Cavitation Problems with Moombas

[Ian Brantford]

Cavitation (http://en.wikipedia.org/wiki/Cavitation) is the formation of water vapour due to a very low-pressure area. It can happen routinely to a prop under heavy load if the prop is not properly cupped or pitched for the load to which it is subjected. The low pressure can damage the prop, although most commonly for small recreational boats, it will be limited to tearing some paint off of aluminum props. Low-quality or heavily-used props may acquire pits.

A completely different phenomenon is slippage due to air getting around the prop. This will typically happen in a tight turn when the water is choppy. The chop provides an opportunity for the hull, as it slides sideways, to close off a large air bubble between wave peaks. It is not a low-pressure phenomenon and does not pose the same risk of prop damage, although there could be small areas of low pressure as the water/air combination foams around the prop. Slippage is typically very short-lived. Since it involves an interaction of the water's surface and the hull, the hull design can play a part.

In real life, I notice no cavitation and only rare slippage in my Moomba XLV. The slippage only occurs when there is no ballast in use. Neither of these things are significant issues at all. Whoever is telling you about "boat quality" causing cavitation is experiencing a problem with "opinion quality".

[smokedog2]

add some weight to the center ballast

If you yank & bank tubes you can cause a little cavitation the weight pushes the prop down

[jester]

I have owned an 08 OBV for over a year now with no issues on Cavitation. Does not matter if the boat is full or empty.

[Razzman]

I'll still say it's not possible, the prop is too far under the boat. If your getting your prop out of the water then you have greater issues like NO water under the boat. Just my two cents.

Cav & slippage is common on outboards & I/O's. Cav & slippage is a problem or issue with large ships. Submarines have fought cav forever, BUT look at the horsepower & torque applied in these instances, our boats could only dream of those kinds of ratios.

[Form&Function]

My lsv does not cavitate but it will suck air under the boat in a sharp turn with the ballast full.It also sucks are past the paddle wheel and freewheels the perfectpass into a false reading at idle.Granted the water has to be a little ruff for it to act up.

[DDL]

We're running an XLV and any combination of tube or rider causes slipping in the corners. The kids like the whip on smooth water and I can't get the backside to stick long enough to get them around. Even doing a couple of quick hard turns to get them outside the wake will cause the slip. When pulling a couple of heavier riders on tubes it's noticably worse.

I also find the boat sticks the corners better doing a left turn over a right.

Is this prop related or the general design of the competition hull?

[mmandley]

Only related to the tubing portion of your post.

I find it best when people want a hard ride or thrown to the flats, take a 45 degree turn and that will toss them outside the wake, then gradually keep turning and it will put so much force on the tube it forces it to glide out as fare as the rope will allow to the flats. I do this till i get the 180 turn done, then you end up crossing your own rollers and thats a bumpy ride also but the tub is behind the boat. Turn the opposite direction and toss the tube to the other side of the boat into the flats and cross your rollers and there bound to catch air.

Ive tried S turns and such and it just seems like the boat is working harder to get the tubers what they really want.

[Sled491]

The left and right thing is the prop rotation.

I ski with a couple of guys who each own MC's, the one does cavitate the other no. My Outback, Never!

[Ian Brantford]

We're running an XLV and any combination of tube or rider causes slipping in the corners. The kids like the whip on smooth water and I can't get the backside to stick long enough to get them around. [...]

I have an XLV, and only one of the 5 tubes that I have available has a problem getting outside the wake. It has an inflated bottom that is ribbed, and this helps it to track too straightly. However, it still gets outside the wake with just a bit more effort.

A good way to get a tube outside the wake is to speed up a bit, go into a turn to give the tube some sideways momentum, then slow down a bit (and possibly stop turning). The tube's momentum will carry it outside the wake, and only marginal speed/turning is needed to keep it there indefinitely.

By "speed", I do not mean a lot. When I upgraded from a smaller boat, I found that I was consistently misjudging speed and going about 5 MPH too fast in the XLV for what the tube and rider required. I learned to pay attention to the speedometer more than "feel". Most spirited tubing (where you are pulling lots of turns and dragging the rider over your own wake from 30 seconds earlier) is best done at 10-20 MPH. 10 MPH to build the wake and 15-20 to drag the tube over it for a good bounce.

That's for braver adults, who dare you to bounce them out of the tube. For lesser folk who want a moderate bounce without the ejection, you can just let off the throttle at the last moment, so the tube goes over the wake under its own momentum, without tension in the tow rope. The rider and tube will bounce up, down, and resume.

[DDL]

Thanks for the comments and advice gang.

I find I'm always having to adjust my speed slower than faster with this XLV. This boat certainly behaves differently than the 20' Bayliner we were running previously. I've never been a driver that tubes at high speeds (for obvious safety reasons) and I have to admit I was pretty good at giving tube riders all they could handle with that old girl.

Even on moderate or extened turns, the backside of this XLV will pop out frequently. This past weekend we were operating on a smaller lake. As we approached the end of the lake I started into a slow right right with my 100 pound nephew on a single, round tube. He was outside the wake and I wanted to keep the momentum going to get his speed up. As I accelerated through the turn, the backend pop out at least three times. I ended up having to slow right down to get the backend settled back in the water.

It will just take a bit of time to learn the the intricacies of this boat.

Thanks again,
DDL