and had the higher drag and steep angle of the bow.

On the other hand, maybe something slipping in the drivetrain. I am assuming these have a torque converter. I have never experienced a failing TC so I dont know what that feels like. I imagine it might shudder when it cant hold the torque!

Anyone have thoughts on this?

[XBIGPUN66]

Check and make sure the prop is tight. Could also be prop cavitation i suppose. Also prop could be causing the shake due to the high load. Did you try to accelerate the boat sacked out without the rider?

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[Joshua J Rowe]

Check and make sure the prop is tight. Could also be prop cavitation i suppose. Also prop could be causing the shake due to the high load. Did you try to accelerate the boat sacked out without the rider?

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Yes, after the rider got back onboard I brought it up to about 30 mph. Never went WOT but some decent acceleration up to plane. No issue.

I am going to give it a good looking over and if nothing is found we will run without the fatsac next time and see if it happens again. If not I will assume the propeller was surging due to the steep angle of the bow. It felt similar to a waverunning that is sucking air into the impeller. Basically the water version of spinning your tires you know, not enough traction!

Much cooler than a mechanical issue.

[Tom_H]

Check your transmission fluid level. These transmissions can slip with low fluid level, and the additional weight, additional angle, and weight behind the boat might have been just enough to push you over the edge.

It's also possible that you had a good batch of weeds/debris on that came off when you shut down. Did you reverse at all before your buddy swam back to the boat and checked the prop/rudder?

[MikeC]

Cavitation - if everything else checks out, which I think it will, with your boat loaded so heavily your prop was spinning but the boat wasn't going anywhere, this creates an air pocket that will cause your prop to "lose traction" so to speak. When you prop is spinning in air it will cause a harsh vibration.

[swatguy]

I am betting it's cavitation. You're weight needed to be distributed more evenly. Your tank is in the back , 600 lb Sac in back , assuming your people were in the back or at least behind the windshield , You need about 400-600 in the bow to even that out. Willing to be that fixes everything. Next time put 2-3 people in the bow just for a test. Especially with a bigger boy on the rope back there

[Joshua J Rowe]

Check your transmission fluid level. These transmissions can slip with low fluid level, and the additional weight, additional angle, and weight behind the boat might have been just enough to push you over the edge.

It's also possible that you had a good batch of weeds/debris on that came off when you shut down. Did you reverse at all before your buddy swam back to the boat and checked the prop/rudder?

I do not recall if I used reverse or not. That was my initial thought though, maybe some weeds caught up in the prop throwing balance off.

[Joshua J Rowe]

Cavitation - if everything else checks out, which I think it will, with your boat loaded so heavily your prop was spinning but the boat wasn't going anywhere, this creates an air pocket that will cause your prop to "lose traction" so to speak. When you prop is spinning in air it will cause a harsh vibration.

That is what I think is most likely. Is that something you have experienced yourself?

[surroundsound64]

Cavitation. I've had it happen on my 81 when we had about 10 people in the boat and had it also happen on my 2000 when I had it sacked out and full of people just gotta lighten the load it really let it work up to speed.

[Joshua J Rowe]

I am betting it's cavitation. You're weight needed to be distributed more evenly. Your tank is in the back , 600 lb Sac in back , assuming your people were in the back or at least behind the windshield , You need about 400-600 in the bow to even that out. Willing to be that fixes everything. Next time put 2-3 people in the bow just for a test. Especially with a bigger boy on the rope back there

Yes everyone but my gf was in the back. She is only 120lbs so weight distribution was pretty bad. The bow was up quite a bit higher than usual.

I'm going to put the bag in the ski locker next time and see what that does.

[Joshua J Rowe]

Sounds like most folks think cavitation is likely.

I had planned to get a second fatsac for the ski locker to get enough ballast to do some surfing. I am going to go ahead and do the plumbing for that and move this bag into the ski locker. I think that should provide better balance for wakeboarding. Both lockers use the same size bag so I can just add the second bag back in the rear locker later on.

[Joshua J Rowe]

Cavitation. I've had it happen on my 81 when we had about 10 people in the boat and had it also happen on my 2000 when I had it sacked out and full of people just gotta lighten the load it really let it work up to speed.

Thanks, I was hoping to hear someone say they had actually experienced this themselves.

[MikeC]

I've had it a few times, it's not unusual, just annoying. If you're pulling an experienced wakeboarder they should be able to get up with some finesse so your don't have to hammer the throttle. If they are not that good then just leave the ballast out until they are

[Joshua J Rowe]

Looks like you guys were correct. Without the additional ballast there was no issue. Going to plumb up the ski locker for ballast and move the bag there!